Hands-On GUI Programming with C++ and Qt5

Build stunning cross-platform applications and widgets with the most powerful GUI framework

Lee Zhi Eng

BIRMINGHAM - MUMBAI

Hands-On GUI Programming with C++ and Qt5

Copyright © 2018 Packt Publishing

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Contributors

About the author

Lee Zhi Eng is a self-taught programmer who has worked as an artist and programmer at several game studios before becoming a part-time lecturer for 2 years at a university, teaching game development subjects related to Unity and Unreal Engine.

He has not only taken part in various projects related to games, interactive apps, and virtual reality, but has also participated in multiple projects that are more oriented toward software and system development. When he is not writing code, he enjoys traveling, photography, and exploring new technologies.

About the reviewer

Nibedit Dey is a technopreneur with multidisciplinary technology background. He holds a bachelor's degree in biomedical engineering and a master's degree in digital design and embedded systems. Before starting his entrepreneurial journey, he worked for L&T and Tektronix for several years in different R&D roles. He has been using Qt to build complex software products for the past 8 years. Currently, he is a healthcare innovation fellow at IIT, Hyderabad, and is involved in the development of a new medical device.

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Table of Contents

1. Title Page

2. Copyright and Credits

1. Hands-On GUI Programming with C++ and Qt5

3. Packt Upsell

1. Why subscribe?

2. PacktPub.com

4. Contributors

1. About the author

2. About the reviewer

3. Packt is searching for authors like you

5. Preface

1. Who this book is for

2. What this book covers

3. To get the most out of this book

1. Download the example code files

2. Download the color images

3. Conventions used

4. Get in touch

1. Reviews

1. Introduction to Qt

1. What is Qt?

1. Why use Qt?

2. Discovering tools in Qt

1. Qt Designer

2. Qt Quick Designer

2. Downloading and installing Qt

1. Setting up the working environment

3. Running our first Hello World Qt program

4. Summary

2. Qt Widgets and Style Sheets

1. Introduction to Qt Designer

2. Basic Qt widgets

3. Qt Style Sheets

4. Summary

3. Database Connection

1. Introducing the MySQL database system

2. Setting up the MySQL database

3. SQL commands

1. SELECT

2. INSERT

3. UPDATE

4. DELETE

5. JOIN

4. Database connection in Qt

5. Creating our functional login page

6. Summary

4. Graphs and Charts

1. Types of charts and graphs in Qt

1. Line and spline charts

2. Bar charts

3. Pie charts

4. Polar charts

5. Area and scatter charts

6. Box-and-whiskers charts

7. Candlestick charts

2. Implementing charts and graphs

3. Creating the dashboard page

4. Summary

5. Item Views and Dialogs

1. Working with item view widgets

1. Creating our Qt Widgets application

1. Making our List Widget functional

2. Adding functionality to the Tree Widget

3. Finally, our Table Widget

2. Working with dialog boxes

3. Creating File Selection Dialogs

4. Image scaling and cropping

5. Summary

6. Integrating Web Content

1. Creating your own web browser

1. Adding the web view widget

2. Creating a UI for a web browser

3. Managing browser history

2. Sessions, cookies, and cache

1. Managing sessions and cookies

2. Managing cache

3. Integrating JavaScript and C++

1. Calling JavaScript functions from C++

2. Calling C++ functions from JavaScript

4. Summary

7. Map Viewer

1. Map display

1. Setting up the Qt location module

2. Creating a map display

2. Marker and shape display

1. Displaying position markers on a map

2. Displaying shapes on a map

3. Obtaining a user's location

4. Geo Routing Request

5. Summary

8. Graphics View

1. Graphics View framework

1. Setting up a new project

2. Movable graphics items

3. Creating an organization chart

2. Summary

9. The Camera Module

1. The Qt multimedia module

1. Setting up a new project

2. Connecting to the camera

3. Capturing a camera image to file

4. Recording a camera video to file

5. Summary

10. Instant Messaging

1. The Qt networking module

1. Connection protocols

2. Setting up a new project

2. Creating an instant messaging server

1. Creating TCP Server

2. Listening to clients

3. Creating an instant messaging client

1. Designing the user interface

2. Implementing chat features

4. Summary

11. Implementing a Graphics Editor

1. Drawing vector shapes

1. Vector versus bitmap

2. Drawing vector shapes using QPainter

3. Drawing text

2. Saving vector images to an SVG File

3. Creating a paint program

1. Setting up a user interface

4. Summary

12. Cloud Storage

1. Setting up the FTP server

1. Introducing FTP

2. Downloading FileZilla

3. Setting up FileZilla

2. Displaying the file list on the list view

1. Setting up a project

2. Setting up user interface

3. Displaying the file list

4. Writing the code

3. Uploading files to the FTP server

4. Downloading files from the FTP server

5. Summary

13. Multimedia Viewers

1. Revisiting the multimedia module

1. Dissecting the module

2. The image viewer

1. Designing a user interface for the image viewer

2. Writing C++ code for image viewers

3. The music player

1. Designing a user interface for music players

2. Writing C++ code for music players

4. The video player

1. Designing a user interface for video players

2. Writing C++ code for video players

5. Summary

14. Qt Quick and QML

1. Introduction to Qt Quick and QML

1. Introducing Qt Quick

2. Introducing QML

2. Qt Quick widgets and controls

3. Qt Quick Designer

4. Qt Quick layouts

5. Basic QML scripting

1. Setting up the project

6. Summary

15. Cross-Platform Development

1. Understanding compilers

1. What is a compiler?

2. Build automation with Make

2. Build settings

1. Qt Project (.pro) File

2. Comment

3. Modules, configurations, and definitions

4. Platform-specific settings

3. Deploying to PC platforms

1. Windows

2. Linux

3. macOS

4. Deploying to mobile platforms

1. iOS

2. Android

5. Summary

16. Testing and Debugging

1. Debugging techniques

1. Identifying the problem

2. Print variables using QDebug

3. Setting breakpoints

2. Debuggers supported by Qt

1. Debugging for PC

2. Debugging for Android devices

3. Debugging for macOS and iOS

3. Unit testing

1. Unit testing in Qt

4. Summary

17. Other Books You May Enjoy

1. Leave a review - let other readers know what you think

Preface

Qt 5, the latest version of Qt, enables you to develop applications with complex user interfaces for multiple targets. It provides you with faster and smarter ways to create modern UIs and applications for multiple platforms. This book will teach you how to design and build graphical user interfaces that are functional, appealing, and user-friendly.

By the end of this book, you will have successfully learned about high-end GUI applications and will be capable of building many more powerful, cross-platform applications.

Who this book is for

This book will appeal to developers and programmers who would like to build GUI-based applications. Basic knowledge of C++ is necessary, and the basics of Qt would be helpful.

What this book covers

Chapter 1, Introduction to Qt, will give you a tour of Qt. In this book, you'll download the SDK, install Qt, and, most importantly, install Qt Creator, which is used as both the user interface designer and the IDE for writing and compiling C++ scripts.

Chapter 2, Qt Widgets and Style Sheets, will introduce you to the different types of widgets generally used in Qt to develop desktop applications. You will learn the first step to create your own application, which uses all kinds of widgets and customizations, using the powerful style sheet mechanism provided by Qt, which is very similar to CSS for web.

Chapter 3, Database Connection, will introduce you to the MariaDB database and teach you how to connect to it using Qt. You will first learn what is MariaDB database and how to call simple SQL commands to fetch and insert data into a MariaDB database. We will then create a simple but fully functional login page.

Chapter 4, Graphs and Charts, explores the Chart feature to allow users to easily render different types of graphs and charts, such as pie chart, bar chart, and line graph. We will make use of the knowledge learned from this chapter to create a dashboard page for an application, which displays all kinds of statistical summary of their company and business.

Chapter 5, Item Views and Dialogs, will teach you how to display a list of information by using three different item view widgets—a list widget, tree widget, and table widget. You will also learn how to prompt a message box to display error messages, warnings, and confirmation. You will then create an application that can load, scale, and crop images.

Chapter 6, Integrating Web Content, will empower you to use the Qt WebEngine module and make a simple web browser, which displays a web page. We will then go through what is sessions, cookies, and cache and subsequently how to manage them with Qt WebEngine. Finally, you will learn how to make your C++ code communicate with JavaScript content through the WebChannel mechanism.

Chapter 7, Map Viewer, will focus on creating a map display. This map will be used to display the location of places and people. You will learn how to implement the QtLocation module, understand the coordinate system, display location markers on the map, and so on.

Chapter 8, Graphics View, will provide a platform to manage and interact with a large number of custom-made graphical items and a view widget to visualize the items with support for zooming and rotation. You will learn how to make an organization's chart page that displays the structure of an organization and the relationships and relative ranks of its parts and positions/jobs.

Chapter 9, The Camera Module, will explore how to display camera images using Qt. Qt provides us with a multimedia module that enables us to easily take advantage of a platform's multimedia capabilities, such as connecting to the camera of the computer.

Chapter 10, Instant Messaging, will discuss the networking module and help us to make our own simple instant messaging program.

Chapter 11, Implementing a Graphics Editor, is divided into two sections, based on the two types of graphics in the rendering world–bitmap graphics and vector graphics. Both are quite different but essential to learning in order to understand 2D graphics rendering. You will learn how to draw graphics using Qt in this chapter.

Chapter 12, Cloud Storage, will teach you how to upload different types of files to the FTP server and display them in a list. The user will be able to download the file and open them with different types of viewers depending on its file format.

Chapter 13, Multimedia Viewers, will deal with creating a media player instead of using the default software on your computer. In this chapter, you will learn how to create an image viewer, a music player, and a video player.

Chapter 14, Qt Quick and QML, will introduce you to the basics of QML scripting, which is one of the most recent trends in the Qt world.

Chapter 15, Cross-Platform Development, will show you how to export applications to different platforms without re-writing the code from scratch. You will learn some of the essential settings and tips to look out for when porting applications to different platforms. Besides PC platforms, you will also learn how to export applications to mobile platforms.

Chapter 16, Testing and Debugging, will teach you the essentials of how to use various techniques to test and debug your Qt application.

To get the most out of this book

In order to successfully execute all the codes and instructions in this book, you would need  the following:

- A basic PC/Laptop

- A working internet connection

- Qt 5.10

- MariaDB 10.2 (or MySQL Connector)

- Filezilla Server 0.9

We will deal with the installation processes and details as we go through each chapter.

Download the example code files

You can download the example code files for this book from your account at www.packtpub.com. If you purchased this book elsewhere, you can visit www.packtpub.com/support and register to have the files emailed directly to you.

You can download the code files by following these steps:

1. Log in or register at www.packtpub.com.

2. Select the SUPPORT tab.

3. Click on Code Downloads & Errata.

4. Enter the name of the book in the Search box and follow the onscreen instructions.

Once the file is downloaded, please make sure that you unzip or extract the folder using the latest version of:

- WinRAR/7-Zip for Windows

- Zipeg/iZip/UnRarX for Mac

- 7-Zip/PeaZip for Linux

The code bundle for the book is also hosted on GitHub at https://github.com/PacktPublishing/Hands-On-GUI-Programming-with-CPP-and-Qt5 In case there's an update to the code, it will be updated on the existing GitHub repository.

We also have other code bundles from our rich catalog of books and videos available at https://github.com/PacktPublishing/. Check them out!

Download the color images

We also provide a PDF file that has color images of the screenshots/diagrams used in this book. You can download it here: https://www.packtpub.com/sites/default/files/downloads/HandsOnGUIProgrammingwithCPPandQt5_ColorImages.pdf.

Conventions used

There are a number of text conventions used throughout this book.

CodeInText: Indicates code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles. Here is an example: "We call the test() function at the MainWindow constructor."

A block of code is set as follows:

void MainWindow::test()
{
int amount = 100;
amount -= 10;
qDebug() << "You have obtained" << amount << "apples!";
}

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
test();
}

Any command-line input or output is written as follows:

********* Start testing of MainWindow *********
Config: Using QtTest library 5.9.1, Qt 5.9.1 (i386-little_endian-ilp32 shared (dynamic) debug build; by GCC 5.3.0)
PASS   : MainWindow::initTestCase()
PASS   : MainWindow::_q_showIfNotHidden()
PASS   : MainWindow::testString()
PASS   : MainWindow::testGui()
PASS   : MainWindow::cleanupTestCase()
Totals: 5 passed, 0 failed, 0 skipped, 0 blacklisted, 880ms
********* Finished testing of MainWindow *********

Bold: Indicates a new term, an important word, or words that you see onscreen. For example, words in menus or dialog boxes appear in the text like this. Here is an example: "The third option is Toggle Bookmark, which lets you set a bookmark for your own reference."

Warnings or important notes appear like this.

Tips and tricks appear like this.

Get in touch

Feedback from our readers is always welcome.

General feedback: Email feedback@packtpub.com and mention the book title in the subject of your message. If you have questions about any aspect of this book, please email us at questions@packtpub.com.

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Introduction to Qt

Qt (pronounced cute) has been used by software engineers and developers for more than two decades to create cross-platform applications since its first release. After several changes of ownership and numerous major code overhauls, Qt has become even more feature rich and supports even more platforms than it used to. Qt not only excels in desktop application development, but is also excellent for both mobile and embedded systems development.

In this chapter, we will cover the following topics :

- What is Qt?

- Why use Qt?

- Using tools in Qt

- Downloading and installing Qt

- Setting up a working environment

- Running our first Hello World Qt program

Throughout this chapter, we will learn more about the history of Qt. Then, we'll proceed to build our first example program using the latest version of Qt, which is Qt version 5. For the convenience of our readers, we will simply refer to it as Qt throughout the book.

What is Qt?

Currently, the latest version of Qt (as this book is being written) is version 5.10. This version incorporated a lot of new features as well as thousands of bug fixes, which makes Qt a really powerful and stable development kit for software developers and system engineers alike. Qt has a huge package of SDK (software development kit) that contains a wide range of tools and libraries for helping developers get their job done without worrying too much about technical issues related to a specific platform. Qt handles all the messy integration and compatibility issues for you behind the curtain so you don't have to deal with them. This will not only improve efficiency but also reduces development costs, especially when you're trying to develop cross-platform applications that cater to a wider range of users.

There are two types of license for Qt:

- The first type is the Open Source License, which is free of charge, but only if your project/product fits their terms and conditions. For example, if you made any changes to the Qt's source code, it is an obligation for you to submit back those changes to Qt developers. Failure to do so could result in serious legal issues, and therefore, you might want to pick the second option instead.

- The second type of license is the Commercial License, which gives you full rights to proprietary Qt source code modifications and keeps your application private. But of course, these privileges come with a set of fees.

If you're just starting to learn Qt, don't get pushed back by these terms, as you're certainly not going to modify the source code of Qt libraries or recompile it from source anyway, at least not now.

For more information regarding Qt's licensing, please visit https://www.qt.io/licensing-comparison.

Why use Qt?

It's not hard to see why Qt stands a chance of winning against all other existing SDKs out there in the market; first of all, cross-platform compatibility. You can hardly find any other development kits that support so many platforms without writing different sets of code for each platform. By eliminating these extra steps, programmers can just focus on developing their applications without the need to worry about the implementation of each and every platform-specific feature. Furthermore, your code will look clean without all the #ifdef macros and having to load different dependencies for different platforms.

Qt generally uses C++, which is a compiled language that generates small and efficient code. It is also well documented and follows a very consistent set of naming conventions, which reduces the learning curve for the developer.

Do be aware that Qt does include a small amount of features that only work on specific platforms. However, these are minimal and often for special use cases, such as Qt Sensors, which only work on mobile platforms; Qt Web Engine, which only works on desktops; Qt NFC, only for Android and Linux; and so on. Those are some very specific functionalities that only exist on certain platforms that support them. Other than that, common features are usually supported on all platforms.

Discovering tools in Qt

Qt comes with a set of tools that make programmers' lives easier. One of the tools is Qt Creator (seen in the following screenshot), which is an IDE (integrated development environment) that consists of a code editor and a GUI (graphical user interface) designer that works hand-in-hand with other Qt tools, such as the compiler, debugger, and so on. The most attractive tool among all is, of course, the GUI designer, which comes with two different types of editors: one for widget-based applications, called Qt Designer, and another for Qt Quick Application, called Qt Quick Designer. Both tools can be accessed directly in Qt Creator when you open up a relevant file format. Qt Creator also includes a built-in documentation viewer called Qt Assistant. It is really handy since you can look for the explanation about a certain Qt class or function by simply hovering the mouse cursor over the class name in your source code, and pressing the F1 key. Qt Assistant will then be opened and show you the documentation related to the Qt class or function:

Qt Designer

Qt Designer is normally used by developers to design GUIs for desktop applications, while Qt Quick Designer is usually used for mobile and embedded platforms. With that being said, both formats run just fine on both desktop and mobile formats, the only difference is the look and feel, and the types of languages used.

The GUI file saved by Qt Designer carries the .ui extension, which is saved in XML format. The file stores the attributes of each and every widget placed by the GUI designer, such as position, size, margin, tooltip, layout direction, and so on. It also saves the signal-and-slot event names within itself for easily connecting with the code in the later stages. This format does not support coding and only works for Qt C++ projects, namely widget-based application projects.

Qt Quick Designer

On the other hand, Qt Quick Designer saves GUI files in both .ui.qml and .qml formats. Qt Quick is a very different type of GUI system in terms of technological concept and development approach, which we will cover in Chapter 14, Qt Quick and QML. Instead of XML format, Qt Quick Designer saves its data in a declarative language similar to JavaScript called QML. QML not only allows the designer to customize their GUI in a CSS-like (Cascading Style Sheets) fashion, it also allows the programmer to write functional JavaScript within the QML file. As we mentioned earlier, .ui.qml is the file format used for visual decoration only while .qml contains application logic.

If you're doing a simple program using Qt Quick, you don't have to touch any C++ coding at all. That's especially welcoming for web developers because they can immediately pick up Qt Quick and develop their own application without a steep learning curve; everything is just so familiar to them. For much more complex software, you can even link C++ functions from QML, and vice versa. Again, if you're interested in learning more about Qt Quick and QML, please head over to Chapter 14, QtQuick and QML.

Since Qt Creator is also written in Qt libraries itself, it is also totally cross-platform. Hence, you can use the same set of tools across different development environments and develop a unified workflow for your team, resulting in better efficiency and cost-effectiveness.

Other than that, Qt comes with many different modules and plugins, which cover a wide range of functionality you need for your projects. There is often no need for you to look for other external libraries or dependencies and try and implement them yourself. The abstraction layer of Qt makes the backend implementation invisible to the users and results in a unified coding style and syntax. If you try to put together a bunch of external dependencies yourself, what you'll find is each library has its own distinctive coding style. It's quite a mess when mixing up all the different coding styles in the same project, unless you make your own abstraction layer, which is a very time-consuming task. Since Qt already includes most, if not all the modules that you need to create feature-rich applications, there is no need for you to implement your own.

For more information regarding the modules that come with Qt, please visit: http://doc.qt.io/qt-5/qtmodules.html.

That being said, there are also many third libraries out there that extend Qt for features that Qt itself does not support, such as libraries that focus on game development or any other features that are designed for the specific user group.

Downloading and installing Qt

Without wasting any time, let's begin with our installation! To get the free installer for Open Source Qt, first go to their website at https://www.qt.io. There, look for the button that says Download Qt (the website might look different if they have updated it). Do note that you might be downloading the free trial version for the Commercial Qt, which you cannot use after 30 days. Make sure that you are downloading the open source version of Qt instead. Also, you may want to pick the right installer for your platform, since there are many different installers of Qt for different operating systemsWindows, macOS, and Linux.

You might wonder why the installer is so small in size—it is only around 19 MB. This is because the unified online installer doesn't actually contain any of the Qt packages, but is rather a downloader client which helps you to download all the relevant files and install them to your computer once the download has completed. Once you have downloaded the online installer, double-click on it and you will be presented with an interface like this (the following example is running on a Windows system):

Click the Next button, and a DRM (Digital Rights Management) page will appear and ask you to log in with your Qt account. If you don't have one, you can also create your account on the same page:

Once you have logged in, you will see a message that says No valid commercial license available in your Qt Account for this host platform. Don't worry about that, just click the Next button to proceed.

Next, you will be asked to specify the installation path. The default path is usually just fine, but you can change it to any other path as you please. Also, you can either leave the Associate this common file types with Qt Creator option checked, or uncheck it manually if otherwise.

After that, you will be presented with a series of checkboxes with which you can select the version(s) of Qt you need to install to your computer. Typically, for new users, the default options are sufficient. If you don't need some of the options, such as support for Qt on Android, you can unselect them here to reduce the size of the download. You can always go back and add or remove Qt components later if needed, using the Maintenance Tool:

Next, you will be presented with the license agreement. Check the first option, which says I have read and agree to the terms contained in the license agreements, and click the Next button. Make sure you do read the terms and conditions stated in the license agreement!

Finally, the installer will ask you to enter a name to create a start menu shortcut for Qt. Once you're done, just click Next and then click Install. The download process will take a couple of minutes to a couple of hours, depending on your internet speed. Once all the files have been downloaded, the installer will automatically proceed to install the files to the installation path that you have just set in one of the previous steps.

Setting up the working environment

Since you have installed the latest version of Qt, let's fire up Qt Creator and start messing around by creating our first project! You should be able to find Qt Creator's shortcut icon either on your desktop or somewhere within your start menu.

Let's look at the steps to set up our environment:

1. When you first start Qt Creator, you should see the following interface:

2. Before you start creating your first project, there are several settings that you might want to tweak. Go to the top menu and select Tools | Options. A window that looks something like this will pop up on the screen:

3. There are quite a number of different categories available on the left of the window. Each category represents a set of options you can set to customize how Qt Creator will look and operate. You may not want to touch the settings at all, but it's good to learn about them first. One of the first settings you might want to change is the Language option, which is available in the Environment category. Qt Creator provides us with an option to switch between different languages. Although it doesn't support all languages, most of the popular ones are available, such as English, French, German, Japanese, Chinese, Russian, and so on. Once you have selected your desired language, click Apply and restart Qt Creator. You must restart Qt Creator in order to see the changes.

4. The next setting you probably need is the setting for indentation. By default, Qt uses space indentation, in which four spaces will be added to your script whenever you press the Tab key on your keyboard. Some people, like me, prefer tab indentation instead. You can change the indentation setting at C++ category.

Do note that if you are contributing to Qt project's source code, it's required that you use space indentation instead of tabs, which is the coding standard and style of the Qt project.

5. Under the C++ category, you can find a Copy button located beside the Edit button, somewhere in the top right position. Click it and a new window will pop up.

6. Insert a code style name of your own, as you can't edit the default built-in coding style. After you have created your own settings, click the Edit button. You can now see the actual Tabs And Indentation settings under the General tab:

7. Do note that even though there is a Tabs And Indentation setting located at the Text Editor category, I believe it's an old setting that no longer has any effect in Qt Creator. There is also a note written on the UI that says Code indentation is configured in C++ and Qt Quick settings. A possible reason for this is that since Qt Creator now supports both C++ project and QML projects, Qt developers probably felt there was a need to separate the settings into two, so therefore the old setting is no longer valid. I'm pretty sure this section on Text Editor will be deprecated in the near future.

8. Next, under the Build and Run category, you'll see a tab labeled Kits.

9. This is where you can set the compile settings for each platform. As you can see from the next screenshot, my Qt does not support desktop build under MSVC (Microsoft Visual Studio Compiler) because I never installed Visual Studio on my computer. Instead, my Qt only supports desktop build under the MinGW (Minimal GNU for Windows) compiler. From this window, you can check and see if your Qt supports the platform and compiler you wanted for your project and make changes to it if necessary. But for now, we'll just leave it as it is. To learn more about what is a kit is and how to configure the build settings, please head over to Chapter 15, Cross-Platform Development:

10. Finally, we can link our project to our version control server at the Version Control category.

11. Version control allows you or your team to submit code changes to a centralized system so that each and every team member can obtain the same code without passing files around manually. When you're working in a big team, it's very difficult to manually keep track of the code changes, and even more so to merge the code done by different programmers. Version control systems are designed to solve these issues. Qt supports different types of version control systems, such as Git, SVN, Mercurial, Perforce, and so on. Although this is a very useful feature, especially if you're working in a team, we don't need to configure it for now:

Running our first Hello World Qt program

A Hello World program is a very simple program that does nothing more than display an output that says Hello, World! (or any other thing, not necessarily this) to show that the SDK is working properly. We don't need to write very long code to produce a Hello World program, we can do it using only the very minimum and the most basic code. In fact, we don't have to write any code in Qt, as it will generate the code when you first create your project!

Let's begin our project by following these steps:

1. To create a new project in Qt, click the New Project button located at the welcome screen on your Qt Creator. Alternatively, you can also go to the top menu and select File | New File or Project.

2. After that, you will be presented with a window which lets you select a template for your project or file. For this demonstration, we will pick Qt Widgets Application:

3. After that, set your project name and project directory. You can also check the checkbox that says Use as default project location so that you can automatically get the same path when you create a new project in Qt next time.

4. Next, Qt Creator will ask you to select one or more kits for your project. For this demonstration, we'll pick Desktop Qt with the MinGW compiler. Don't worry, as you are allowed to add or remove kits from your project later on during development:

5. After that, you will be presented with a page that says Class Information. This is basically where you set the class name for your base window, but we're not going to change anything, so just click the Next button to proceed:

6. Finally, it will ask you to link your project to your version control server. If you have not added any to Qt before, you can click the Configure button, which will bring you to the settings dialog that I showed you in the previous section of this chapter.

7. For this demonstration, however, we'll keep the settings as <None> and press the Finish button. Qt Creator will then proceed to generate the necessary files for your project. After a second or two, Qt Creator will automatically switch to Edit mode and you should be able to see the files it created for you under the Project panel. You can open up any of the files by double-clicking on them in Qt Creator and they will be shown in the editor located on the right-hand side:

8. Before we start compiling the project, let's open up the mainwindow.ui file under the Forms directory in your project panel. Don't worry too much about the user interface as we will cover it in the following chapter. What we need to do is to click and drag the Label icon under the Display Widgets category to the center of the window on the right, as shown in the following screenshot:

9. After that, double-click on the Text Label widget and change the text to Hello World!. Once you're done, hit the Enter button on your keyboard:

10. The final step is to press the Run button located at the bottom left corner that looks like this:

11. We would normally build the program first and then run the program, but Qt Creator is smart enough to figure out that it needs to build it. However, it is still a good habit to build and run your application separately. After a few seconds of compiling, ... voila! You have created your first Hello World program using Qt!:

Summary

The existence of tools such as Qt Creator has made designing applications' user interfaces an easy and fun job for the developers. We no longer need to write a bunch of code just to create a single button, or change a bunch just to adjust the position of a text label, since Qt Designer will generate that code for us when we design our GUI. Qt has applied the WYSIWYG (what you see is what you get) philosophy into the working pipeline and it provides us with all the convenience and efficiency we need to get our jobs done.

In the next chapter, we will learn the ins and outs of Qt Creator and start designing our first GUI with Qt!

Qt Widgets and Style Sheets

One of the advantages of using Qt for software development is that it's very easy to design a program's graphical user interface (GUI ) using the tools provided by Qt. Throughout this book, we will try and create a single project that involves many different components and modules of Qt. We will go through each section of the project in each chapter, so that you will eventually be able to grasp the entire Qt Framework and at the same time complete demo projects, which is a really valuable item to add to your portfolio. You can find all the source code at https://github.com/PacktPublishing/Hands-On-GUI-Programming-with-C-QT5.

In this chapter, we will cover the following topics:

- Introduction to Qt Designer

- Basic Qt widgets

- Qt Style Sheets

In this chapter, we will take a deep look into what Qt can offer us when it comes to designing sleek-looking GUIs with ease. At the beginning of this chapter, you will be introduced to the types of widgets provided by Qt and their functionalities. After that, we will walk through a series of steps and design our first form application using Qt.

Introduction to Qt Designer

There are two types of GUI applications in Qt, namely Qt Quick Application and Qt Widgets Application. In this book, we will cover mostly the latter, as it is the standard way of designing a GUI for desktop applications, and Qt Quick is more widely used for mobile and embedded systems:

1. The first thing we need to do is to open up Qt Creator and create a new project. You can do so by either going to File | New File or Project, or by clicking the New Project button located at the welcome screen:

2. After that, a new window will pop up and ask you to pick the type of project you want to create. Choose Qt Widgets Application under the Application category and click Choose..., Then, create a name for your project (I have chosen Chapter2 for mine) and select the project directory by clicking the Browse... button:

3. Next, you will be asked to select a kit for your project. If you are running this on a Windows system and you have Microsoft Visual Studio installed, you can pick the relevant kit with the MSVC compiler; otherwise, choose the one running MinGW compiler. Qt normally comes with MinGW compiler pre-installed so you don't need to download it separately. If you're running this on a Linux system, then you will see the GCC kit, or the Clang kit if you're running this on macOS. To learn more about Kits and Builds Settings, please check out Chapter 15, Cross-Platform Development:

4. After that, the new project wizard will ask you to name your main window class. We'll just go with the default settings and click the Next button to proceed:

5. Finally, you will be asked to link your version control tool to your project. By linking a version control tool to your project, you will be able to keep every revision of your code on a remote server and keep track of all the changes being made to the project. This is especially useful if you're working in a team. In this tutorial, however, we will not be using any version control, so let's just proceed by clicking the Finish button:

6. Once you're done with that, Qt Creator will open up your new project and you will be able to see your project directory displayed at the top left corner, like so:

7. Now, open up mainwindow.ui by double-clicking on it on the project directory panel. Qt Creator will then switch to another mode, called Qt Designer, which is essentially a tool used to design widget-based GUIs for your program. Once Qt Designer is activated, you will see a list of widgets available on the left panel and a place for you to design your GUI on the right. Let's take a bit of time to get ourselves familiar with Qt Designer's interface before we start learning how to design our own UI:

The following numbers represent the UI shown in the preceding screenshot:

1. Menu bar: The menu bar is where you find all the basic functions of Qt Creator, such as to create new projects, save files, change compiler settings, and so on.

2. Widget box: The widget box is sort of like a toolbox, where all the different widgets provided by Qt Designer are being displayed and are ready to be used. You can drag-and-drop any of the widgets from the widget box directly onto the canvas in the form editor and they will appear in your program.

3. Mode selector: The mode selector is where you can quickly and easily switch between source code editing or UI design by clicking the Edit or Design buttons. You can also easily navigate to the debugger and profiler tools by clicking on their respective buttons located on the mode selector panel.

4. Build shortcuts: There are three different shortcut buttons being displayed here—Build, Run, and Debug. You can easily build and test run your application by pressing the buttons here instead of doing so on the menu bar.

5. Form editor: This is where you apply your creative idea and design your application's UI. You can drag and drop any of the widgets from the Widget Box onto the canvas in the Form Editor for it to appear in your program.

6. Form toolbar: The form toolbar is where you can quickly select a different form to edit. You can change to a different form by clicking on the drop-down box located above the widget box and selecting the UI file you want to open with Qt Designer. There are also buttons that allow you to switch between different modes for the form editor and layout of your UI.

7. Object inspector: This is where all the widgets in your current .ui file are being listed in a hierarchical fashion. The widgets are being arranged in the tree list in accordance to its parent-child relationship with other widgets. The widgets' hierarchy can be easily re-arranged by moving it in the form editor.

8. Property editor: When you select a widget from the object inspector window (or from the form editor window), the properties of that particular widget will be displayed on the property editor. You can change any of the properties here and the result will instantly show up on the form editor.

9. Action editor and signals and slots editor: Both the action editor and signals and slots editor are located in this window. You can create actions that are linked to your menu bar and toolbar buttons by using the action editor. The signal and slots editor is where you

10. Output panes: The output panes are where you look for issues or debugging information when testing your application. It consists of several windows that display different information, such as Issues, Search Results, Application Output, and so on.

In a nutshell, Qt provides an all-in-one editor called Qt Creator. Qt Creator works hand-in-hand with several different tools that come with Qt, such as the script editor, compiler, debugger, profiler, and UI editor. The UI editor, which you can see in the preceding screenshot, is called Qt Designer. Qt Designer is the perfect tool for designers to design their program's UI without writing any code. This is because Qt Designer adopted the WYSIWYG (what you see is what you get) approach by providing an accurate visual representation of the final result, which means whatever you design with Qt Designer will turn out exactly the same when the program is compiled and run. Do note that each tool that comes with Qt can, in fact, be run individually, but if you're a beginner or just doing a simple project, it's recommended to just use the Qt Creator, which connects all those tools together in one interface.

Basic Qt widgets

Now, we will take a look at the default set of widgets available in Qt Designer. You can actually create custom widgets by yourself, but that's an advanced topic which is out of the scope of this book. Let's take a look at the first two categories listed on the widget box—Layouts and Spacers:

Layouts and Spacers are not really something that you can directly observe, but they can affect the positions and orientations of your widgets:

1. Vertical Layout: A vertical layout widget lays out widgets in a vertical column, from top to bottom.

2. Horizontal Layout: A horizontal layout widget lays out widgets in a horizontal row, from left to right (or right to left for right-to-left languages).

3. Grid Layout: A grid layout widget lays out widgets in a two-dimensional grid. Each widget can occupy more than one cell.

4. Form Layout: A form layout widget lays out widgets in a two-column field style. Just as the name implies, this type of layout is best suited for forms of input widgets.

Layouts provided by Qt are very important for creating quality applications and are really powerful. Qt programs don't typically lay elements out using the fixed position because layouts allow dialogs and windows to be dynamically resized in a sensible manner while handling a varying length of text when it's localized in different languages. If you don't make use of layouts in your Qt programs, its UI may very look very different on different computers or devices, which in most cases will create an unpleasant user experience.

Next, let's take a look at the spacer widget. A spacer is a non-visible widget that pushes widgets along a specific direction until it reaches the limit of the layout container. Spacers must be used within a layout, otherwise they will not carry any effect.

There are two types of spacer, namely the Horizontal Spacer and Vertical Spacer:

1. Horizontal Spacer: A horizontal spacer widget is a widget that occupies the space within a layout and pushes other widgets within the layout along a horizontal space.

2. Vertical Spacer: A vertical spacer is similar to a horizontal spacer, except it pushes the widgets along the vertical space.

It's kind of hard to imagine how the Layouts and Spacers work without actually working with them. Don't worry about that, as we will be trying it out in a moment. One of the most powerful features of Qt Designer is that you can experiment with and test your layouts without have to change and compile your code after each change.

Besides Layouts and Spacers, there are a few more categories, namely Buttons, Item Views, Containers, Input Widgets, and Display Widgets. I won't go and explain every single one of them as their names are pretty much self-explanatory. You can also drag and drop the widget on the Form Editor to see what it does. Let's do it:

1. Click and drag the Push Button widget from the Widget Box to the Form Editor, as shown in the following screenshot:

2. Then, select the newly added Push Button widget, and you will see that all the information related to this particular widget is now appearing on the Properties Editor panel:

3. You can change the properties of the widget, such as appearance, focus policy, tooltip, and so on programmatically in C++ code. Some properties can also be edited directly in the Form Editor. Let's double-click on the Push Button and change the text of the button, and then resize the button by dragging its edge:

4. Once you're done with that, let's drag and drop a Horizontal Layout to the Form Editor. Then, drag the Push Button to the newly added layout. You will now see that the button automatically fits into the layout:

5. By default, the main window does not carry any layout effect, and therefore the widgets will stay where they were originally placed, even when the window is being resized, which does not look very good. To add a layout effect to the main window, right-click on the window in the Form Editor, select Lay out, and finally select Lay Out Vertically. You will now see the Horizontal Layout widget we added previously is now automatically expanding to fit the entire window. This is the correct behavior of a layout in Qt:

6. Next, we can play around with the spacer and see what effect it has. We will drag and drop a Vertical Spacer to the top of the layout containing the Push Button, and then we'll place two Horizontal Spacers on both sides of the button, within its layout:

The spacers will push all of the widgets located on both of their ends and occupy the space itself. In this example, the Submit button will always stay at the bottom of the window and keep its middle position, regardless of the size of the window. This makes the GUI look good, even on different screen sizes.

Ever since we added the spacers to the window, our Push Button has been squeezed to its minimum size. Let's enlarge the button by setting its minimumSize property to 120 x 40, and you'll see that the button appears a lot bigger now:

7. After that, let's add a Form Layout above the layout of the Push Button and a Vertical Spacer below it. You will now see that the Form Layout is really thin because it has been squeezed by the Vertical Spacers we placed earlier onto the main window, which can be troublesome when you want to drag and drop a widget into the Form Layout. To solve this problem, temporarily set the layoutTopMargin property to 20 or higher:

8. Then, drag and drop two Labels to the left side of the Form Layout and two Line Edits to its right side. Double click on both of the labels and change their display texts to Username: and Password:, respectively. Once you're done with that, set the layoutTopMargin property of the Form Layout back to 0:

Currently, the GUI looks pretty great, but the Form Layout is now occupying the entire spacing in the middle, which is not very pleasant when the main window is maximized. To keep the form compact, we'll do the following steps, which are a little tricky:

9. First, drag and drop a Horizontal Layout above the form, and set its layoutTopMargin and layoutBottomMargin to 20 so that the widgets that we place in it, later on, are not too close to the Submit button. Next, drag and drop the entire Form Layout, which we placed earlier into the Horizontal Layout. Then, place Horizontal Spacers on both sides of the form to keep it centered. The following screenshot illustrates these steps:

10. After that, we can make further adjustments to the GUI to make it look tidy before we proceed to the next section, where we will be customizing the widgets' style. Let's start off by setting the minimumSize property of the two Line Edit widgets to 150 x 25. Then, set the layoutLeftMargin, layoutRightMargin, layoutTopMargin, and layoutBottomMargin properties of the Form Layout to 25. The reason why we want to do this is that we will be adding an outline to the Form Layout in the following section.

11. Since the Push Button is now way too distanced from the Form Layout, let's set the layoutBottomMargin property of the Horizontal Layout, which sets the Form Layout to 0. This will make the Push Button move slightly above and closer to the Form Layout. After that, we'll adjust the size of the Push Button to make it align with the Form Layout. Let's set the minimumSize property of the Push Button to 260 x 35, and we're done!:

You can also preview your GUI without building your program by going to Tools | Form Editor | Preview. Qt Designer is a very handy tool when it comes to designing sleek GUIs for Qt programs without a steep learning curve. In the following section, we will learn how to customize the appearance of the widgets using Qt Style Sheets.

Qt Style Sheets

Qt's Widgets Application uses a styling system called Qt Style Sheets, which is similar to the web technology's styling system—CSS (Cascading Style Sheet). All you need to do is write the style description of the widget and Qt will render it accordingly. The syntax of Qt Style Sheets is pretty much the same as CSS.

Qt Style Sheets has been inspired by CSS and thus they are both very similar to each other:

- Qt Style Sheets:

QLineEdit { color: blue; background-color: black; }

- CSS:

h1 { color: blue; background-color: black; }

In the preceding example, both Qt Style Sheet and CSS contain a declaration block and a selector. Each declaration consists of a property and value, which are separated by a colon.

You can change a widget's style sheet by using two methods—using C++ code directly or by using the properties editor. If you're using C++ code, you can call the QObject::setStyleSheet() function, like so:

myButton->setStyleSheet("background-color: green");

The preceding code changes the background color of our push button widget to green. You can also achieve the same result by writing the same declaration into the styleSheet property of the widget in Qt Designer:

QPushButton#myButton { background-color: green }

To learn more about the syntax and properties of Qt Style Sheets, please refer to the following link: http://doc.qt.io/qt-5/stylesheet-reference.html

Let's continue with our project and apply a custom Qt Style Sheet to our GUI!

1. First, right-click on the Submit button and select Change styleSheet... A window will pop up for you to edit the widget's Style Sheet:

2. Then, add the following to the Style Sheet Editor window:

border: 1px solid rgb(24, 103, 155);
border-radius: 5px;
background-color: rgb(124, 203, 255);
color: white;

3. Once you're done, click the OK button and you should be able to see that the Submit button changes its appearance to this:

The Style Sheet we used earlier is pretty much self-explanatory. It enables the borderline of the Push Button and sets the border color to dark blue using RGB values. Then, it also applies a rounded corner effect to the button and changes its background color to light blue. Finally, the Submit text has also been changed to white.

4. Next, we want to apply a custom Style Sheet to the Form Layout. However, you will notice that there is no Change styleSheet... option when right clicking on it. This is because layouts do not carry that property with it. In order to apply styling to the Form Layout, we must first convert it into a QWidget or QFrame object. To do so, right-click on the Form Layout and select Morph into | QFrame:

5. Once you're done with that, you will notice it is now carrying the styleSheet property and thus we are now able to customize its appearance. Let's right-click on it and select Change styleSheet... to open up the Style Sheet Editor window. Then, insert the following script:

#formFrame {
border: 1px solid rgb(24, 103, 155);
border-radius: 5px;
background-color: white; }

The word formFrame is referring to the widget's objectName property and it must match the exact name of the widget, otherwise the style will not be applied to it. The reason why we define the widget name for this example (which we didn't do in the previous one) is because the style will also be applied to all its children if we don't specify the widget name. You can try and remove #formFrame {} from the preceding script and see what happens—now, even the Labels and Line Edits have borderlines, and that is not what we intended to do. The GUI now looks like this:

6. Lastly, we want to have a nice-looking background, and we can do this by attaching a background image. To do so, we first need to import the image into Qt's resource system. Go to File | New File or Project...Then, select Qt under the Files and Classes category. After that, pick the Qt Resource File and click the Choose... button. The Qt resource system is a platform-independent mechanism for storing binary files in the application's executable. You can basically store all of those important files here, such as icon images or language files, directly into your executable by using the Qt resource file. These important files will be directly embedded into your program during the compilation process.

7. Then, key in the file name and set its location before pressing the Next button, and follow this by clicking the Finish button. Now, you will see a new resource file being created, which I named resource.qrc:

8. Open up resource.qrc with Qt Creator and select Add | Add Prefix. After that, key in your preferred prefix, for example, /images. Once you're done with that, select Add again and this time, pick Add Files. Add the image file provided by the sample project called login_bg.png. Then, save resource.qrc and right-click on the image and select Copy Resource Path to Clipboard. After that, close resource.qrc and open up mainwindow.ui again:

9. The next thing we need to do is to right-click on the centralWidget object from the Object Inspector and select Change styleSheet..., and then insert the following script:

#centralWidget {
border-image: url(:/images/login_bg.png); }

10. The text within url() can be inserted by pressing Ctrl + V (or paste) because it was copied to the clipboard when we selected Copy Resource Path to Clipboard in the previous step. The final outcome looks like this:

Please make sure that you also build and run the application, and then check whether the final outcome looks the same, as intended. There are a lot more things to tweak in order to make it look truly professional, but so far it's looking pretty great!

Summary

Qt Designer really revolutionized the way we design program GUIs. Not only does it include all the common widgets but it also has handy stuff like the layout and spacer, which makes our program run perfectly fine on different types of monitors and screen sizes. Also, notice that we have successfully created a working application with a beautiful user interface without writing a single line of C++ code!

What we've learned in this chapter merely scratches the surface of Qt, as there are many more features that we are yet to cover! Join us in the next chapter to learn how we can make our program truly functional!

Database Connection

In the previous chapter, we learned how to create a login page from scratch. However, it is not functional yet, as the login page is not connected to a database. In this chapter, you will learn how to connect your Qt application to a MySQL (or MariaDB) database that validates login credentials.

In this chapter, we will cover the following topics:

- Introducing the MySQL database system

- Setting up the MySQL database

- SQL commands

- Database connection in Qt

- Functional login page

We will walk through this chapter in a step-by-step approach to discover the powerful features that come with Qt and allow your application to connect directly to a database without any additional third-party dependencies. Database querying is a huge topic by itself, but we will be able to learn the most basic commands from scratch through examples and practical methods.

Qt supports multiple different types of database systems:

- MySQL (or MariaDB)

- SQLite (version 2 and 3)

- IBM DB2

- Oracle

- ODBC

- PostgreSQL

- Sybase Adaptive Server

Two of the most popular ones are MySQL and SQLite. The SQLite database is usually used offline and it doesn't require any setup as it uses an on-disk file format for storing data. Therefore, in this chapter, we will learn how to set up a MySQL database system instead, and at the same time learn how to connect our Qt application to a MySQL database. The C++ code used to connect to the MySQL database can be reused for connecting to other database systems without many alterations.

Introducing the MySQL database system

MySQL is an open source database management system based on the relational model, which is the most common method used by modern database systems to store information for various purposes.

Unlike some other legacy models—such as an object database system or a hierarchical database system—the relational model has been proven to be more user friendly and performs well beyond the other models. That's the reason why most of the modern database systems we see today are mostly using this method.

MySQL was originally developed by a Swedish company called MySQL AB, and its name is the combination of My, the name of the daughter of the company's co-founder, and SQL, the abbreviation for Structured Query Language.

Similar to Qt, MySQL has also been owned by multiple different people throughout its history. The most notable acquisition happened in 2008, where Sun Microsystems bought MySQL AB for $1 billion. One year later in 2009, Oracle Corporation acquired Sun Microsystems, and so MySQL is owned by Oracle up to this day. Even though MySQL changed hands several times, it still remains as an open source software that allows users to change the code to suit their own purposes.

Due to its open source nature, there are also other database systems out there that were derived/forked from the MySQL project, such as MariaDB, Percona Server, and so on. However, these alternatives are not fully compatible with MySQL as they have modified it to suit their own needs, and therefore some of the commands may be varied among these systems.

According to a 2017 survey carried out by Stack Overflow, MySQL is the most widely used database system among web developers, as we can see in the following screenshot:

The survey result indicates that what you learn in this chapter can be applied to not just Qt projects but also web, mobile app, and other types of applications.

Furthermore, MySQL and its variants are being used by big corporations and project groups such as Facebook, YouTube, Twitter, NASA, Wordpress, Drupal, Airbnb, Spotify, and so on and so forth. This means that you can easily get answers when encountering any technical issues during development.

For more information regarding MySQL, please visit:
https://www.mysql.com

Setting up the MySQL database

There are many different ways to set up your MySQL database. It really depends on the type of platforms you are running, whether it is Windows, Linux, Mac, or any other type of operating system; it will also depend on the purpose of your database—whether it's for development and testing, or for a large-scale production server.

For large scale services (such as social media), the best way is to compile MySQL from the source, because such as project requires a ton of optimization, configuration, and sometimes customization in order to handle the large amount of users and traffic.

However, you can just download the pre-compiled binaries if you're going for normal use, as the default configuration is pretty sufficient for that. You can install a standalone MySQL installer from their official website or the download installation packages that come with several other pieces of software besides MySQL.

In this chapter, we will be using a software package called XAMPP, which is a web server stack package developed by a group called Apache Friends. This package comes with Apache, MariaDB, PHP, and other optional services that you can add on during the installation process. Previously, MySQL was part of the package, but it has since been replaced with MariaDB starting from version 5.5.30 and 5.6.14. MariaDB works almost the same as MySQL, except those commands involving advanced features, which we will not be using in this book.

The reason why we use XAMPP is that it has a control panel that can easily start and stop the services without using Command Prompt, and provides easy access to the configuration files without you having to dig into the installation directory by yourself. It is very quick and efficient for application development that involves frequent testings. However, it is not recommended that you use XAMPP on a production server as some of the security features have been disabled by default.

Alternatively, you may also install MySQL through other similar software packages such as AppServ, AMPPS, LAMP (Linux only), WAMP (Windows only), ZendServer, and so on.

Now, let's learn how to install XAMPP:

1. First, go to their website at https://www.apachefriends.org and click on one of the download buttons located at the bottom of your screen, which displays the icon of your current operating system:

2. Once you click on the Download button, the download process should start automatically within a few seconds, and it should proceed to install the program once it's done. Make sure that Apache and MySQL/MariaDB are included before the installation process starts.

3. After you have installed XAMPP, launch the control panel from the start menu or from the desktop shortcut. After that, you may notice that nothing has happened. This is because the XAMPP control panel is hidden within the taskbar by default. You may display the control panel window by right-clicking on it and selecting the Show / Hide option in the pop-up menu. The following screenshot shows you what this looks like on a Windows machine. For Linux, the menu may look slightly different, but overall it is very similar. For macOS, you must launch XAMPP from the launchpad or from the dock:

4. Once you have clicked the Show / Hide option, you will finally see the control panel window displayed on your screen. If you click the Show / Hide option again, the window will be hidden away:

5. Their control panel is pretty much self-explanatory at first glance. On the left, you can see the names of the services that are available in XAMPP, and on the right, you will see the buttons that indicate Start, Config, Logs, and so on. For some reason, XAMPP is showing MySQL as the module name but it is in fact running MariaDB. Don't worry; both work pretty much the same since MariaDB is a fork of MySQL.

6. In this chapter, we'll only need Apache and MySQL (MariaDB), so let's click the Start buttons of these services. After a second or two, you'll see that the Start buttons are now labeled as Stop, which means the services have been launched!:

7. To verify this, let's open up the browser and type localhost as the website address. If you see something like the following image, it means that the Apache web server has been successfully launched!:

8. Apache is very important here as we'll be using it to configure the database using a web-based administrative tool called phpMyAdmin. phpMyAdmin is an administrative tool for MySQL written in PHP scripting language, hence its name. Even though it was originally designed for MySQL, it works pretty well for MariaDB as well.

9. To access the phpMyAdmin control panel, type localhost/phpmyadmin on your browser. After that, you should see something like this:

10. On the left-hand side of the page, you will see the navigation panel, which allows you access to the different databases available in your MariaDB database. On the right-hand side of the page are various tools that let you view table, edit table, run SQL command, export data to spreadsheet, set privileges, and so on.

11. By default, you can only modify the General settings of the database on the setting panel located on the right. You must select a database from the navigation panel on the left before you are able to modify the settings of a particular database.

12. A database is like a cabinet that you can store log books within. Each log book is called a table and each table contains data, which is sorted like a spreadsheet. When you want to obtain a data from MariaDB, you must specify which cabinet (database) and log book (table) you would like to access before getting the data from it. Hopefully, this will make you better understand the concept behind MariaDB and other similar database systems.

13. Now, let's get started by creating our very first database! To do so, you can either click the New button located above the database names on the navigation panel or click the Databases button located at the top of the menu. Both buttons will bring you to the Databases page, and you should be able to see this located below the menu buttons:

14. After that, let's create our very first database! Type in your desired database name and click the Create button. Once the database has been created, you will be redirected to the Structure page, which will list down all the tables contained in this database. By default, your newly created database doesn't contain any tables, so you will see a line of text that says No tables found in database:

15. Guess what we'll be doing next? Correct, we will create our first table! First, let's insert the name of the table you want to create. Since we'll be doing a login page later in this chapter, let's name our table user. We'll leave the default number of columns as it is and click Go.

16. After that, you will be redirected to another page, which contains many columns of input fields for you to fill in. Each column represents a data structure which will be added to your table after it's been created.

17. The first thing you need to add to the table structure is an ID that will automatically increase upon each new data insertion. Then, add a timestamp column to indicate the date and time of the data insertion, which is good for debugging. Last but not least, we will add a username column and password column for login validation. If you're unsure on how to do this, please refer to the following image. Make sure you follow the settings that are being circled in the image:

18. The type of the structure is very important and must be set according to its intended purpose. For example, the id column must be set as INT (integer number) as it must be a full number, while username and password must be set as either VARCHAR or other similar data types (CHAR, TEXT, and so on) in order for it to save the data correctly.

19. The timestamp, on the other hand, must be set to TIMESTAMP type, and must set the default value to CURRENT_TIMESTAMP, which notifies MariaDB to automatically generate the current timestamp upon data insertion.

20. Please note that the index setting for the ID column must be set to PRIMARY, and make sure that the A_I (auto increment) checkbox is ticked. When you check the A_I checkbox, an Add Index window will appear. You can keep the default settings as they are and then you can click the Go button to complete the steps and start creating the table:

21. After you have created the new table, you should be able to see something similar like the following image. You can still edit the structure settings anytime by clicking the Change button; you can also remove any of the columns by clicking on the Drop button located at the right-hand side of the column. Please note that deleting a column will also remove all the existing data belonging to that column, and this action cannot be undone:

22. Even though we'll usually add data to the database through our programs or web pages, we can also add data directly on phpMyAdmin for testing purposes. To add data using phpMyAdmin, first, you must create a database and table, which we have done in the previous steps. Then, click the Insert button located at the top of the menu:

23. After that, you'll see that a form has appeared, which resembles the data structure that we created previously:

24. You can simply ignore the ID and timestamp values as they will be automatically generated when you save the data. In this case, only username and password need to be filled in. For the sake of testing, let's put test as the username and 123456 as the password. Then, click the Go button to save the data.

Please note that you should not save your password in a human-readable format on your actual production server. You must encrypt the password with a cryptographic hash function such as SHA-512, RIPEEMD-512, BLAKE2b, and so on before passing it to the database. This will ensure that the password is not readable by hackers in case your database is being compromised. We will cover this topic at the end of this chapter.

Now that we have finished setting up our database and inserted our first test data, let's proceed to learn some of the SQL commands!

SQL commands

Most of the popular relational database management systems, such as MySQL, MariaDB, Oracle SQL, Microsoft SQL, and so on, use a type of declarative language called SQL (Structured Query Language) to interact with the database. SQL was initially developed by IBM engineers in the 1970s, but later on, it was further enhanced by Oracle Corporation and other emerging tech companies of that era.

Today, SQL has become a standard of the American National Standards Institute (ANSI) and of the International Organization for Standardization (ISO). SQL language has since been adopted by many different database systems and has become one of the most popular database languages in the modern era.

In this section, we will learn what some of the basic SQL commands you can use to interact with your MariaDB database are, specifically for obtaining, saving, modifying, and deleting your data from/to the database. These basic commands can be used in other types of SQL-based database systems as well as under the ANSI and ISO standards. Only, some of the more advanced/customized features could be different across different systems, so make sure that you read the system manual before using these advanced features.

Alright, let's get started!

SELECT

Most of the SQL statements are one-word short and self-explanatory. This statement, for example, is used to select one or more columns from a specific table and to obtain the data from the said columns. Let's check out some of the sample commands that use the SELECT statement.

The following command retrieves all the data of all the columns from the user table:

SELECT * FROM user;

The following command retrieves only the username column from the user table:

SELECT username FROM user;

The following command retrieves the username and password columns from the user table with the condition that the id equals 1:

SELECT username, password FROM user WHERE id = 1;

You can try out these commands by yourself using phpMyAdmin. To do that, click the SQL button located at the top of the menu in phpMyAdmin. After that, you can type the command in the text field below and click Go to execute the query:

To learn more about the SELECT statement, please refer to the following link:
https://dev.mysql.com/doc/refman/5.7/en/select.html

INSERT

Next, the INSERT statement is used to save new data into a database table. For example:

INSERT INTO user (username, password) VALUES ("test2", "123456");

The preceding SQL command inserts username and password data into the user table. There are some other statements that can be used together with INSERT, such as LOW_PRIORITY, DELAYED, HIGH_PRIORITY, and so on.

Please refer to the following link to learn more about these options:
https://dev.mysql.com/doc/refman/5.7/en/insert.html

UPDATE

The UPDATE statement modifies existing data in the database. You must specify a condition for the UPDATE command as otherwise, it will modify every single piece of data in a table, which is not our intended behavior. Try the following command, which will change the username and password of the first user:

UPDATE user SET username = "test1", password = "1234321" WHERE id = 1;

The command will fail, however, if the user with ID 1 does not exist. The command will also return the status 0 rows affected if the username and password data you provided matches exactly with the one stored in the database (nothing to change). For more information regarding the UPDATE statement, please refer to the following link:

https://dev.mysql.com/doc/refman/5.7/en/update.html

DELETE

The DELETE statement deletes data from a specific table of a database. For example, the following command deletes a data from the user table that carries the ID 1:

DELETE FROM user WHERE id = 1;

Even though you can use this statement to delete unwanted data, it is not recommended to delete any data from your database because the action cannot be undone. It is better to add another column to your table called status and use that to indicate whether data should be shown or not. For example, if your user deletes data on the front end application, set the status of that data to (let's say) 1 instead of 0. Then, when you want to display data on the front end, display only the data that carries a status of 0:

This way, any data that has been accidentally deleted can be recovered with ease. You can also use a BOOLEAN type for this if you only plan to use true or false. I usually use TINYINT just in case I need a third or fourth status in the future. For more information regarding the DELETE statement, you can refer to the following link:

https://dev.mysql.com/doc/refman/5.7/en/delete.html

JOIN

The advantage of using a relational database management system is that the data can be easily joined together from different tables and can be returned to the user in a single bulk. This greatly improves the productivity of the developers as it allows fluidity and flexibility when it comes to designing a complex database structure.

There are many types of JOIN statements in MariaDB/MySQL—INNER JOIN, FULL OUTER JOIN, LEFT JOIN, and RIGHT JOIN. All of these different JOIN statements behave differently when executed, which you can see in the following image:

Most of the time, we'll be using the INNER JOIN statement, as it only returns the data that has matching values in both tables, and thus only returns a small amount of the data that is needed. The JOIN command is much more complicated than the others as you need to design the tables to be join-able in the first place. Before we start testing the JOIN command, let's create another table to make this possible. We will call this new table department:

After that, add two departments, like so:

Then, go to the user table, and at the structure page, scroll all the way to the bottom and look for the form shown, then click the Go button:

Add a new column called deptID (which stands for department ID) and set its data type to int (integer number):

After that, set up a few test users and put each of their deptID as either 1 or 2:

Please notice that I have also added the status column here for checking whether the user has been deleted or not. Once you have done with that, let's try to run a sample command!:

SELECT my_user.username, department.name FROM (SELECT * FROM user WHERE deptID = 1) AS my_user INNER JOIN department ON department.id = my_user.deptID AND my_user.status = 0

That looks quite complicated at first glance, but it really isn't if you separate it into a few parts. We'll start from the command within the () bracket first, in which we asked MariaDB/MySQL to select all columns within the user table that carry deptID =  1:

SELECT * FROM user WHERE deptID = 1

After that, contain it within a () bracket and name this entire command as my_user. After that, you can start joining your user table (now called my_user) with the department table by using the INNER JOIN statement. Here, we also added some conditions for it to look up the data, such as the ID of the department table must match the deptID of my_user, and the status value of my_user must be 0, indicating that the data is still valid and not tagged as removed:

(SELECT * FROM user WHERE deptID = 1) AS my_user INNER JOIN department ON department.id = my_user.deptID AND my_user.status = 0

Lastly, add the following code in front to complete the SQL command:

SELECT my_user.username, department.name FROM

Let's try the preceding command and see if the result is what you expected.

You can join infinite numbers of tables using this method as long as the tables are linked to each another through matching columns.

To find out more about the JOIN statement, please visit the following link:
https://dev.mysql.com/doc/refman/5.7/en/join.html

There are many other SQL statements that we have not covered in this chapter, but the ones that we have covered are pretty much all you need to get started.

One last thing before we move on to the next section—we must create a user account for the application to access to our MariaDB/MySQL database. First of all, go to your phpMyAdmin home page and click User accounts on the top menu:

Then, go to the bottom and look for this link called Add user account:

Once you're in the Add user account page, type in the User name and Password information in the Login Information form. Make sure that the Host name is set to Local:

Then, scroll down and set the Global privileges of the user. Enabling the options within the Data section is well enough, but do not enable the other options as it might give hackers the privilege to alter your database structure once your server has been compromised:

Once you have created the user account, follow the following steps to allow the newly-created user access to the database called test (or any other table name of your choice):

After you have clicked the Go button, you have now given the user account the privilege to access the database! In the next section, we'll be learning how to connect our Qt application to the database.

Database connection in Qt

Now that we have learned how to set up a functional MySQL/MariaDB database system, let's move a step further and discover the database connection module in Qt!

Before we continue working on our login page from the previous chapter, let's start off with a new Qt project first so that it's easier to demonstrate the functionality solely related to database connection and so that we don't get distracted by the other stuff. This time, we'll go for the Terminal-style application called Qt Console Application, as we don't really need any GUI for this demonstration:

After you have created the new project, you should only see two files in the project, that is, [project_name].pro and main.cpp:

The first thing you need to do is to open up your project file (.pro), which in my case is DatabaseConnection.pro, and add the sql keyword at the back of the first line, like so:

QT += core sql

As simple as that, we have successfully imported the sql module into our Qt project! Then, open up main.cpp and you should see a very simple script that contains only eight lines of code. This is basically all you need to create an empty console application:

#include <QCoreApplication>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
return a.exec();
}

In order for us to connect to our database, we must first import the relevant headers to main.cpp, like so:

#include <QCoreApplication>
#include <QtSql>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QDebug>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
return a.exec();
}

Without these header files, we won't be able to use the functions provided by Qt's sql module, which we have imported previously. Additionally, we also added the QDebug header so that we can easily print out any text on the console display (similar to the std::cout function provided by C++'s standard library).

Next, we'll add some code to the main.cpp file. Add the following highlighted code before return a.exec():

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName("127.0.0.1");
db.setPort(3306);
db.setDatabaseName("test");
db.setUserName("testuser");
db.setPassword("testpass");
if (db.open())
{
qDebug() << "Connected!";
}
else
{
qDebug() << "Failed to connect.";
return 0;
}
return a.exec();
}

Do note that the database name, username, and password could be different from what you have set in your database, so please make sure they are correct before compiling the project.

Once you are done with that, let's click the Run button and see what happens!:

If you see the following error, don't worry:

That is simply because you must install the MariaDB Connector (or MySQL Connector if you're running MySQL) to your computer and copy the DLL file over to your Qt installation path. Please make sure that the DLL file matches your server's database library. You can open up your phpMyAdmin home page and see which library it is currently using.

For some reason, even though I'm running XAMPP with MariaDB, the library name here shows libmysql instead of libmariadb, so I had to install MySQL Connector instead:

If you're using MariaDB, please download the MariaDB Connector at the following link:
https://downloads.mariadb.org/connector-c
If you're using MySQL instead (or are having the same issue as I did), please visit the other link and download MySQL Connector:
https://dev.mysql.com/downloads/connector/cpp/

After you have downloaded the MariaDB Connector, install it on your computer:

The preceding screenshot shows the installation process for a Windows machine. If you're running Linux, you must download the right package for your Linux distribution. If you're running Debian, Ubuntu, or one of its variants, download the Debian and Ubuntu packages. If you're running Red Hat, Fedora, CentOS, or one of its variants, download the Red Hat, Fedora, and CentOS packages. The installation for these packages are automated, so you're good to go. However, if you're running neither of those, you'll have to download one of the gzipped tar files listed on the download page that fits your system requirement.

For more information about installing MariaDB binary tarballs on Linux, please refer to the following link:
https://mariadb.com/kb/en/library/installing-mariadb-binary-tarballs/

As for macOS, you need to use a package manager called Homebrew to install MariaDB server.

For more information, check out the following link:
https://mariadb.com/kb/en/library/installing-mariadb-on-macos-using-homebrew/

Once you have installed it, go to its installation directory and look for the DLL file (libmariadb.dll for MariaDB or libmysql.dll for MySQL). For Linux and macOS, it's libmariadb.so or libmysql.so instead of DLL.

Then, copy the file over to your application's build directory (the same folder as your application's executable file). After that, try and run your application again:

If you still getting Failed to connect but without the QMYSQL driver not loaded message, please check your XAMPP control panel and make sure that your database service is running; also make sure that the database name, username, and password that you put in the code is all the correct information.

Next, we can start playing around with SQL commands! Add the following code before return a.exec():

QString command = "SELECT name FROM department";
QSqlQuery query(db);
if (query.exec(command))
{
while(query.next())
{
QString name = query.value("name").toString();
qDebug() << name;
}
}

The preceding code sends the command text to the database and synchronously waits for the result to return from the server. After that, use a while loop to go through every single result and convert it to a string format. Then, display the result on the console window. If everything went right, you should see something like this:

Let's try out something more complex:

QString command = "SELECT my_user.username, department.name AS deptname FROM (SELECT * FROM user WHERE status = 0) AS my_user INNER JOIN department ON department.id = my_user.deptID";
QSqlQuery query(db);
if (query.exec(command))
{
while(query.next())
{
QString username = query.value("username").toString();
QString department = query.value("deptname").toString();
qDebug() << username << department;
}
}

This time, we used INNER JOIN to combine two tables to select the username and department name. To avoid confusion regarding the variable called name, rename it to deptname using the AS statement. After that, display both the username and department name on the console window:

We're done... for now. Let's move on to the next section, where we will learn how to make our login page functional!

Creating our functional login page

Since we have learned how to connect our Qt application to the MariaDB/MySQL database system, it's time to continue working on the login page! In the previous chapter, we learned how to set up the GUI of our login page. However, it didn't have any functionality at all as a login page since it doesn't connect to the database and verify login credentials. Therefore, we will learn how to achieve that by empowering Qt's sql module.

Just to recap—this is what the login screen looks like:

The very first thing we need to do now is to name the widgets that are important in this login page, which are the Username input, Password input, and the Submit button. You can set these properties by selecting the widget and looking for the property in the property editor:

Then, set the echoMode of the password input as Password. This setting will hide the password visually by replacing it with dots:

After that, right-click on the Submit button and select Go to slot... A window will pop up and ask you which signal to use. Select clicked() and click OK:

A new function called on_loginButton_clicked() will be automatically added to the MainWindow class. This function will be triggered by Qt when the Submit button is pressed by the user, and thus you just need to write the code here to submit the username and password to the database for login verification. The signal and slots mechanism is a special feature provided by Qt which is used for communication between objects. When one widget is emitting a signal, another widget will be notified and will proceed to run a specific function that is designed to react to the particular signal.

Let's check out the code.

First, add in the sql keyword at your project (.pro) file:

QT += core gui
sql

Then, proceed and add the relevant headers to mainwindow.cpp:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

#include <QtSql>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QDebug>
#include <QMessageBox>

Then, go back to mainwindow.cpp and add the following code to the on_loginButton_clicked() function:

void MainWindow::on_loginButton_clicked()
{
QString username = ui->userInput->text();
QString password = ui->passwordInput->text();
qDebug() << username << password;
}

Now, click the Run button and wait for the application to start. Then, key in any random username and password, followed by clicking on the submit button. You should now see your username and password being displayed on the application output window in Qt Creator.

Next, we'll copy the SQL integration code we have written previously into mainwindow.cpp:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName("127.0.0.1");
db.setPort(3306);
db.setDatabaseName("test");
db.setUserName("testuser");
db.setPassword("testpass");

if (db.open())
{
qDebug() << "Connected!";
}
else
{
qDebug() << "Failed to connect.";
}
}

Do note that I've used some random text for the database name, username, and password. Please make sure you enter the correct details here and that they match with what you've set in the database system.

One minor thing we have changed for the preceding code is that we only need to call db = QSqlDatabase::addDatabase("QMYSQL") in mainwindow.cpp without the class name as the declaration QSqlDatabase db has now been relocated to mainwindow.h:

private:
Ui::MainWindow *ui;
QSqlDatabase db;

Lastly, we add in the code that combines the username and password information with the SQL command, and send the whole thing to the database for execution. If there is a result that matches the login information, then it means that the login has been successful, otherwise, it means the login has failed:

void MainWindow::on_loginButton_clicked()
{
QString username = ui->userInput->text();
QString password = ui->passwordInput->text();

qDebug() << username << password;

QString command = "SELECT * FROM user WHERE username = '" + username
+ "' AND password = '" + password + "' AND status = 0";
QSqlQuery query(db);
if (query.exec(command))
{
if (query.size() > 0)
{
QMessageBox::information(this, "Login success.", "You
have successfully logged in!");
}
else
{
QMessageBox::information(this, "Login failed.", "Login
failed. Please try again...");
}
}
}

Click the Run button again and see what happens when you click the Submit button:

Hip hip hooray! The login page is now fully functional!

Summary

In this chapter, we learned how to set up a database system and make our Qt application connect to it. In the next chapter, we will learn how to draw graphs and charts using the powerful Qt Framework.

Graphs and Charts

In the previous chapter, we learned how to retrieve data from a database using Qt's sql module. There are many ways to present this data to the users, such as displaying it in the form of tables or diagrams. In this chapter, we will learn how to do the latter—presenting data with different types of graphs and charts using Qt's charts module.

In this chapter, we will cover the following topics:

- Types of charts and graphs in Qt

- Charts and graphs implementation

- Creating the dashboard page

Since Qt 5.7, several modules that were only available for commercial users before have become free for all the open source package users, which includes the Qt Charts module. Therefore, it is considered a very new module for most Qt users who don't own the commercial license.

Do note that, unlike most of the Qt modules that are available under an LGPLv3 license, the Qt Chart module is offered under an GPLv3 license. Unlike LGPLv3, a GPLv3 license requires you to release the source code of your application, while your application must also be licensed under GPLv3. This means that you are not allowed to static-link Qt Chart with your application. It also prevents the module from being used in proprietary software.

To learn more about the GNU licenses, please head over to the following link: https://www.gnu.org/licenses/gpl-faq.html.

Let's get started!

Types of charts and graphs in Qt

Qt supports most commonly used diagrams, and even allows the developer to customize the look and feel of them so that they can be used for many different purposes. The Qt Charts module provides the following chart types:

- Line and spline charts

- Bar charts

- Pie charts

- Polar charts

- Area and scatter charts

- Box-and-whiskers charts

- Candlestick charts

Line and spline charts

The first type of chart is the line and spline chart. These charts are typically presented as a series of points/markers that are connected by lines. In a line chart, the points are connected by straight lines to show the changes of the variables over a period of time. On the other hand, spline charts are very similar to line charts except the points are connected by a spline/curve line instead of straight lines:

Bar charts

Bar charts are one of the most commonly used diagrams beside line charts and pie charts. A bar chart is quite similar to a line chart, except it doesn't connect the data along an axis. Instead, a bar chart displays its data using individual rectangular shapes, where its height is determined by the value of the data. This means that the higher the value, the taller the rectangular shape will become:

Pie charts

A pie chart, as its name implies, is a type of chart that looks like a pie. A pie chart presents its data in the form of pie slices. The size of each slice of pie will be determined by the overall percentage of its value compared to the rest of the data. Therefore, pie charts are normally used to display fraction, ratio, percentage, or a share of a set of data:

Sometimes, a pie chart can also be displayed in a donut shape (also known as donut chart):

Polar charts

Polar charts present data in a circular graph, where the placement of the data is based on both the angle and the distance from the center of the graph, which means the higher the value of the data, the further away the point is from the center of the chart. You can display multiple types of graphs within the polar chart, such as line, spline, area, and scatter to visualize the data:

If you are a gamer, you should have noticed this type of graph being used in some video games to display the in-game character's attributes:

Area and scatter charts

An area chart displays its data as an area or shape to indicate volume. It's usually used to compare the differences between two or more datasets.

Scatter charts, on the other hand, are used to display a collection of data points, and for showing the non-linear relationship between two or more datasets.

Box-and-whiskers charts

Box-and-whiskers charts present data as quartiles extended with whiskers that show the variability of the values. The boxes may have lines extending vertically called whiskers. These lines indicate variability outside the upper and lower quartiles, and any point outside those lines or whiskers is considered an outlier. Box-and-whisker charts are most commonly used in statistical analysis, such as stock market analysis:

Candlestick charts

Candlestick charts are visually quite similar to the box-and-whiskers charts, except they are used to represent the difference between the opening and closing value, while showing the direction of the value (whether increasing or decreasing) through different colors. If the value of a particular piece of data stays the same, the rectangular shape will not be shown at all:

For more information regarding the different types of charts supported by Qt, please head over to the following link: https://doc.qt.io/qt-5/qtcharts-overview.html.

Qt supports most of the diagram types you need for your project. It is also extremely easy to implement these diagrams in Qt. Let's see how we can do it!

Implementing charts and graphs

Qt makes drawing different types of diagrams easy by putting the complex drawing algorithms behind different abstraction layers, and providing us with a set of classes and functions that can be used to easily create these diagrams without knowing how the drawing algorithm works behind the scenes. These classes and functions are all included in the chart module that comes together with Qt.

Let's create a new Qt Widgets Application project and try to create our first chart in Qt.

After you have created the new project, open up the project file (.pro) and add the charts module to your project, like so:

QT += core gui charts

Then, open up mainwindow.h and add the following to include the header files that are required for using the charts module:

#include <QtCharts>
#include <QChartView>
#include <QBarSet>
#include <QBarSeries>

The QtCharts and QtChartView headers are both essential for Qt's charts module. You must include both of them for any type of chart to work at all. The other two headers, namely QBarSetand QBarSeries, are used here because we're going to create a bar chart. The headers that get included in your project will be different depending on the type of chart you want to create.

Next, open mainwindow.ui and drag either Vertical Layout or Horizontal Layout to the central widget. Then, select the central widget and click either Layout Horizontally or Layout Vertically. The layout direction is not particularly important, as we will only create a single chart here:

After that, right-click on the layout widget you just dragged to the central widget, and select Morph into | QFrame. This will change the layout widget into a QFrame widget while still maintaining its layout properties. If you create a QFrame from Widget Box, it won't have the layout properties that we need. This step is important so that we can set it as the parent of our chart later:

Now open up mainwindow.cpp and add the following code:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

QBarSet *set0 = new QBarSet("Jane");
QBarSet *set1 = new QBarSet("John");
QBarSet *set2 = new QBarSet("Axel");
QBarSet *set3 = new QBarSet("Mary");
QBarSet *set4 = new QBarSet("Samantha");

*set0 << 10 << 20 << 30 << 40 << 50 << 60;
*set1 << 50 << 70 << 40 << 45 << 80 << 70;
*set2 << 30 << 50 << 80 << 13 << 80 << 50;
*set3 << 50 << 60 << 70 << 30 << 40 << 25;
*set4 << 90 << 70 << 50 << 30 << 16 << 42;

QBarSeries *series = new QBarSeries();
series->append(set0);
series->append(set1);
series->append(set2);
series->append(set3);
series->append(set4);
}

The code above initializes all the categories that will be displayed in the bar chart. Then, we also added six different items of data to each category, which will later be represented in the form of bars/rectangular shapes.

The QBarSet class represents a set of bars in the bar chart. It groups several bars into a bar set, which can then be labeled. QBarSeries, on the other hand, represents a series of bars grouped by category. In other words, bars that have the same color belong to the same series.

Next, initiate the QChart object and add the series to it. We also set the chart's title and enable animation:

QChart *chart = new QChart();
chart->addSeries(series);
chart->setTitle("Student Performance");
chart->setAnimationOptions(QChart::SeriesAnimations);

After that, we create a bar chart category axis and apply it to the bar chart's x axis. We used a QStringList variable, which is similar to an array, but explicitly for storing strings. The QBarCategoryAxis will then take the string list and populate it over the x axis:

QStringList categories;
categories << "Jan" << "Feb" << "Mar" << "Apr" << "May" << "Jun";
QBarCategoryAxis *axis = new QBarCategoryAxis();
axis->append(categories);
chart->createDefaultAxes();
chart->setAxisX(axis, series);

Then, we create a chart view for Qt to render the bar chart and set it as a child of the frame widget in the main window; otherwise, it won't be rendered on the main window:

QChartView *chartView = new QChartView(chart);
chartView->setParent(ui->verticalFrame);

Click the Run button in Qt Creator, and you should see something like this:

Next, let's do a pie chart; it's really easy. First, instead of QBarSet and QBarSeries, we include QPieSeries and QPieSlice:

#include <QPieSeries>
#include <QPieSlice>

Then, create a QPieSeries object and set up the name and value of each data. After that, set one of the slices to a different visual style and make it pop out from the rest. Then, create a QChart object and link it with the QPieSeriesobject that we have created:

QPieSeries *series = new QPieSeries();
series->append("Jane", 10);
series->append("Joe", 20);
series->append("Andy", 30);
series->append("Barbara", 40);
series->append("Jason", 50);

QPieSlice *slice = series->slices().at(1);
slice->setExploded(); // Explode this chart
slice->setLabelVisible(); // Make label visible
slice->setPen(QPen(Qt::darkGreen, 2)); // Set line color
slice->setBrush(Qt::green); // Set slice color

QChart *chart = new QChart();
chart->addSeries(series);
chart->setTitle("Students Performance");

Last, but not least, create the QChartView object and link it with the QChart object we just created. Then, set it as a child of the frame widget, and we're good to go!

QChartView *chartView = new QChartView(chart);
chartView->setParent(ui->verticalFrame);

Press the Run button now, and you should be able to see something like this:

For more examples of how to create different charts in Qt, please check out their sample code at the following link: https://doc.qt.io/qt-5/qtcharts-examples.html.

Now that we've seen that it is easy to create graphs and charts with Qt, let's expand the project we started in the previous chapters and create a dashboard for it!

Creating the dashboard page

In the previous chapter, we created a functional login page that allows the user to sign in using their username and password. What we need to do next is to create the dashboard page, which the user will automatically get directed to upon successful login.

The dashboard page usually serves as a quick overview for the user about the status of their company, business, project, assets, and/or other statistics. The following image shows an example of what a dashboard page could look like:

As you can see, there are quite a number of charts and graphs that are being used on the dashboard page because it is the best way for displaying a huge number of data without making the users feel overwhelmed. Moreover, graphs and charts can let the users understand the overall situation easily without digging too much into the details.

Let's open up our previous project and open the mainwindow.ui file. The user interface should look something like this:

As you can see, we already have the login page now, but we need to add in another page for the dashboard as well. For multiple pages to co-exist in the same program and to be able to switch between different pages at any time, Qt provides us with something called QStackedWidget.

A stacked widget is just like a book that you can add more and more pages to, but it shows only one page at a time. Each page is a completely different GUI, so it won't interfere with other pages in the stacked widget.

Since the previous login page was not made with a stacked widget in mind, we have to make some adjustments to it. First, drag and drop a stacked widget from the Widget Box to the central widget of your application, and then, we need to move everything previously under the central widget into the first page of the stacked widget, which we renamed loginPage:

Next, set all the layout settings of the central widget to 0, so that it contains no margin at all, like so:

After that, we must cut away the code in the style sheet property of the central widget, and paste it to the login page's style sheet property. In other words, the background image, button style, and other visual settings are now only applied to the login page.

Once you're done, you should be getting two completely different GUIs (the dashboard page is empty for now) when switching between pages on the stacked widget:

Next, drag and drop a grid layout to the dashboard page, and apply Layout Vertically to the dashboard page:

After that, drag and drop six Vertical Layout into the Grid Layout, like so:

Then, select each of the vertical layouts we just added to the grid layout, and turn it into QFrame:

Just as we did in the charts implementation examples, we must turn the layout into a QFrame (or QWidget) so that we can attach the chart on it as a child object. If you directly drag a QFrame from the widget box and don't use morphing, the QFrame objects do not have the layout properties, and hence the charts may not be resizing themselves to fit the QFrame's geometry. Also, name those QFrame objects as chart1 to chart6 as we're going to need them in the following steps. Once you're done with that, let's proceed to the code.

First, open your project (.pro) file and add the charts module, just as we did in the earlier example in this chapter. Then, open up mainwindow.h and include all the headers required. This time around, we also include the QLineSeries header for creating the line chart:

#include <QtCharts>
#include <QChartView>

#include <QBarSet>
#include <QBarSeries>

#include <QPieSeries>
#include <QPieSlice>

#include <QLineSeries>

After that, declare the pointers for the charts, like so:

QChartView *chartViewBar;
QChartView *chartViewPie;
QChartView *chartViewLine;

Then, we'll add the code for creating a bar chart. This is the same bar chart we created earlier in the chart implementation example, except it's now attached to the QFrame object called chart1, and is set to enable anti-aliasing when rendering. The anti-aliasing feature removes the jagged edges of all charts and thus makes the rendering appear smoother:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

////////BAR CHART/////////////
QBarSet *set0 = new QBarSet("Jane");
QBarSet *set1 = new QBarSet("John");
QBarSet *set2 = new QBarSet("Axel");
QBarSet *set3 = new QBarSet("Mary");
QBarSet *set4 = new QBarSet("Samantha");

*set0 << 10 << 20 << 30 << 40 << 50 << 60;
*set1 << 50 << 70 << 40 << 45 << 80 << 70;
*set2 << 30 << 50 << 80 << 13 << 80 << 50;
*set3 << 50 << 60 << 70 << 30 << 40 << 25;
*set4 << 90 << 70 << 50 << 30 << 16 << 42;

QBarSeries *seriesBar = new QBarSeries();
seriesBar->append(set0);
seriesBar->append(set1);
seriesBar->append(set2);
seriesBar->append(set3);
seriesBar->append(set4);

QChart *chartBar = new QChart();
chartBar->addSeries(seriesBar);
chartBar->setTitle("Students Performance");
chartBar->setAnimationOptions(QChart::SeriesAnimations);

QStringList categories;
categories << "Jan" << "Feb" << "Mar" << "Apr" << "May" << "Jun";
QBarCategoryAxis *axis = new QBarCategoryAxis();
axis->append(categories);
chartBar->createDefaultAxes();
chartBar->setAxisX(axis, seriesBar);

chartViewBar = new QChartView(chartBar);
chartViewBar->setRenderHint(QPainter::Antialiasing);
chartViewBar->setParent(ui->chart1);
}

Next, we also add the code for the pie chart. Again, this is the same pie chart from the previous example:

QPieSeries *seriesPie = new QPieSeries();
seriesPie->append("Jane", 10);
seriesPie->append("Joe", 20);
seriesPie->append("Andy", 30);
seriesPie->append("Barbara", 40);
seriesPie->append("Jason", 50);

QPieSlice *slice = seriesPie->slices().at(1);
slice->setExploded();
slice->setLabelVisible();
slice->setPen(QPen(Qt::darkGreen, 2));
slice->setBrush(Qt::green);

QChart *chartPie = new QChart();
chartPie->addSeries(seriesPie);
chartPie->setTitle("Students Performance");

chartViewPie = new QChartView(chartPie);
chartViewPie->setRenderHint(QPainter::Antialiasing);
chartViewPie->setParent(ui->chart2);

Finally, we also add a line graph to the dashboard, which is something new. The code is very simple and very similar to the pie chart:

QLineSeries *seriesLine = new QLineSeries();
seriesLine->append(0, 6);
seriesLine->append(2, 4);
seriesLine->append(3, 8);
seriesLine->append(7, 4);
seriesLine->append(10, 5);
seriesLine->append(11, 10);
seriesLine->append(13, 3);
seriesLine->append(17, 6);
seriesLine->append(18, 3);
seriesLine->append(20, 2);

QChart *chartLine = new QChart();
chartLine->addSeries(seriesLine);
chartLine->createDefaultAxes();
chartLine->setTitle("Students Performance");

chartViewLine = new QChartView(chartLine);
chartViewLine->setRenderHint(QPainter::Antialiasing);
chartViewLine->setParent(ui->chart3);

Once you're done with that, we must add a resize-event slot to the main window class, and make the charts follow the size of their respective parent when the main window is being resized. This can be done by first going to the mainwindow.h and adding in the event-handler declaration:

protected:
void resizeEvent(QResizeEvent* event);

Then, open up mainwindow.cpp and add the following code:

void MainWindow::resizeEvent(QResizeEvent* event)
{
QMainWindow::resizeEvent(event);

chartViewBar->resize(chartViewBar->parentWidget()->size());
chartViewPie->resize(chartViewPie->parentWidget()->size());
chartViewLine->resize(chartViewLine->parentWidget()->size());
}

Do note that the QMainWindow::resizeEvent(event) must be called first so that the default behavior will be triggered before you call your custom methods below it. resizeEvent() is one of the many event handlers provided by Qt for reacting to its events, such as mouse events, window events, paint events, and so on. Unlike the signal-and-slots mechanism, you need to replace the virtual function of the event handler to make it do what you want it to do when the event is being called.

If we build and run the project now, we should be getting something like this:

Looks pretty neat, doesn't it! However, for the sake of simplicity and so as not to confuse the readers, the charts are all hard-coded and are not using any data from the database. If you intend to use data from the database, don't make any SQL query during program startup, as this will make your program freeze if the data you're loading is very large, or your server is very slow.

The best way to do it is to load the data only when you're switching from the login page to the dashboard page (or upon switching to any other pages) so that the loading time is less obvious to the user. To do this, right-click on the stacked widget and select Go to slot. Then, select currentChanged(int) and click OK.

After that, a new slot function will be created automatically by Qt. This function will be called automatically when the stacked widget is switching between pages. You can check which page it is currently switching over to by checking the arg1 variable. The arg1 value will be 0 if the target page is the first page within stacked widget, or 1 if the target is the second page, and so on.

You can submit the SQL query only when the stacked widget is showing the dashboard page, which is the second page (arg1equals to 1):

void MainWindow::on_stackedWidget_currentChanged(int arg1)
{
if (arg1 == 1)
{
// Do it here
}
}

Phew! That's a lot to digest for this chapter! Hopefully, this chapter will help you understand how to create a beautiful and informative page for your project.

Summary

The chart module in Qt is the combination of feature and visual aesthetic. Not only is it easy to implement without the need to write a very long code just to display the chart, but it is also customizable to suit your visual requirements. We really need to be thankful to Qt developers for opening up this module and allowing non-commercial users to use it for free!

In this chapter, we have learned how to create a really nice-looking dashboard, and display different types of charts on it using the Qt Chart module. In the coming chapter, we will learn how to use view widget, dialog boxes, and file-selection dialogs.

Item Views and Dialogs

In the previous chapter, we learned how to display data using different types of chart. Charts are one of many ways to present information to the users on screen. It is very important for your application to present vital information to the users so that they know exactly what's happening to the application—whether data has been saved successfully, or the application is waiting for the user's input, or warning/error messages that the users should be aware of, and so on—it's all very important to ensure your application's user-friendliness and usability.

In this chapter, we will cover the following topics :

- Working with item view widgets

- Working with dialog boxes

- Working with file selection dialogs

- Image scaling and cropping

Qt provides us with many types of widget and dialog that we can easily use to display important information to the users. Let's check out what these widgets are!

Working with item view widgets

Other than displaying data using different types of chart, we can also display this data using different types of item view. An item view widget presents data by rendering it visually, usually along the vertical axis.

A two-dimensional item view, often known as a table view, displays data in both vertical and horizontal directions. That allows it to display huge volumes of data within a compact space, and enables the users to search for an item very quickly and easily.

There are two ways to display data in an item view. The most common method is to use the model-view architecture, which uses three different components, model, view, and delegate, to retrieve data from a data source and display it in the item view. These components all make use of the signal-slot architecture provided by Qt to communicate with each other:

- Signals from the model inform the view about changes to the data held by the data source

- Signals from the view provide information about the user's interaction with the items being displayed

- Signals from the delegate are used during editing to tell the model and view about the state of the editor

The other method is the manual way, in which the programmer must tell Qt which data goes into which column and row. This method is much simpler than the model-view, but much slower when compared to its performance. However, for small amounts of data, the performance issue can be negligible, making this a good approach.

If you open up Qt Designer, you will see the two different categories for Item View Widgets, namely Item Views (Model-Based) and Item Widgets (Item-Based):

Even though they might look the same, in actual fact the widgets within the two categories work very differently. In this chapter, we will learn how to use the latter category, as it is more straightforward and easy to understand, and able to serve as prerequisite knowledge for the former category.

Under the Item Widgets (Item-Based) category are three different widgets called List Widget, Tree Widget, and Table Widget. Each of these item widgets displays data in a different way. Pick the one that suits your needs:

As you can see from the preceding diagram, the List Widget displays its items in a one-dimensional list, while the Table Widget displays its item in a two-dimensional table. Even though the Tree Widget works almost similar to the List Widget, its items are displayed in a hierarchical structure, in which each item can have multiple children items under it, recursively. One good example of this is the filesystem in our operating system, which displays the directory structure using the tree widget.

To illustrate the differences, let's create a new Qt Widgets application project and try it out ourselves.

Creating our Qt Widgets application

Once you have created the project, open up mainwindow.ui and drag the three different item widgets to your main window. After that, select the main window and click the Layout Vertically button located at the top:

Then, double-click on the List Widget and a new window will pop out. Here, you can add a few dummy items to the List Widget by clicking the + icon, or remove them by selecting an item from the list and clicking the - icon. Click the OK button to apply the final result to the widget:

You can do the same to the Tree Widget. It's almost the same as the List Widget, except that you can add sub-items to an item, recursively. You can also add columns to the Tree Widget and name the columns:

Finally, double-click on the Table Widget to open the Edit Table Widget window. Unlike the other two item views, the Table Widget is a two-dimensional item view, which means you can add columns and rows to it just like a spreadsheet. Each column and row can be labeled with the desired name by setting it in the Columns or Rows tab:

It's really easy to understand how a widget works by using the Qt Designer. Just drag and drop the widget into the window and play around with its settings, then build and run the project to see the result for yourself.

In this case, we have demonstrated the differences between the three item views widgets without writing a single line of code:

Making our List Widget functional

Writing code, however, is still required in order for the widgets to be fully functional in your application. Let's learn how to add items to our item view widgets using C++ code!

First, open up mainwindow.cpp and write the following code to the class constructor, right after ui->setupui(this):

ui->listWidget->addItem("My Test Item");

As simple as that, you have successfully added an item to the List Widget!

There is another way to add an item to the List Widget. But before that, we must add the following headers to mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QDebug>
#include <QListWidgetItem>

The QDebug header is for us to print out debug message, and the QListWidgetItem header is for us to declare List Widget Item objects. Next, open up mainwindow.cpp and add the following code:

QListWidgetItem* listItem = new QListWidgetItem;
listItem->setText("My Second Item");
listItem->setData(100, 1000);
ui->listWidget->addItem(listItem);

The preceding code does the same as the previous one-line code. Except, this time, I've added an extra data to the item. The setData() function takes in two input variables—the first variable is the data-role of item, which indicates how it should be treated by Qt. If you put a value that matches the Qt::ItemDataRole enumerator, the data will affect the display, decoration, tooltip, and so on, and that may change its appearance.

In my case, I just simply set a number that doesn't match any of the enumerators in Qt::ItemDataRole so that I can store it as a hidden data for later use. To retrieve the data, you can simply call data() and insert the number that matches the one you've just set:

qDebug() << listItem->data(100);

Build and run the project; you should be able to see that the new item is now being added to the List Widget:

For more information about Qt::ItemDataRole enumerators, please check out the following link: http://doc.qt.io/qt-5/qt.html#ItemDataRole-enum

As mentioned earlier, hidden data can be attached to a list item for later use. For example, you could use the list widget to display a list of products ready to be purchased by the user. Each of these items can be attached with its product ID so that when the user selects the item and places it on the cart, your system can automatically identify which product has been added to the cart by identifying the product ID stored as the data role.

In the preceding example, I stored custom data, 1000, in my list item and set its data role as 100, which does not match any of the Qt::ItemDataRole enumerators. This way, the data won't be shown to the users, and thus it can only be retrieved through C++ code.

Adding functionality to the Tree Widget

Next, let's move on to the Tree Widget. It is actually not that different from the List Widget. Let's take a look at the following code:

QTreeWidgetItem* treeItem = new QTreeWidgetItem;
treeItem->setText(0, "My Test Item");
ui->treeWidget->addTopLevelItem(treeItem);

It's pretty much the same as the List Widget, except we have to set the column ID in the setText() function. This is because the Tree Widget is somewhere between a List Widget and a Table Widget—it can have more than one column but can't have any rows.

The most obvious distinction between a Tree Widget and other view widgets is that all its items can contain children items, recursively. Let's look at the following code to see how we can add a child item to an existing item in the Tree Widget:

QTreeWidgetItem* treeItem2 = new QTreeWidgetItem;
treeItem2->setText(0, "My Test Subitem");
treeItem->addChild(treeItem2);

It's really that simple! The final result looks like this:

Finally, our Table Widget

Next, let's do the same for the Table Widget. Technically, the items already exist and are reserved in the Table Widget when the columns and rows are being created. What we need to do is to create a new item and replace it with the (currently empty) item located at a specific column and row, which is why the function name is called setItem(), instead of addItem() used by the List Widget.

Let's take a look at the code:

QTableWidgetItem* tableItem = new QTableWidgetItem;
tableItem->setText("Testing1");
ui->tableWidget->setItem(0, 0, tableItem);

QTableWidgetItem* tableItem2 = new QTableWidgetItem;
tableItem2->setText("Testing2");
ui->tableWidget->setItem(1, 2, tableItem2);

As you can see from the code, I have added two sections of data to two different locations, which translates into the following result:

That's it! It's all that simple and easy to display data using item views in Qt. If you are looking for more examples related to item views, please visit the following link: http://doc.qt.io/qt-5/examples-itemviews.html

Working with dialog boxes

One very important aspect of creating a user-friendly application is the ability to display vital information regarding the status of the application when a certain event (intended or unintended) occurs. To display such information, we need an external window that can be dismissed by the user once he/she has acknowledged the information.

Qt comes with this functionality, and it's all residing in the QMessageBox class. There are several types of message box you can use in Qt; the most basic one uses just a single line of code, like so:

QMessageBox::information(this, "Alert", "Just to let you know, something happened!");

There are three parameters you need to provide for this function. The first one is the parent of the message box, which we have set as the main window. The second parameter is for the window title, and the third parameter is for the message we want to deliver to the user. The preceding code will produce the following result:

The appearance shown here is running on a Windows system. The appearance may look different on different operating systems (Linux, macOS, and so forth). As you can see, the dialog box even comes with an icon located before the text. There are a few types of icon you can use, such as information, warning, and critical. The following code shows you the code for calling all the different message boxes with icons:

QMessageBox::question(this, "Alert", "Just to let you know, something happened!");
QMessageBox::warning(this, "Alert", "Just to let you know, something happened!");
QMessageBox::information(this, "Alert", "Just to let you know, something happened!");
QMessageBox::critical(this, "Alert", "Just to let you know, something happened!");

The preceding code produces the following results:

If you don't need any icons, just call the QMessageBox::about() function instead. You can also set the buttons you want by picking from a list of standard buttons provided by Qt, for example:

QMessageBox::question(this, "Serious Question", "Am I an awesome guy?", QMessageBox::Ignore, QMessageBox::Yes);

The preceding code will produce the following result:

Since these are the built-in functions provided by Qt to create message boxes with ease, it doesn't give developers the freedom to fully customize a message box. However, Qt does allow you to create your message boxes manually using another method, which is much more customizable than the built-in method. It takes a couple more lines of code, but is still pretty simple to write:

QMessageBox msgBox;
msgBox.setWindowTitle("Alert");
msgBox.setText("Just to let you know, something happened!");
msgBox.exec();

The preceding code will produce the following result:

It looks just the same, you're telling me. What about adding our own icon and customized buttons? No problem with that:

QMessageBox msgBox;
msgBox.setWindowTitle("Serious Question");
msgBox.setText("Am I an awesome guy?");
msgBox.addButton("Seriously Yes!", QMessageBox::YesRole);
msgBox.addButton("Well no thanks", QMessageBox::NoRole);
msgBox.setIcon(QMessageBox::Question);
msgBox.exec();

The preceding code produces the following result:

In the preceding code example, I have loaded the question icon that comes with Qt, but you can also load your own icon from the resource file if you intended to do so:

QMessageBox msgBox;
msgBox.setWindowTitle("Serious Question");
msgBox.setText("Am I an awesome guy?");
msgBox.addButton("Seriously Yes!", QMessageBox::YesRole);
msgBox.addButton("Well no thanks", QMessageBox::NoRole);
QPixmap myIcon(":/images/icon.png");
msgBox.setIconPixmap(myIcon);
msgBox.exec();

Build and run the project now, and you should be able to see this fantastic message box:

Once you have understood how to create your own message boxes, let's proceed to learn about the event system that comes with the message box.

When a user is presented with a message box with multiple different choices, he/she would expect a different reaction from the application when pressing a different button.

For example, when a message box pops up and asks the user whether they wish to quit the program or not, the button Yes should make the program terminate, while the No button will do nothing.

Qt's QMessageBox class provides us with a simple solution for checking the button event. When the message box is being created, Qt will wait for the user to pick their choice; then, it will return the button that gets triggered. By checking which button is being clicked, the developer can then proceed to trigger the relevant event. Let's take a look at the example code:

if (QMessageBox::question(this, "Question", "Some random question. Yes or no?") == QMessageBox::Yes)
{
QMessageBox::warning(this, "Yes", "You have pressed Yes!");
}
else
{
QMessageBox::warning(this, "No", "You have pressed No!");
}

The preceding code will produce the following result:

If you prefer the manual way to create your message box, the code for checking the button event is slightly longer:

QMessageBox msgBox;
msgBox.setWindowTitle("Serious Question");
msgBox.setText("Am I an awesome guy?");
QPushButton* yesButton = msgBox.addButton("Seriously Yes!", QMessageBox::YesRole);
QPushButton* noButton = msgBox.addButton("Well no thanks", QMessageBox::NoRole);
msgBox.setIcon(QMessageBox::Question);
msgBox.exec();

if (msgBox.clickedButton() == (QAbstractButton*) yesButton)
{
QMessageBox::warning(this, "Yes", "Oh thanks! :)");
}
else if (msgBox.clickedButton() == (QAbstractButton*) noButton)
{
QMessageBox::warning(this, "No", "Oh why... :(");
}

Even though the code is slightly longer, the basic concept is pretty much the same—the clicked button will always be able to be retrieved by the developer for triggering the appropriate action. This time, however, instead of checking the enumerator, Qt directly checks the button pointer instead, since the preceding code does not use the built-in standard buttons from the QMessageBox class.

Build the project, and you should be able to get the following result:

For more information regarding the dialog boxes, please visit the API documents located at the following link: http://doc.qt.io/qt-5/qdialog.html

Creating File Selection Dialogs

Since we have covered the topic about message boxes, let's also learn about the other type of dialog—the File Selection Dialog. The File Selection Dialog is also very useful, especially if your application frequently deals with files. It is extremely unpleasant to ask users to key in the absolute path of the file they wanted to open, so the File Selection Dialog is very handy in this kind of situation.

Qt provides us with a built-in File Selection Dialog that looks exactly the same as the one we see in our operating system, and therefore, it won't feel unfamiliar to the users. The File Selection Dialog essentially only does one thing—it lets the user pick the file(s) or folder they want and return the path(s) of the selected file(s) or folder; that's all. In fact, it is not in charge of opening the file and reading its content.

Let's look at how we can trigger the File Selection Dialog. First, open up mainwindow.h and add in the following header files:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QFileDialog>
#include <QDebug>

Next, open up mainwindow.cpp and insert the following code:

QString fileName = QFileDialog::getOpenFileName(this);
qDebug() << fileName;

It's that simple! Build and run the project now, and you should get this:

If the user has selected a file and pressed Open, the fileName variable will be filled with the absolute path of the selected file. If the user clicked the Cancel button, the fileName variable will be an empty string.

The File Selection Dialog also contains several options that can be set during the initialization step. For example:

QString fileName = QFileDialog::getOpenFileName(this, "Your title", QDir::currentPath(), "All files (*.*) ;; Document files (*.doc *.rtf);; PNG files (*.png)");
qDebug() << fileName;

There are three things that we have set in the preceding code they are as follows:

- The window title of the File Selection Dialog

- The default path that the users see when the dialog is being created

- File type filters

The file type filter is very handy when you only allow the users to select a specific type of file (for example, only JPEG image files) and hide the rest. Besides getOpenFileName(), you can also use getSaveFileName(), which will allow the user to specify a filename that does not already exist.

For more information regarding the File Selection Dialog, please visit the API documents located at the following link: http://doc.qt.io/qt-5/qfiledialog.html

Image scaling and cropping

Since we learned about the File Selection Dialog in the previous section, I'd thought we should learn something fun this time!

First off, let's create a new Qt Widgets Application. Then, open up mainwindow.ui and create the following user interface:

Let's dissect this user interface into three parts:

- Top—Image preview:

- First, add a Horizontal Layout to the window.

- Then, add a Label widget into the Horizontal Layout we just added, then set the text property to empty. Set both the label's minimumSize and maximumSize properties to 150x150. Finally, set the frameShape property under the QFrame category to Box.

- Add two Horizontal Spacers to the sides of the label to make it centered.

- Middle—Sliders for adjustments:

- Add a Form Layout to the window, below the Horizontal Layout we just added previously in step 1.

- Add three Labels to the Form Layout, and set their text property to Scale:, Horizontal:, and Vertical: respectively.

- Add three Horizontal Sliders to the Form Layout. Set the minimum property to 1 and maximum to 100. Then, set the pageStep property to 1.

- Set the value property of the scale slider to 100.

- Bottom—Browse button and Save button:

- Add a Horizontal Layout to the window, below the Form Layout we previously added during step 2.

- Add two Push Buttons to the Horizontal Layout and set their text property to Browse and Save respectively.

- Lastly, delete the Menu Bar, Tool Bar, and Status Bar from the central widget.

Now that we have created the user interface, let's dive into the coding! First, open up mainwindow.h and add in the following headers:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QMessageBox>
#include <QFileDialog>
#include <QPainter>

After that, add the following variables to mainwindow.h:

private:
Ui::MainWindow *ui;
bool canDraw;
QPixmap* pix;
QSize imageSize;
QSize drawSize;
QPoint drawPos;

Then, go back to mainwindow.ui and right-click on the Browse button, followed by selecting Go to slot. Then, a window will pop up and ask you to select a signal. Pick the clicked() signal located at the top of the list, and then press the OK button:

A new slot function will be automatically added to your source file. Now, add the following code to open up the File Selection Dialog when the Browse button is clicked. The dialog only lists JPEG images and hides the other files:

void MainWindow::on_browseButton_clicked()
{
QString fileName = QFileDialog::getOpenFileName(this, tr("Open
Image"), QDir::currentPath(), tr("Image Files (*.jpg *.jpeg)"));

if (!fileName.isEmpty())
{
QPixmap* newPix = new QPixmap(fileName);

if (!newPix->isNull())
{
if (newPix->width() < 150 || newPix->height() < 150)
{
QMessageBox::warning(this, tr("Invalid Size"),
tr("Image size too small. Please use an image
larger than 150x150."));
return;
}

pix = newPix;
imageSize = pix->size();
drawSize = pix->size();

canDraw = true;

}
else
{
canDraw = false;

QMessageBox::warning(this, tr("Invalid Image"),
tr("Invalid or corrupted file. Please try again with
another image file."));
}
}
}

As you can see, the code checks whether any image has been selected by the user. If it has its checks again and see whether the image resolution is at least 150 x 150. If no problem is found, we will save the image's pixel map to a pointer called pix, then save the image size to the imageSize variable, and the initial drawing size to the drawSize variable. Finally, we set the canDraw variable to true.

After that, open up mainwindow.h again and declare these two functions:

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
virtual void paintEvent(QPaintEvent *event);
void paintImage(QString fileName, int x, int y);

The first function, paintEvent(), is a virtual function that automatically gets called whenever Qt needs to refresh the user interface, such as when the main window is being resized. We'll override this function and draw the newly loaded image onto the image preview widget. In this case, we'll call the paintImage() function within the paintEvent() virtual function:

void MainWindow::paintEvent(QPaintEvent *event)
{
if (canDraw)
{
paintImage("", ui->productImage->pos().x(), ui->productImage-
>pos().y());
}
}

After that, we'll write the paintImage() function in mainwindow.cpp:

void MainWindow::paintImage(QString fileName, int x, int y)
{
QPainter painter;
QImage saveImage(150, 150, QImage::Format_RGB16);

if (!fileName.isEmpty())
{
painter.begin(&saveImage);
}
else
{
painter.begin(this);
}

if (!pix->isNull())
{
painter.setClipRect(x, y, 150, 150);
painter.fillRect(QRect(x, y, 150, 150), Qt::SolidPattern);
painter.drawPixmap(x - drawPos.x(), y - drawPos.y(),
drawSize.width(), drawSize.height(), *pix);
}

painter.end();

if (fileName != "")
{
saveImage.save(fileName);
QMessageBox::information(this, "Success", "Image has been
successfully saved!");
}
}

This function does two things—if we don't set the fileName variable, it will proceed to draw the image on top of the image preview widget, otherwise, it will crop the image based on the dimension of the image preview widget and save it to the disk following the fileName variable.

We'll call this function again when the save button is being clicked. This time, we'll set the fileName variable as the desired directory path and filename, so that the QPainter class can save the image correctly:

void MainWindow::on_saveButton_clicked()
{
if (canDraw)
{
if (!pix->isNull())
{
// Save new pic from painter
paintImage(QCoreApplication::applicationDirPath() +
"/image.jpg", 0, 0);
}
}
}

Lastly, right-click on each of the three sliders and select Go to slot. Then, select valueChanged(int) and click OK.

After that, we'll write the code for the slot functions resulting from the previous step:

void MainWindow::on_scaleSlider_valueChanged(int value)
{
drawSize = imageSize * value / 100;
update();
}

void MainWindow::on_leftSlider_valueChanged(int value)
{
drawPos.setX(value * drawSize.width() / 100 * 0.5);
update();
}

void MainWindow::on_topSlider_valueChanged(int value)
{
drawPos.setY(value * drawSize.height() / 100 * 0.5);
update();
}

The scale slider is basically for users to resize the image to their desired scale within the image preview widget. The left slider is for the users to move the image horizontally, while the top slider is used by the users to move the image vertically. By combining these three different sliders, users can adjust and crop the image to their liking before proceeding to upload the image to the server, or use it for other purposes.

If you build and run the project now, you should be able to get this result:

You can click on the Browse button to select a JPG image file to load. After that, the image should appear on the preview area. You can then move the sliders around for adjusting the cropping size. Once you're satisfied with the result, click the Save button to save your image in the current directory.

Do check out the sample code that comes together with this book if you want to learn more about it in detail. You can find the source code at the following GitHub page: https://github.com/PacktPublishing/Hands-On-GUI-Programming-with-C-QT5

Summary

Input and Output (I/O) are the essence of modern computer software. Qt allows us to display our data in many different ways that are both intuitive and engaging to the end users. Other than that, the event system that comes with Qt makes our life as a programmer a lot easier, as it tends to automatically capture the user inputs through the powerful signal-and-slot mechanism and in-response triggering custom-defined behaviors. Without Qt, we would have a hard time trying to figure out how to reinvent the proverbial wheel, and might eventually end up creating a less user-friendly product.

In this chapter, we have learned how to make use of the fantastic features that are provided by Qt—view widgets, dialog boxes, and file selection dialogs used to display important information to the users. Furthermore, we also went through a fun little project that taught us how to scale and crop an image using Qt widgets for user inputs. In the next chapter, we will go for something more advanced (and fun too), which is creating our very own web browser using Qt!

Integrating Web Content

In the previous chapter, we learned how to use item views and dialogs in Qt. In this chapter, we will learn how to integrate web content into our Qt application.

Starting from the dotcom era in the late 90s and early 2000s, our world has become more and more connected by the internet. Naturally, the applications running on our computers are also evolving in that direction. Nowadays, most—if not all—of our software is in some way connected to the internet, usually to retrieve useful information and display it to their users. The easiest way to do this is to embed a web browser display (also known as a web view) into the application's user interface. That way, the users can not only view the information, but do so in an aesthetic way.

By using the web view, developers can take advantage of its rendering capability and decorate their contents using the powerful combination of HTML (Hypertext Markup Language) and CSS (Cascading Style Sheets). In this chapter, we will explore Qt's web engine module and create our very own web browser.

In this chapter, we will cover the following topics:

- Creating your own web browser

- Sessions, cookies, and cache

- Integrating JavaScript and C++

Without further ado, let's check out how to create our own web browser in Qt!

Creating your own web browser

Once upon a time, Qt used a different module called WebKit to render web contents on its user interface. However, the WebKit module has been completely deprecated since version 5.5 and replaced by a new module called WebEngine.

The new WebEngine module is based on the Chromium framework built by Google, and it will only work on the Visual C++ compiler on the Windows platform. Therefore, if you're running Windows, please make sure that you have installed Microsoft Visual Studio on your computer as well as all the MSVC components for Qt that match the version of Visual Studio installed on your computer. Other than that, the Qt WebEngine component is also required for this particular chapter. If you have skipped the components during Qt's installation, all you need to do is to run the same installer again and install it there:

Adding the web view widget

Once you are ready, let's get started! First, open up Qt Creator and create a new Qt Widgets Application project. After that, open up the project (.pro) file and add in the following text to enable the modules:

QT += core gui webengine webenginewidgets

If you didn't install the MSVC component (on Windows) or the Qt WebEngine component, error messages will appear at this point if you are trying to build the project. Please run the Qt installer again if that's the case.

Next, open up mainwindow.h and add the following header files:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QWebEngineView>

After that, open up mainwindow.h and add the following code:

private:
Ui::MainWindow *ui;
QWebEngineView* webview;

Then, add the following code:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

webview = new QWebEngineView(ui->centralWidget);
webview->load(QUrl("http://www.kloena.com"));
}

Build and run the program now and you should see the following result:

It's actually that simple. You have now successfully placed a web view on your application!

The reason why we're writing C++ code to create the web view is that the default Qt Designer used by the Qt Creator doesn't have web view in the widget box. The preceding code simply creates the QWebEngineView object, sets its parent object (in this case, the central widget), and sets the URL of the web page before showing the web view widget. If you want to use Qt Designer to place a web engine view on your UI, you must run the standalone Qt Designer located in your Qt installation directory. For example, if you're running Windows, it's located in C:QtQt5.10.25.10.2msvc2017_64bin. Please note that it's located in the directory with the compiler name that supports the web engine:

Creating a UI for a web browser

Next, we are going to turn this into a proper web browser. First, we need to add a few layout widgets so that we can put other widgets in place afterwards. Drag a Vertical Layout (1) onto the centralWidget and select the centralWidget from the object list. Then, click the Lay Out Vertically button (2) located at the top:

After that, select the newly added vertical layout, right-click and select Morph into | QFrame. The reason why we're doing this is that we want to place the web view widget under this QFrame object instead of the central widget. We must convert the layout widget to a QFrame (or any QWidget-inherited) object so that it can adopt the web view as its child. Finally, rename the QFrame object to webviewFrame:

Once you're done with that, let's drag and drop a Horizontal Layout widget above the QFrame object. Now we can see that the size of both the Horizontal Layout widget and the QFrame object are the same, and we don't want that. Next, select the QFrame object and set its Vertical Policy to Expanding:

Then, you will see the top layout widget is now very thin. Let's temporarily set its height to 20, like so:

After that, drag and drop three push buttons to the horizontal layout and we can now set its top margin back to 0:

Set the buttons' labels to Back, Forward, and Refresh respectively. You may also use icons instead of text to display on these buttons. If you wish to do that, simply set the text property to empty and select an icon from the icon property. For the sake of simplicity, we'll just display texts on the buttons for this tutorial.

Next, place a line edit widget on the right-hand side of the three buttons, followed by adding another push button with a Golabel:

After that, right-click on each of the buttons and select Go to slot. A window will pop up, select clicked() and press OK.

The signal functions for these buttons will look something like this:

void MainWindow::on_backButton_clicked()
{
webview->back();
}

void MainWindow::on_forwardButton_clicked()
{
webview->forward();
}

void MainWindow::on_refreshButton_clicked()
{
webview->reload();
}

void MainWindow::on_goButton_clicked()
{
loadPage();
}

Basically, the QWebEngineViewclass already provided us with functions such as back(), forward() and reload(), so we just have to call these functions when the respective button is pressed. The loadPage() function, however, is a custom function that we will write:

void MainWindow::loadPage()
{
QString url = ui->addressInput->text();
if (!url.startsWith("http://") && !url.startsWith("https://"))
{
url = "http://" + url;
}
ui->addressInput->setText(url);
webview->load(QUrl(url));
}

Remember to add the declaration for loadPage() in mainwindow.h as well.

Instead of just calling the load() function, I think we should do something more. Normally, users will not include the http:// (or https://) scheme when typing the URL of the web page, but it is required when we are passing the URL to the web view. To solve this problem, we automatically check for the existence of the scheme. If none has been found, we will manually add the http:// scheme to the URL. Also, don't forget to call it at the beginning to replace the load() function:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

webview = new QWebEngineView(ui->webviewFrame);
loadPage();
}

Next, right-click on the text input and select Go to slot. Then, select returnPressed() and click the OK button:

This slot function will be called when the user presses the Return key on the keyboard once they have finished typing the web page URL. Logically, the user would expect the page to start loading not have to press the Go button every time they are done typing the URL. The code is really simple, we'll just call the loadPage() function we just created in the previous step:

void MainWindow::on_addressInput_returnPressed()
{
loadPage();
}

Now that we have done a significant amount of code, let's build and run our project and see how it turns out:

The result shown doesn't really look that great. For some reason, the new web view doesn't seem to scale properly even on an expanding size policy, at least on Qt version 5.10, which is being used when writing this book. It might be fixed in the future version, but let's find a way to solve this issue. What I did was to override an inherited function in the main window called paintEvent(). In mainwindow.h, simply add the function declaration, like this:

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
void paintEvent(QPaintEvent *event);

Then, write its definition in mainwindow.cpp like so:

void MainWindow::paintEvent(QPaintEvent *event)
{
QMainWindow::paintEvent(event);
webview->resize(ui->webviewFrame->size());
}

This paintEvent() function will be automatically called by Qt whenever the main window needs to re-render its widgets (such as when the window is being resized). Since this function will be called when the application is being initialized and also when the window is being resized, we will use this function to manually resize the web view to fit with its parent widget.

Build and run the program again and you should be able to get the web view to fit nicely, regardless of how you resize the main window. Additionally, I also removed the menu bar, toolbar, and status bar to make the whole interface look more tidy, since we're not using any of those in this application:

Next, we need a progress bar to show users the current progression of the page load. To do that, first we need to place a progress bar widget below the web view:

Then, add these two slot functions to mainwindow.h:

private slots:
void on_backButton_clicked();
void on_forwardButton_clicked();
void on_refreshButton_clicked();
void on_goButton_clicked();
void on_addressInput_returnPressed();
void webviewLoading(int progress);
void webviewLoaded();

Their function definition in mainwindow.cpp looks like this:

void MainWindow::webviewLoading(int progress)
{
ui->progressBar->setValue(progress);
}

void MainWindow::webviewLoaded()
{
ui->addressInput->setText(webview->url().toString());
}

The first function, webviewLoading() simply takes the progression level (in the form of a percentage value) from the web view and directly supplies it to the progress bar widget.

The second function webviewLoaded() will replace the URL text on the address input with the actual URL of the web page loaded by the web view. Without this function, the address input will not display the correct URL after you've pressed the back button or the forward button. Once you're done, let's compile and run the project again. The result looks amazing:

You will ask me, what's the actual use of this if I'm not making a web browser using Qt? There are many other uses for embedding a web view into your application, for instance, showing the latest news and updates of your product to the users through a nicely decorated HTML page, which is a common method used by most of the online games in the gaming market. The stream client, for example, also uses a web view to display the latest games and discounts to their players.

These are often called hybrid applications, which combine web content with native x, so you can leverage both dynamic contents from the web as well as code running natively that has the benefits of high performance and a consistent look and feel.

Other than that, you can also use it to display the printable report in HTML format. You can easily send the report to the printer, or save it as a PDF file by calling webview->page()->print() or webview->page()->printToPdf().

To learn more about printing from the web view, check out the following link: http://doc.Qt.io/Qt-5/qwebenginepage.html#print.

You might also want to create the entire user interface of your program using HTML and embed all the HTML, CSS and image files into Qt's resource package and run it locally from the web view. The possibilities are endless, the only limit is your imagination!

To learn more about Qt WebEngine, check out the documentation here: https://doc.Qt.io/Qt-5/qtwebengine-overview.html.

Managing browser history

Qt's web engine stores all the links which the user has visited into an array structure for later use. The web view widget uses this to move back and forth between history by calling back() and forward().

If you need to manually access this browsing history, add the following header to mainwindow.h:

#include <QWebEnginePage>

After that, use the following code to obtain the browsing history in the form of a QWebEngineHistory object:

QWebEngineHistory* history = QWebEnginePage::history();

You can get the entire list of visited links from history->items() or navigate between history using functions such as back() or forward(). To clear the browsing history, call history->clear(). Alternatively, you can also do this:

QWebEngineProfile::defaultProfile()->clearAllVisitedLinks();

To learn more about the QWebEngineHistory class, visit the following link: http://doc.Qt.io/Qt-5/qwebenginehistory.html.

Sessions, cookies, and cache

Just like any other web browser, the WebEngine module also supports mechanisms used to store temporary data and persistent data for session and cache. Sessions and cache are very important as they allow websites to remember your last visit and associate you with data, such as a shopping cart. The definitions of a session, a cookie, and a cache are shown as follows:

- Session: Normally, sessions are server-side files that contain user information with a unique identifier, which gets sent from the client side to map them to a specific user. In Qt, however, a session simply means a cookie that doesn't have any expiration date, and hence it will be gone when the program is closed.

- Cookie: Cookies are client-side files that contain user information or any other information that you want to save. Unlike sessions, cookies have an expiration date which means they will remain valid and can be retrieved before reaching the expiration date, even if the program has been closed and re-opened again.

- Cache: Caching is a method used to speed up page loading by saving the page and its resources to a local disk during its first load. If the user loads the same page again on the next visit, the web browser will reuse the cached resources instead of waiting for the download to complete, which can significantly speed up the page loading time.

Managing sessions and cookies

By default, WebEngine doesn't save any cookie and treats all user information as temporary sessions, which means when you close the program, your login session on the web page will automatically become invalid.

To enable cookies on Qt's WebEngine module, first add the following header to mainwindow.h:

#include <QWebEngineProfile>

Then, simply call the following function to force persistent cookies:

QWebEngineProfile::defaultProfile()->setPersistentCookiesPolicy(QWebEngineProfile::ForcePersistentCookies);

After calling the preceding function, your login session will continue to exist after closing the program. To revert it to non-persistent cookies, we simply call:

QWebEngineProfile::defaultProfile()->setPersistentCookiesPolicy(QWebEngineProfile::NoPersistentCookies);

Other than that, you can also change the directory in which your Qt program stores the cookies. To do that, add the following code to your source file:

QWebEngineProfile::defaultProfile()->setPersistentStoragePath("your folder");

If, for some reason, you want to manually delete all the cookies, use the following code:

QWebEngineProfile::defaultProfile()->cookieStore()->deleteAllCookies();

Managing cache

Next, let's talk about a cache. There are two types of cache which you can use in the web engine module, namely, Memory Cache and Disk Cache. Memory Cache uses the computer's memory to store the cache, which will be gone once you've closed the program. On the other hand, Disk Cache saves all the files in the hard disk, and hence they will still remain, even after you've turned off your computer.

By default, the web engine module saves all the cache to the disk, if you need to change them to Memory Cache, call the following function:

QWebEngineProfile::defaultProfile()->setHttpCacheType(QWebEngineProfile::MemoryHttpCache);

Alternatively, you can also disable caching completely by calling:

QWebEngineProfile::defaultProfile()->setHttpCacheType(QWebEngineProfile::NoCache);

As for changing the folder to which your program saves the cache files, call the setCachePath() function:

QWebEngineProfile::defaultProfile()->setCachePath("your folder");

Lastly, to delete all the cache files, call clearHttpCache():

QWebEngineProfile::defaultProfile()->clearHttpCache();

There are many other functions that you can use to change the settings related to cookies and cache.

You can read more about it at the following link: https://doc.Qt.io/Qt-5/qwebengineprofile.html

Integrating JavaScript and C++

One powerful feature of using Qt's web engine module is that it can call JavaScript functions from C++, as well as calling C++ functions from JavaScript. This makes it more than just a web browser. You can use this to access features that are not supported by the web browser standard, such as file management and hardware integration. Things like that are not possible with W3C standards; hence, it is not possible to do it in native JavaScript. However, you can implement these features using C++ and Qt, then simply call the C++ functions from your JavaScript. Let's take a look at how we can achieve this with Qt.

Calling JavaScript functions from C++

After that, add in the following code to the HTML file we just created:

<!DOCTYPE html><html>
<head>
<title>Page Title</title>
</head>
<body>
<p>Hello World!</p>
</body>
</html>

These are the basic HTML tags which show you nothing other than a line of words that says Hello World!. You can try and load it using your web browser:

After that, let's go back to our Qt project and go to File | New File or Project and create a Qt Resource File:

Then, open up the Qt resource file we just created and add in an /html prefix followed by adding the HTML file to the resource file, like so:

Right-click on text.html while the resource file is still opened, then select Copy Resource Path to Clipboard. Right after that, change the URL of your web view to:

webview->load(QUrl("qrc:///html/test.html"));

You can use the link you just copied from the resource file, but make sure you add the URL scheme qrc:// at the front of the link. Build and run your project now and you should be able to see the result instantly:

Next, we need to set up a function in JavaScript that will be called by C++ in just a moment. We'll just create a simple function that pops up a simple message box and changes the Hello World! text to something else when called:

<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
<script>
function hello()
{
document.getElementById("myText").innerHTML =
"Something happened!";
alert("Good day sir, how are you?");
}
</script>
</head>
<body>
<p id="myText">Hello World!</p>
</body>
</html>

Note that I have added an ID to the Hello World! text so that we are able to find it and change its text. Once you're done, let's go to our Qt project again.

Let's proceed to add a push button to our program UI, and when the button is pressed, we want our Qt program to call the hello() function we just created in JavaScript. It's actually very easy to do that in Qt; you simply call the runJavaScript() function from the QWebEnginePage class, like so:

void MainWindow::on_pushButton_clicked()
{
webview->page()->runJavaScript("hello();");
}

The result is pretty astounding, as you can see from the following screenshot:

You can do a lot more than just change the text or call a message box. For example, you can start or stop an animation in an HTML canvas, show or hide an HTML element, trigger an Ajax event to retrieve information from a PHP script, and so on and so forth... endless possibilities!

Calling C++ functions from JavaScript

Next, let's take a look at how we can call C++ functions from JavaScript instead. For the sake of demonstration, I'll put a text label above the web view and we will change its text using a JavaScript function:

Normally, JavaScript can only work within the HTML environment and hence, is only able to alter HTML elements and not something outside the web view. However, Qt allows us to do just that by using the web channel module. So let's open up our project (.pro) file and add the web channel module to the project:

QT += core gui webengine webenginewidgets webchannel

After that, open up mainwindow.h and add in the QWebChannel header:

#include <QMainWindow>
#include <QWebEngineView>
#include <QWebChannel>

At the same time, we also declare a function called doSomething(), with a Q_INVOKABLE macro in front of it:

Q_INVOKABLE void doSomething();

The Q_INVOKABLE macro tells Qt to expose the function to the JavaScript engine, and thus the function can then be called from JavaScript (and QML, since QML is also based on JavaScript).

Then in mainwindow.cpp, we'll have to first create a QWebChannel object and register our main window as a JavaScript object. You can register any Qt object as a JavaScript object as long as it is derived from the QObject class.

Since we're going to call the doSomething() function from JavaScript, we must register the main window to the JavaScript engine. After that, we also need to set the QWebChannel object we just created as the web channel of our web view. The code looks like the following:

QWebChannel* channel = new QWebChannel(this);
channel->registerObject("mainwindow", this);
webview->page()->setWebChannel(channel);

Once you're done with that, let's define the doSomething() function. We're just going to do something simple—change the text label on our Qt GUI, and that's all:

void MainWindow::doSomething()
{
ui->label->setText("This text has been changed by javascript!");
}

We're done with the C++ code, let's open up our HTML file. There are couple of things we need to do to make this work. First, we need to include the qwebchannel.js script that is embedded in your Qt program by default, so you don't have to search for that file in your Qt directory. Add the following code in between the head tags:

<script type="text/javascript" src="qrc:///qtwebchannel/qwebchannel.js"></script>

Then, we create a QWebChannel object in JavaScript when the document is successfully being loaded by web view and link the mainwindow variable to the actual main window object from Qt (which we registered earlier in C++). This step must only be done after the web page has been loaded (through window.onload callback); otherwise, there might be problems creating the web channel:

var mainwindow;
window.onload = function()
{
new QWebChannel(Qt.webChannelTransport,function(channel)
{
mainwindow = channel.objects.mainwindow;
});
}

After that, we create a JavaScript function that calls the doSomething() function:

function myFunction()
{
mainwindow.doSomething();
}

Finally, add a button to the HTML body and make sure myFunction() is called when the button is pressed:

<body>
<p id="myText">Hello World!</p>
<button onclick="myFunction()">Do Something</button>
</body>

Build and run the program now and you should be able to get the following result:

You can do a lot of useful things using this method other than altering the properties of a Qt widget. For example, saving a file to the local hard disk, getting scanned data from a barcode scanner, and so on. There is no longer any barrier between native and web technology. However, do be extra aware of any possible security implications of this technique. As the old saying goes:

"With great power comes great responsibility."

Summary

In this chapter, we have learned how to create our own web browser and make it interact with the native code. Qt provides us with the web channel technology that makes Qt a very powerful platform for software development.

It takes advantage of both the power of Qt and the beauty of web technology, which means you can have a lot more options when it comes to development and not just be limited to Qt's methods. I'm really excited and can't wait to see what you can achieve with this!

Join us in the next chapter to learn how to create a map viewer similar to Google Maps, using Qt!

Map Viewer

User location and map display are two features that have become more common these days and have been used in various types of applications. They are commonly used for both backend analytics and frontend display purposes.

The map viewer can be used for navigation, nearby point-of-interest lookup, location-based services (such as calling for a taxi), and so on. You can use Qt to achieve most of it but you will require an advanced database system, if you're going for something more complex.

In the previous chapter, we learned how to embed a web browser into your application. In this chapter, we will try something more fun, which covers the following topics:

- Creating a map display

- Marker and shape display

- Obtaining a user's location

- Geo Routing Request

Let's proceed to create our own map viewer!

Map display

The Qt Location module provides developer access to geocoding and navigation information. It can also allow the user to do a place search for which the data needs to be retrieved, either from a server or from the user's device.

At the moment, Qt's map view does not support C++, only QML. This means that we can only use QML script to alter anything related to the visual—displaying a map, adding a marker, and so on; on the other hand, we can use the C++ classes provided by the module to obtain information from a database or from a service provider, before displaying it to the user via QML.

Just a quick note, QML (Qt Modeling Language) is a user interface markup language for Qt Quick applications. Since QML is powered by the JavaScript framework, its coding syntax is almost similar to the JavaScript. If you need an in-depth learning on QML and Qt Quick, please proceed to Chapter 14, Qt Quick and QML, as it is an entire chapter dedicated to it.

There are many tutorials out there that teach you how to create a fully fledged map viewer using Qt Quick and QML language, but there isn't a lot that teaches you how to combine C++ with QML. Let's get started!

Setting up the Qt location module

1. First, create a new Qt Widgets Application project.

2. After that, open up your project file (.pro) and add the following modules to your Qt project:

QT += core gui location qml quickwidgets

Besides the location module, we also added qml and quickwidgets modules, which are required by the map display widget in the next section. That is all we need to do for enabling the Qt Location module in our project. Next, we will proceed to add the map display widget to our project.

Creating a map display

Once you are ready, let's open up mainwindow.ui and remove the menuBar, toolBar, and statusBar as we don't need any of those in this project:

After that, drag a QQuickWidget from the widget box to the UI canvas. Then, click on the Lay Out Horizontally button located at the top of the canvas to add layout properties to it:

Then, set all the margin properties of the central widget to 0:

Next, we need to create a new file called mapview.qml by going to File | New File or Project.... After that, select Qt category and follow QML File (Qt Quick 2):

Once the QML file has been created, open it up and add the following code to include the location and positioning modules to this file so that we can use its functions later:

import QtQuick 2.0
import QtLocation 5.3
import QtPositioning 5.0

After that, we create a Plugin object and name it osm (Open Street Map), we then create a Map object and apply the plugin to its plugin property. We also set the starting coordinates to (40.7264175,-73.99735), which is somewhere in New York. Other than that, the default zoom level is set to 14, which is enough to have a good view of the city:

Item
{
Plugin
{
id: mapPlugin
name: "osm"
}

Map
{
id: map
anchors.fill: parent
plugin: mapPlugin
center: QtPositioning.coordinate(40.7264175,-73.99735)
zoomLevel: 14
}
}

Before we're able to display the map on our application, we must first create a resource file and add the QML file to it. This can be done by going to File | Create New File or Project.... Then, select Qt category and pick Qt Resource File.

Once the resource file has been created, add a prefix called qml, and add the QML file to the prefix, like so:

We can now open up mainwindow.ui and set the source property of the QQuickWidget to qrc:/qml/mapview.qml. You may also click the button behind the source property to select the QML file straight from the resources.

Once you're done, let's compile and run our project and see what we've got! You can try panning around and zooming in and out from the map using the mouse, too:

Even though we can achieve the same result by using the web view widget, it will make us write a ton of JavaScript code just to display a map like this. By using Qt Quick, we only need to write a few simple lines of QML code and that's it.

Marker and shape display

In the previous section, we successfully created a map display, but that is just the beginning of this project. We need to be able to display custom data in the form of markers or shapes layered on top of the map, so that the user can understand the data.

Displaying position markers on a map

If I tell you my favorite restaurant is located at (40.7802655, -74.108644), you won't be able to make sense of it. However, if those coordinates are being displayed on the map view in the form of a location marker, instantly, you will have an idea of where it is. Let's see how we can add position markers to our map view!

First of all, we need a marker image that should look something like this, or even better, design your own marker:

After that, we need to register this image to our project's resource file. Open up resource.qrc with Qt Creator and create a new prefix called images. Then, add the marker image to the newly created prefix. Do make sure that the image has a transparent background to look good on the map:

Next, open up mapview.qml and replace the code with the following:

Item
{
id: window

Plugin
{
id: mapPlugin
name: "osm"
}

Image
{
id: icon
source: "qrc:///images/map-marker-icon.png"
sourceSize.width: 50
sourceSize.height: 50
}

MapQuickItem
{
id: marker
anchorPoint.x: marker.width / 4
anchorPoint.y: marker.height
coordinate: QtPositioning.coordinate(40.7274175,-73.99835)

sourceItem: icon
}

Map
{
id: map
anchors.fill: parent
plugin: mapPlugin
center: QtPositioning.coordinate(40.7264175,-73.99735)
zoomLevel: 14

Component.onCompleted:
{
map.addMapItem(marker)
}
}
}

In the preceding code, we first added an image object that will be used as the marker's image. Since the original image is really huge, we have to resize it by setting the sourceSize property to 50x50. We must also set the anchor point of the marker image to the center-bottom of the image because that is where the tip of the marker is located.

After that, we create a MapQuickItem object that will be served as the marker itself. Set the marker image as the sourceItem of the MapQuickItem object, then add the marker to the map by calling map.addMapItem(). This function must be called after the map has been created and is ready to be displayed, which means we can only call it after the Component.onCompleted event has been triggered.

Now that we're done with the code, let's compile and look at the result:

Even though it's now looking all good, we don't want to hardcode the marker in QML. Imagine adding hundreds of markers to the map, it's simply impossible to manually add each marker using a distinct set of code.

In order to create a function that allows us to dynamically create position markers, we need to first separate the marker QML code from mapview.qml to a new QML file. Let's create a new QML file called marker.qml and add it to the resource file:

Next, remove both the MapQuickItem and Image objects from mapview.qml and move it to marker.qml:

import QtQuick 2.0
import QtLocation 5.3

MapQuickItem
{
id: marker
anchorPoint.x: marker.width / 4
anchorPoint.y: marker.height
sourceItem: Image
{
id: icon
source: "qrc:///images/map-marker-icon.png"
sourceSize.width: 50
sourceSize.height: 50
}
}

As you can see from the preceding code, I have merged the Image object with the MapQuickItem object. The coordinate property has also been removed as we will only set it when putting the marker on the map.

Now, open up mapview.qml again, and add this function to the Item object:

Item
{
id: window

Plugin
{
id: mapPlugin
name: "osm"
}

function addMarker(latitude, longitude)
{
var component = Qt.createComponent("qrc:///qml/marker.qml")
var item = component.createObject(window, { coordinate:
QtPositioning.coordinate(latitude, longitude) })
map.addMapItem(item)
}

From the preceding code, we first created a component by loading the marker.qml file. Then, we created an object/item from the component by calling createObject(). In the createObject() function, we made the window object as its parent and set its position to the coordinate supplied by the addMarker() function. Finally, we added the item to the map for it to be rendered.

Whenever we want to create a new position marker, we'll just have to call this addMarker() function. To demonstrate this, let's create three different markers by calling addMarker() three times:

Map
{
id: map
anchors.fill: parent
plugin: mapPlugin
center: QtPositioning.coordinate(40.7264175,-73.99735)
zoomLevel: 14

Component.onCompleted:
{
addMarker(40.7274175,-73.99835)
addMarker(40.7276432,-73.98602)
addMarker(40.7272175,-73.98935)
}
}

Build and run the project again, and you should be able to see something like this:

We can go even further by adding a text label to each of the markers. To do that, first open up marker.qml, then add another module called QtQuick.Controls:

import QtQuick 2.0
import QtQuick.Controls 2.0
import QtLocation 5.3

After that, add a custom property to the MapQuickItem object called labelText:

MapQuickItem
{
id: marker
anchorPoint.x: marker.width / 4
anchorPoint.y: marker.height
property string labelText

Once you're done, change its sourceItem property into this:

sourceItem: Item
{
Image
{
id: icon
source: "qrc:///images/map-marker-icon.png"
sourceSize.width: 50
sourceSize.height: 50
}

Rectangle
{
id: tag
anchors.centerIn: label
width: label.width + 4
height: label.height + 2
color: "black"
}

Label
{
id: label
anchors.centerIn: parent
anchors.horizontalCenterOffset: 20
anchors.verticalCenterOffset: -12
font.pixelSize: 16
text: labelText
color: "white"
}
}

From the preceding code, we created an Item object to group multiple objects together. Then, we created a Rectangle object to serve as the label background and a Label object for the text. The text property of the Label object will get linked to the labelText property of the MapQuickItem object. We can add another input to the addMarker() function for setting the labelText property, like so:

function addMarker(name, latitude, longitude)
{
var component = Qt.createComponent("qrc:///qml/marker.qml")
var item = component.createObject(window, { coordinate: QtPositioning.coordinate(latitude, longitude), labelText: name })
map.addMapItem(item)
}

Therefore, when we create the markers, we can call the addMarker() function like this:

Component.onCompleted:
{
addMarker("Restaurant", 40.7274175,-73.99835)
addMarker("My Home", 40.7276432,-73.98602)
addMarker("School", 40.7272175,-73.98935)
}

Build and run the project again and you should see this:

Pretty awesome isn't it? However, we're not done yet. Since we're most likely using C++ to obtain data from the database through Qt's SQL module, we need to find a way to call the QML function from C++.

To achieve that, let's comment out the three addMarker() functions in mapview.qml and open up mainwindow.h and the following headers:

#include <QQuickItem>
#include <QQuickView>

After that, open up mainwindow.cpp and call the QMetaObject::invokeMethod() function, like this:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

QObject* target = qobject_cast<QObject*>(ui->quickWidget->rootObject());
QString functionName = "addMarker";

QMetaObject::invokeMethod(target, functionName, Qt::AutoConnection, Q_ARG(QVariant, "Testing"), Q_ARG(QVariant, 40.7274175), Q_ARG(QVariant, -73.99835));
}

The preceding code might seem complex, but it's actually really simple if we dissect it and analyze each of its arguments. The first argument of the preceding function is the object that we want to call the function from, and in this case, it is the root object (the Item object in mapview.qml) of the map view widget. Next, we want to tell which function name we want to call, and it is the addMarker() function. After that, the third argument is the connection type used by the signal and slot system to invoke this method. For this, we'll just let it be the default setting, which is Qt::AutoConnection. The rest are the arguments that are needed by the addMarker() function. We used the Q_ARG macro for indicating the type and value of the data.

Finally, build and run the application again. You will see a marker with the label has been added to the map, but this time, it's called from our C++ code instead of QML:

Displaying shapes on a map

Besides adding markers to the map, we can also draw different types of shapes on the map to indicate an area of interest or serve as geofences, which give out warnings whenever a target is entering or leaving the area covered by the shape. A geofence is a polygonal shape that defines an area of interest or virtual geographic boundary on a map for location-based services. Usually, geofences are used to trigger an alarm when a device is entering and/or exiting a geofence. A good example of using a geofence is when you need a shopping reminder, you can draw a geofence around the supermarket and attach a shopping list along with the geofence. When you (and your phone) are entering the area of the geofence, you will get a notification on your phone that reminds you what to buy. Wouldn't that be great?

For more information about geofences, please visit: https://en.wikipedia.org/wiki/Geo-fence

We won't be creating a functional geofence in this chapter as it is quite an advanced topic, and it usually runs as a server-side service for checking and triggering an alarm. We will only use Qt to draw the shape and display it visually on the screen.

To draw shapes on the map view widget, we'll create a few more QML files for each type of shape and add them to the program's resources:

For each of the newly created QML files, we'll do something similar to the position marker. For circle.qml, it looks like this:

import QtQuick 2.0
import QtLocation 5.3

MapCircle
{
property int borderWidth
border.width: borderWidth
}

We only declare borderWidth in this file because we can directly set the other properties later, when calling the createCircle() function. The same goes for rectangle.qml:

import QtQuick 2.0
import QtLocation 5.3

MapRectangle
{
property int borderWidth
border.width: borderWidth
}

Repeat a similar step for polygon.qml:

import QtQuick 2.0
import QtLocation 5.3

MapPolygon
{
property int borderWidth
border.width: borderWidth
}

You can set other properties if you want, but for the sake of demonstration, we only change a few of the properties such as color, shape, and border width. Once you're done, let's open up mapview.qml and define a few functions for adding the shapes:

Item
{
id: window

Plugin
{
id: mapPlugin
name: "osm"
}

function addCircle(latitude, longitude, radius, color, borderWidth)
{
var component = Qt.createComponent("qrc:///qml/circle.qml")
var item = component.createObject(window, { center:
QtPositioning.coordinate(latitude, longitude), radius: radius,
color: color, borderWidth: borderWidth })
map.addMapItem(item)
}

function addRectangle(startLat, startLong, endLat, endLong, color,
borderWidth)
{
var component = Qt.createComponent("qrc:///qml/rectangle.qml")
var item = component.createObject(window, { topLeft:
QtPositioning.coordinate(startLat, startLong), bottomRight:
QtPositioning.coordinate(endLat, endLong), color: color,
borderWidth: borderWidth })
map.addMapItem(item)
}

function addPolygon(path, color, borderWidth)
{
var component = Qt.createComponent("qrc:///qml/polygon.qml")
var item = component.createObject(window, { path: path, color:
color, borderWidth: borderWidth })
map.addMapItem(item)
}

These functions are very similar to the addMarker() function, except it takes in different arguments that are later passed to the createObject() function. After that, let's try and create the shapes using the preceding function:

addCircle(40.7274175,-73.99835, 250, "green", 3);
addRectangle(40.7274175,-73.99835, 40.7376432, -73.98602, "red", 2)
var path = [{ latitude: 40.7324281, longitude: -73.97602 },
{ latitude: 40.7396432, longitude: -73.98666 },
{ latitude: 40.7273266, longitude: -73.99835 },
{ latitude: 40.7264281, longitude: -73.98602 }];
addPolygon(path, "blue", 3);

The following are the shapes created using the functions we have just defined. I have called each of the functions separately to demonstrate its outcome, hence the three different windows:

Obtaining a user's location

Qt provides us with a set of functions to retrieve a user's location information, but it will only work if the user's device supports geopositioning. This should work on all modern smartphones and might work on some of the modern computers as well.

To obtain the user's location using the Qt Location module, first let's open up mainwindow.h and add the following header files:

#include <QDebug>
#include <QGeoPositionInfo>
#include <QGeoPositionInfoSource>

After that, declare the following slot function in the same file:

private slots:
void positionUpdated(const QGeoPositionInfo &info);

Right after that, open up mainwindow.cpp and add the following code to the place where you want it to start getting the user's location. For demonstration purposes, I'll just call it within the MainWindow constructor:

QGeoPositionInfoSource *source = QGeoPositionInfoSource::createDefaultSource(this);
if (source)
{
connect(source, &QGeoPositionInfoSource::positionUpdated,
this, &MainWindow::positionUpdated);
source->startUpdates();
}

Then, implement the positionUpdated() function we declared earlier, like this:

void MainWindow::positionUpdated(const QGeoPositionInfo &info)
{
qDebug() << "Position updated:" << info;
}

If you build and run the application now, you may or may not get any location information, depending on the device you use to run the test. If you get debug messages like these:

serialnmea: No serial ports found
Failed to create Geoclue client interface. Geoclue error: org.freedesktop.DBus.Error.Disconnected

Then you probably need to find some other devices for the test. Otherwise, you may get a result similar to this:

Position updated: QGeoPositionInfo(QDateTime(2018-02-22 19:13:05.000 EST Qt::TimeSpec(LocalTime)), QGeoCoordinate(45.3333, -75.9))

I will leave you an assignment here which you can try and do by making use of the functions that we have created thus far. Since you can now obtain the coordinates of your location, try and further enhance your application by adding a marker to the map display to show where you are currently located. That should be fun to work with!

Geo Routing Request

There is another important feature called Geo Routing Request, which is a set of functions that help you plot out the route (often the shortest route) from point A to point B. This feature requires a service provider; in this case, we will be using Open Street Map (OSM) as it is completely free.

Do note that OSM is an online collaborative project, which means that if no one from your area contributed the route data to the OSM server, then you won't be able to get an accurate result. Optionally, you can also use paid services such as Mapbox or ESRI.

Let's see how we can implement Geo Routing Request in Qt! First, include the following headers to our mainwindow.h file:

#include <QGeoServiceProvider>
#include <QGeoRoutingManager>
#include <QGeoRouteRequest>
#include <QGeoRouteReply>

After that, add two slot functions to MainWindow class, namely, routeCalculated() and routeError():

private slots:
void positionUpdated(const QGeoPositionInfo &info);
void routeCalculated(QGeoRouteReply *reply);
void routeError(QGeoRouteReply *reply, QGeoRouteReply::Error error, const QString &errorString);

Once you're done, open up mainwindow.cpp and create a service provider object in the MainWindow constructor method. We will be using the OSM service so we'll place the acronym "osm" when initiating the QGeoServiceProvider class:

QGeoServiceProvider* serviceProvider = new QGeoServiceProvider("osm");

Right after that, we'll get the pointer of the routing manager from the service provider object we just created:

QGeoRoutingManager* routingManager = serviceProvider->routingManager();

Then, connect the finished() signal and error() signal from the routing manager with the slot functions we just defined:

connect(routingManager, &QGeoRoutingManager::finished, this, &MainWindow::routeCalculated);
connect(routingManager, &QGeoRoutingManager::error, this, &MainWindow::routeError);

These slot functions will be triggered when there is a reply from the service provider upon a successful request, or when the request is failed and returned with an error message instead. The routeCalculated() slot function looks something like this:

void MainWindow::routeCalculated(QGeoRouteReply *reply)
{
qDebug() << "Route Calculated";
if (reply->routes().size() != 0)
{
// There could be more than 1 path
// But we only get the first route
QGeoRoute route = reply->routes().at(0);
qDebug() << route.path();
}
reply->deleteLater();
}

As you can see, the QGeoRouteReply pointer contains route information sent by the service provider upon a successful request. Sometimes it comes with more than one route, so in the example, we just obtain the first route and display it through Qt's application output window. Alternatively, you can use these coordinates to draw a path or animate your marker along the route.

As for routeError() slot function, we'll just output the error string sent by the service provider:

void MainWindow::routeError(QGeoRouteReply *reply, QGeoRouteReply::Error error, const QString &errorString)
{
qDebug() << "Route Error" << errorString;
reply->deleteLater();
}

Once you're done with that, let's initiate a Geo Routing Request in the MainWindow constructor method and send it to the service provider:

QGeoRouteRequest request(QGeoCoordinate(40.675895,-73.9562151), QGeoCoordinate(40.6833154,-73.987715));
routingManager->calculateRoute(request);

Build and run the project now and you should see results like the following:

Here comes another challenging assignment for you—try to put all these coordinates into an array and create an addLine() function that takes in the array and draws a series of straight lines that represent the route described by the Geo Routing service.

Geo Routing has been one of the most important features ever since GPS navigator systems were invented. Hopefully, you will be able to create something useful after going through the tutorial!

Summary

In this chapter, we have learned how to create our own map view similar to Google Maps. We have learned how to create a map display, placing markers and shapes on the map, and finally finding a user's location. Do note that you can also use the web view and call Google's JavaScript mapping API to create a similar map display. However, using QML is much simpler, lightweight (we don't have to load the entire web engine module just to use the map), works very well on mobile and touch screens, and it can also be easily ported to other map services. Hopefully, you can make use of this knowledge and create something really awesome and useful.

In the next chapter, we will look into how to display information using graphical items. Let's move on!

Graphics View

In the previous chapter, we learned about the importance of visual presentation for the user by displaying coordinate data on a map. In this chapter, we will further explore the possibility of graphics data representation using Qt's Graphics View framework.

In this chapter, we will cover the following topics:

- Graphics View framework

- Moveable graphics items

- Creating an organization chart

At the end of this chapter, you will be able to create an organization chart display using C++ and Qt's API. Let's get started!

Graphics View framework

The Graphics View framework is part of the widgets module in Qt so it is already supported by default, unless you're running Qt console application instead, which does not need the widgets module.

The Graphics View view in Qt works pretty much like a whiteboard, where you can draw anything on it using C/C++ code, such as drawing shapes, lines, text, and even images. This chapter may be a little hard to follow for beginners but it will definitely be a fun project to work with. Let's get started!

Setting up a new project

First, create a new Qt Widgets Application project. After that, open mainwindow.ui and drag and drop the Graphics View widget onto the main window, like this:

Then, create a layout for the graphics view by clicking on the Lay Out Vertically button at the top of the canvas. After that, open up mainwindow.h and add the following headers and variables:

#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QGraphicsEllipseItem>
#include <QGraphicsTextItem>
#include <QBrush>
#include <QPen>

private:
Ui::MainWindow *ui;
QGraphicsScene* scene;

After that, open mainwindow.cpp. Once it is opened, add the following code:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);

QBrush greenBrush(Qt::green);
QBrush blueBrush(Qt::blue);
QPen pen(Qt::black);
pen.setWidth(2);

QGraphicsRectItem* rectangle = scene->addRect(80, 0, 80, 80, pen, greenBrush);
QGraphicsEllipseItem* ellipse = scene->addEllipse(0, -80, 200, 60, pen, blueBrush);
QGraphicsTextItem* text = scene->addText("Hello World!", QFont("Times", 25));
}

Build and run the program now, and you should see something like this:

The code is a bit long so let me explain to you what it does and how it draws the graphics onto the screen.

As I said earlier, the Graphics View widget is like a canvas or whiteboard that allows you to draw anything you want on it. However, we also need something called Graphics Scene, which is essentially a scene graph that stores all the graphical components in a parent–child hierarchy before displaying them on the Graphics View, accordingly. Scene graph hierarchy is something that the image that appears in the previous screenshot, where each object could have a parent or children that link together:

In the preceding code, we first created a QGraphicsScene object and set it as the Graphics Scene for our Graphics View widget:

scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);

In this example, however, we don't have to link the graphics items together so we'll just create them independently, like so:

QBrush greenBrush(Qt::green);
...
QGraphicsTextItem* text = scene->addText("Hello World!", QFont("Times", 25));

The QPen and QBrush classes are used to define the rendering style of these graphics items. QBrush is usually for defining the background color and pattern for the item, while QPen normally affects the outline of the item.

Qt provides many types of graphics items for the most common shapes, including:

- QGraphicsEllipseItem – ellipse item

- QGraphicsLineItem – line item

- QGraphicsPathItem – arbitrary path item

- QGraphicsPixmapItem – pixmap item

- QGraphicsPolygonItem – polygon item

- QGraphicsRectItem – rectangular item

- QGraphicsSimpleTextItem – simple text label item

- QGraphicsTextItem – advanced formatted text item

For more information, please visit this link: http://doc.qt.io/archives/qt-5.8/qgraphicsitem.html#details.

Movable graphics items

In the previous example, we have successfully drawn some simple shapes and text onto the Graphics View widget. However, these graphics items are not interactive and thus don't suit our purpose. What we want is an interactive organization chart where the user can move the items around using mouse. It is actually really easy to make these items movable under Qt; let's see how we can do that by continuing our previous project.

First, make sure you don't change the default interactive property of our Graphics View widget, which is set to enabled (checkbox is checked):

After that, add the following code below each of the graphics items we just created in the previous Hello World example:

QGraphicsRectItem* rectangle = scene->addRect(80, 0, 80, 80, pen, greenBrush);
rectangle->setFlag(QGraphicsItem::ItemIsMovable);
rectangle->setFlag(QGraphicsItem::ItemIsSelectable);

QGraphicsEllipseItem* ellipse = scene->addEllipse(0, -80, 200, 60, pen, blueBrush);
ellipse->setFlag(QGraphicsItem::ItemIsMovable);
ellipse->setFlag(QGraphicsItem::ItemIsSelectable);

QGraphicsTextItem* text = scene->addText("Hello World!", QFont("Times", 25));
text->setFlag(QGraphicsItem::ItemIsMovable);
text->setFlag(QGraphicsItem::ItemIsSelectable);

Build and run the program again, and this time you should be able to select and move the items around the Graphics View. Do note that ItemIsMovable and ItemIsSelectable both give you a different behavior—the former flag will make the item movable by mouse, and the latter makes the item selectable, which typically gives it a visual indication using dotted outline when selected. Each of the flags works independently and will not affect the other.

We can test out the effect of ItemIsSelectable flag by using the signal and slot mechanism in Qt. Let's go back to our code and add the following line:

ui->setupUi(this);
scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);
connect(scene, &QGraphicsScene::selectionChanged, this, &MainWindow::selectionChanged);

The selectionChanged() signal will be triggered whenever you selected an item on the Graphics View widget and the selectionChanged() slot function under our MainWindow class will then be called (which we need to write). Let's open up mainwindow.h and add in another header for displaying debug messages:

#include <QDebug>

Then, we declare the slot function, like this:

private:
Ui::MainWindow *ui;

public slots:
void selectionChanged();

After that open mainwindow.cpp and define the slot function, like this:

void MainWindow::selectionChanged()
{
qDebug() << "Item selected";
}

Now try and run the program again; you should see a line of debug messages that say Item selection which appears whenever a graphics item has been clicked. It's really simple, isn't it?

As for the ItemIsMovable flag, we won't be able to test it using the signal and slot method. This is because all classes inherited from QGraphicsItem class are not inherited from the QObject class, and therefore the signal and slot mechanism doesn't work on these classes. This is intentionally done by Qt developers to make it lightweight, which improves the performance, especially when rendering thousands of items on the screen.

Even though signal and slot is not an option for this, we can still use the event system, which requires an override to the itemChange() virtual function, which I will demonstrate in the next section.

Creating an organization chart

Let's proceed to learn how to create an organization chart using Graphics View. An organization chart is a diagram that shows the structure of an organization and the relationship hierarchy of its employee positions. It is easy to understand a company's structure by using graphical representation; therefore it's best to use Graphics View instead of, say, a table.

This time, we need to create our own classes for the graphics items so that we can make use of Qt's event system, as well as have more control of how it's grouped and displayed.

First, create a C/C++ class by going to File | New File or Project:

Next, name our class as profileBox before clicking the Next and Finish button:

After that, open mainwindow.h and add in these headers:

#include <QWidget>
#include <QDebug>
#include <QBrush>
#include <QPen>
#include <QFont>
#include <QGraphicsScene>
#include <QGraphicsItemGroup>
#include <QGraphicsItem>
#include <QGraphicsRectItem>
#include <QGraphicsTextItem>
#include <QGraphicsPixmapItem>

Then, open profilebox.h and make our profileBox class inherit QGraphicsItemGroup instead:

class profileBox : public QGraphicsItemGroup
{
public:
explicit profileBox(QGraphicsItem* parent = nullptr);

After that, open profilebox.cpp and at the constructor of the class, set up QBrush, QPen and QFont, which will be used for rendering in a moment:

profileBox::profileBox(QGraphicsItem *parent) : QGraphicsItemGroup(parent)
{
QBrush brush(Qt::white);
QPen pen(Qt::black);
QFont font;
font.setFamily("Arial");
font.setPointSize(12);
}

After that, also in the constructor, create a QGraphicsRectItem, QGraphicsTextItem and a QGraphicsPixmapItem:

QGraphicsRectItem* rectangle = new QGraphicsRectItem();
rectangle->setRect(0, 0, 90, 100);
rectangle->setBrush(brush);
rectangle->setPen(pen);

nameTag = new QGraphicsTextItem();
nameTag->setPlainText("");
nameTag->setFont(font);

QGraphicsPixmapItem* picture = new QGraphicsPixmapItem();
QPixmap pixmap(":/images/person-icon-blue.png");
picture->setPixmap(pixmap);
picture->setPos(15, 30);

Then, add these items to the group, which is the current class, since this class is inherited from the QGraphicsItemGroup class:

this->addToGroup(rectangle);
this->addToGroup(nameTag);
this->addToGroup(picture);

Finally, set three flags for the current class, which are ItemIsMovable, ItemIsSelectable and ItemSendsScenePositionChanges:

this->setFlag(QGraphicsItem::ItemIsMovable);
this->setFlag(QGraphicsItem::ItemIsSelectable);
this->setFlag(QGraphicsItem::ItemSendsScenePositionChanges);

These flags are very important because they are all disabled by default for performance reasons. We have covered both ItemIsMovable and ItemIsSelectable in the previous section, while ItemSendsPositionChanges is something new. This flag makes the graphics item notify Graphics Scene when it's being moved by the user, hence the name.

Next, create another function called init() for setting up the employee profile. For the sake of simplicity, we only set the employee name, however, you can do more if you wish, such as setting a different background color based on the rank, or changing their profile picture:

void profileBox::init(QString name, MainWindow *window, QGraphicsScene* scene)
{
nameTag->setPlainText(name);
mainWindow = window;
scene->addItem(this);
}

Do notice that we also set the main window and Graphics Scene pointers here so that we can use them later on. We must add the QGraphicsItem to a scene before it will render on screen. In this case, we group all the graphics items into a QGraphicsItemGroup so we only need to add the group to the scene instead of an individual item.

Do note that you must do a forward declaration for the MainWindow class in profilebox.h after #include "mainwindow.h" to avoid the error that says recursive header inclusion. At the same time, we also placed the MainWindow and QGraphicsTextItem pointers in profilebox.h so that we can call them later:

#include "mainwindow.h"

class MainWindow;

class profileBox : public QGraphicsItemGroup
{
public:
explicit profileBox(QGraphicsItem* parent = nullptr);
void init(QString name, MainWindow* window, QGraphicsScene* scene);

private:
MainWindow* mainWindow;
QGraphicsTextItem* nameTag;

You will also notice that I have used an icon in the QGraphicsPixmapItem as a decorative icon:

This icon is a PNG image that is stored within the resource file. You can get this image from our sample project files on our GitHub page: http://github.com/PacktPublishing/Hands-On-GUI-Programming-with-C-QT5

Let's create a resource file for your project. Go to File | New File or Project and select the Qt Resource File option under Qt category:

After you have created an empty resource file, add a new prefix by going to Add | AddPrefix. We will just call this prefix images:

Then, select the newly created images prefix and click Add | Add Files. Add the icon image to your resource file and save. You have now successfully added the image to your project.

If your prefix name or filename is different than the prefix name or filename in this book, you may right-click on your image in the resource file and select Copy Resource Path to Clipboard and replace the one in the code with your path.

After that, open mainwindow.h and add in:

#include "profilebox.h"

Then, open mainwindow.cpp and add the following code to create a profile box manually:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);

connect(scene, &QGraphicsScene::selectionChanged, this, &MainWindow::selectionChanged);

profileBox* box = new profileBox();
box->init("John Doe", this, scene);
}

Build and run the project now and you should see something like this:

Looks neat; but we're far from done. There are a few things left to be done—we must allow the user to add or delete profile boxes with a user interface, and not by using code. At the same time, we also need to add lines that connect different profile boxes to showcase the relationship between different employees and their position within the company.

Let's start with the easy part. Open mainwindow.ui again and add a push button to the bottom of the Graphics View widget and name it addButton:

Then, right-click on the push button and select Go to slot... After that, select the clicked option and click Ok. A new slot function will be created for you automatically, called on_addButton_clicked(). Add the following code to allow the user to create a profile box when they click the Add button:

void MainWindow::on_addButton_clicked()
{
bool ok;
QString name = QInputDialog::getText(this, tr("Employee Name"),
tr("Please insert employee's full name here:"), QLineEdit::Normal,
"John Doe", &ok);
if (ok && !name.isEmpty())
{
profileBox* box = new profileBox();
box->init(name, this, scene);
}
}

Instead of creating each profile box using code, users can now easily create any number of profile boxes they want by clicking the Add button. A message box will also appear and let the user type in the employee name before creating the profile box:

Next, we'll create another class called profileLine. This time, we will make this class inherit QGraphicsLineItem. The profileline.h basically looks like this:

#include <QWidget>
#include <QGraphicsItem>
#include <QPen>

class profileLine : public QGraphicsLineItem
{
public:
profileLine(QGraphicsItem* parent = nullptr);
void initLine(QGraphicsItem* start, QGraphicsItem* end);
void updateLine();

QGraphicsItem* startBox;
QGraphicsItem* endBox;

private:
};

Similar to profileBox class, we also create an init function for profileLine class, called the initLine() function. This function takes in two QGraphicsItem objects as the starting point and ending point for rendering the line. Besides that, we also create an updateLine() function to redraw the line whenever the profile boxes move.

Next, open profileline.cpp and add the following code to the constructor:

profileLine::profileLine(QGraphicsItem *parent) : QGraphicsLineItem(parent)
{
QPen pen(Qt::black);
pen.setWidth(2);
this->setPen(pen);

this->setZValue(-999);
}

We used QPen to set the color of the line to be black and its width to be 2. After that, we also set the Zvalue of the line to be -999 so that it will always remain at the back of the profile boxes.

After that, add the following code to our initLine() function so that it looks something like this:

void profileLine::initLine(QGraphicsItem* start, QGraphicsItem* end)
{
startBox = start;
endBox = end;

updateLine();
}

What it does is basically set the boxes for it to position its starting point and ending point. After that, call updateLine() function to render the line.

Finally, the updateLine() function looks like this:

void profileLine::updateLine()
{
if (startBox != NULL && endBox != NULL)
{
this->setLine(startBox->pos().x() + startBox->boundingRect().width() / 2, startBox->pos().y() + startBox->boundingRect().height() / 2, endBox->pos().x() + endBox->boundingRect().width() / 2, endBox->pos().y() + endBox->boundingRect().height() / 2);
}
}

The preceding code looks a little complicated, but it's really simple if I put it this way:

this->setLine(x1, y1, x2, y2);

The values x1 and y1 are basically the center position of the first profile box while x2 and y2 are the center position of the second profile box. Since the position value we get from calling pos() starts from the top-left corner, we must get the bounding size of the profile box and divide it by two to get its center position. Then, add that value to the top-left corner position to offset it to the center.

Once you're done, let's open mainwindow.cpp again and add the following code to the on_addButton_clicked() function:

void MainWindow::on_addButton_clicked()
{
bool ok;
QString name = QInputDialog::getText(this, tr("Employee Name"), tr("Please insert employee's full name here:"), QLineEdit::Normal, "John Doe", &ok);
if (ok && !name.isEmpty())
{
profileBox* box = new profileBox();
box->init(name, this, scene);

if (scene->selectedItems().size() > 0)
{
profileLine* line = new profileLine();
line->initLine(box, scene->selectedItems().at(0));
scene->addItem(line);

lines.push_back(line);
}
}
}

In the preceding code, we check whether there is any profile box selected by the user. If there is none, we don't have to create any line. Otherwise, create a new profileLine object and set the newly created profile box and the currently selected profile box as the startBox and endBox properties.

After that, add the line to our Graphics Scene so that it will appear on the screen. Lastly, store this profileLine object to a QList array so that we can use it later. The array declaration looks like this in mainwindow.h:

private:
Ui::MainWindow *ui;
QGraphicsScene* scene;
QList<profileLine*> lines;

Build and run the project now. You should be able to see the line appear when you created the second profile box by clicking on the Add button, entering a name, and selecting OK while the first box remains selected. However, you may notice a problem whenever you move the profile box away from its original position—the lines simply won't update themselves!:

That is the main reason we put the lines into a QList array, so that we can update these lines whenever a profile box has been moved by the user.

To do that, first, we need to override the virtual function in the profileBox class called itemChanged(). Let's open profilebox.h and add the following line of code:

class profileBox : public QGraphicsItemGroup
{
public:
explicit profileBox(QGraphicsItem* parent = nullptr);
void init(QString name, MainWindow* window, QGraphicsScene* scene);
QVariant itemChange(GraphicsItemChange change, const QVariant
&value) override;

Then, open profilebox.cpp and add the code for itemChanged():

QVariant profileBox::itemChange(GraphicsItemChange change, const QVariant &value)
{
if (change == QGraphicsItem::ItemPositionChange)
{
qDebug() << "Item moved";

mainWindow->updateLines();
}

return QGraphicsItem::itemChange(change, value);
}

The itemChanged() function is a virtual function in QGraphicsItem class which will automatically be called by Qt's event system when something has changed in the graphics item, be it position change, visibility change, parent change, selection change, and so on.

Therefore, all we need to do is to override the function and add in our own custom behavior to the function. In the preceding sample code, all we did was to call the updateLines() function in our main window class.

Next, open mainwindow.cpp and define the updateLines() function. As the function name implies, what you're going to do in this function is to loop through all the profile line objects stored in the lines array and update every single one of them, like so:

void MainWindow::updateLines()
{
if (lines.size() > 0)
{
for (int i = 0; i < lines.size(); i++)
{
lines.at(i)->updateLine();
}
}
}

Once you're done, build and run the project again. This time, you should be able to create an organization chart, such as the following:

This is just a simpler version that shows you how you can make use of Qt's powerful Graphics View system to display graphical representation of a set of data that can be easily understood by an average Joe.

One last thing before it's done–we have yet to cover how to delete the profile box yet. It's actually pretty simple, let's open mainwindow.h and add the keyReleaseEvent() function, which looks like this:

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

void updateLines();
void keyReleaseEvent(QKeyEvent* event);

This virtual function will also get called by Qt's event system automatically when a keyboard button is being pressed and released. The content of the function looks like this in mainwindow.cpp:

void MainWindow::keyReleaseEvent(QKeyEvent* event)
{
qDebug() << "Key pressed: " + event->text();

if (event->key() == Qt::Key_Delete)
{
if (scene->selectedItems().size() > 0)
{
QGraphicsItem* item = scene->selectedItems().at(0);
scene->removeItem(item);

for (int i = lines.size() - 1; i >= 0; i--)
{
profileLine* line = lines.at(i);

if (line->startBox == item || line->endBox ==
item)
{
lines.removeAt(i);
scene->removeItem(line);
delete line;
}
}
delete item;
}
}
}

What we did in this function is first to detect the keyboard button that's being pressed by the user. If the button is Qt::Key_Delete (delete button), then we'll check if the user has selected any profile box by checking whether scene->selectedItems().size() is empty. If the user has indeed selected a profile box, then remove that item from the Graphics Scene. After that, loop through the lines array and check whether any profile line has connected to the profile box that has been deleted. Remove any lines that are connected to the profile box from the scene and we're done:

This screenshot shows the result of deleting the Jane Smith profile box from the organization chart. Notice that the lines connecting the profile box have been correctly removed. That's it for this chapter; I hope you found this interesting and will perhaps go on to create something even better than this!

Summary

In this chapter, we have learned how to create an application using Qt that allows the user to easily create and edit an organization chart. We have learned about classes such as QGraphicsScene, QGrapicsItem, QGraphicsTextItem, QGraphicsPixmapItem and so on that help us to create an interactive organization chart in a short period of time. In the upcoming chapter, we will learn how to capture images using our webcam!

The Camera Module

After working your way through so many chapters with increasing difficulty, let's try out something simpler and more fun for this chapter instead! We will learn how to access our camera through Qt's multimedia module and take photos using it.

In this chapter, we will cover the following topics:

- The Qt multimedia module

- Connecting to the camera

- Capturing a camera image to file

- Recording a camera video to file

You can use this to create a video conference app, a security camera system, and more. Let's get started!

The Qt multimedia module

The multimedia module in Qt is the module that handles a platform's multimedia capabilities, such as media playback and the use of camera and radio devices. This module covers a wide range of topics, but we will just focus on the camera for this chapter.

Setting up a new project

First, create a new Qt Widgets Application project.

Then, the first thing we need to do is to open up the project file (.pro) and add two keywords—multimedia and multimediawidgets:

QT += core gui multimedia multimediawidgets

By detecting these keywords in the project file, Qt will include the multimedia module and all the widgets that are related to multimedia into your project when it compiles. The multimedia module includes four major components which are listed as follows:

- Audio

- Video

- Camera

- Radio

Each component includes a range of classes that provide respective functionality. By using this module, you no longer have to implement low-level, platform-specific code yourself. Let Qt do the job for you. It's really that easy.

After you have finished adding the multimedia module, let's open mainwindow.ui and drag and drop a Horizontal Layout on to the main window, shown as follows:

Then, add a Label, Combo Box (name it deviceSelection), and a Push Button into the Horizontal Layout we just added in the previous step. After that, add a Horizontal Spacer between the combo box and a push button to push them apart from each other. Once you're done, select the central widget and click on the Layout Vertically button located above the workspace.

Then, add another Horizontal Layout to the bottom of the previous horizontal layout and right-click on it and select Morph into | QFrame. After that, set its sizePolicy (Horizontal Policy and Vertical Policy) settings to Expanding. Refer the following screenshot:

Your program's user interface should look something like this by now:

The reason we convert the layout to a frame is so that we can set the sizePolicy (both Horizontal policy and Vertical policy) to Expanding. However, if we just add a Frame widget (which is essentially a QFrame) from the widget box, we don't get the layout component on it which is needed for attaching the viewfinder later.

Next, right click on the QFrame again and select Change styleSheet. A window will pop up for setting the style sheet of that widget. Add the following style sheet code to make the background black:

This step is optional; we made its background black just to indicate the viewfinder's location. Once this is done, let's put another Horizontal Layout above the QFrame, such as the following:

After that, add two Push Buttons to the Horizontal Layout and a Horizontal Spacer to keep them aligned to the right:

That's it; we have finished setting up our project with the multimedia module and laid out the user interface nicely for our next sections.

Connecting to the camera

Here comes the most exciting part. We are going to learn how to access our camera(s) using Qt's multimedia module! First, open mainwindow.h and add the following headers:

#include <QMainWindow>
#include <QDebug>
#include <QCameraInfo>
#include <QCamera>
#include <QCameraViewfinder>
#include <QCameraImageCapture>
#include <QMediaRecorder>
#include <QUrl>

Next, add the following variable, as shown here:

private:
Ui::MainWindow *ui;
QCamera* camera;
QCameraViewfinder* viewfinder;
bool connected;

Then, open up mainwindow.cpp and add the following code to the class constructor to initiate the QCamera object. We then use the QCameraInfo class to retrieve a list of connected cameras and fill in that information in the combo box widget:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

connected = false;
camera = new QCamera();

qDebug() << "Number of cameras found:" << QCameraInfo::availableCameras().count();

QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
foreach (const QCameraInfo &cameraInfo, cameras)
{
qDebug() << "Camera info:" << cameraInfo.deviceName() <<
cameraInfo.description() << cameraInfo.position();

ui->deviceSelection->addItem(cameraInfo.description());
}
}

Let's build and run the project now. After that, check the debug output for any detected cameras on your computer. The cameras that have been detected should also be displayed in the drop-down box. If you are running on a laptop with a supported camera, you should see it listed. If you're running a system with no built-in camera, then the debug output may not display anything and the drop-down box will remain empty as well. If that's the case, try plugging in an inexpensive USB camera and run the program again:

After that, open up mainwindow.ui and right click on the Connect button, and select Go to slot.... Select the clicked() option and click OK. Qt Creator will automatically create a slot function for you; add the following code into the function, like so:

void MainWindow::on_connectButton_clicked()
{
if (!connected)
{
connectCamera();
}
else
{
camera->stop();
viewfinder->deleteLater();
ui->connectButton->setText("Connect");
connected = false;
}
}

When the Connect button is being clicked, we first check whether the camera is already connected by checking the connect variable. If it's not connected yet, we run the connectCamera() function which we will define in the next step. If the camera is already connected, we stop the camera, delete the viewfinder and set the Connect button's text to Connect. Finally, set the connected variable to false. Do note that we're using deleteLater() here instead of delete(), which is the recommended way to delete a memory pointer. deleteLater() is called on an object that lives in a thread with no running event loop, the object will be destroyed when the thread finishes.

Next, we will add a new function in our MainWindow class called connectCamera(). The function looks like the following code block:

void MainWindow::connectCamera()
{
QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
foreach (const QCameraInfo &cameraInfo, cameras)
{
qDebug() << cameraInfo.description() << ui->deviceSelection-
>currentText();

if (cameraInfo.description() == ui->deviceSelection-
>currentText())
{
camera = new QCamera(cameraInfo);
viewfinder = new QCameraViewfinder(this);
camera->setViewfinder(viewfinder);
ui->webcamLayout->addWidget(viewfinder);

connected = true;
ui->connectButton->setText("Disconnect");

camera->start();

return;
}
}
}

In the connectCamera() function, we repeat what we did in the construction and get the current list of connected cameras. Then, we loop through the list and compare the name of the camera (stored in the description variable) with the currently selected device name on the combo box widget.

If there's a matching name, it means the user is intending to connect to that particular camera, and thus we will proceed to connect to that camera by initializing a QCamera object and a new QCameraViewFinder object. We then link the viewfinder to the camera and add the viewfinder to the layout with the black color background. Then, we set the connected variable to true and set the Connect button's text to Disconnect. Finally, call the start() function to start running the camera.

Build and run the project now. Select the camera you are intending to connect to and click the Connect button. You should be able to connect to your camera and see yourself in the program:

If your camera is unable to connect, do the following steps to display any errors returned by the operating system. First, open up mainwindow.h and add in the following slot function:

private slots:
void cameraError(QCamera::Error error);

After that, open mainwindow.cpp and add the following code to connectCamera() function to connect the error() signal to the cameraError(), slot function:

void MainWindow::connectCamera()
{
QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
foreach (const QCameraInfo &cameraInfo, cameras)
{
qDebug() << cameraInfo.description() << ui->deviceSelection-
>currentText();

if (cameraInfo.description() == ui->deviceSelection-
>currentText())
{
camera = new QCamera(cameraInfo);
viewfinder = new QCameraViewfinder(this);
camera->setViewfinder(viewfinder);
ui->webcamLayout->addWidget(viewfinder);

connect(camera, SIGNAL(error(QCamera::Error)), this,
SLOT(cameraError(QCamera::Error)));

connected = true;
ui->connectButton->setText("Disconnect");

camera->start();

return;
}
}
}

The cameraError() slot function looks like this:

void MainWindow::cameraError(QCamera::Error error)
{
qDebug() << "Camera error:" << error;

connected = false;
camera->stop();
ui->connectButton->setText("Connect");
}

In the preceding code, we display the error message and make sure the camera has completely stopped, just in case. By looking at the error message, you should be able to debug the problem more easily.

Capturing a camera image to file

We have learned how to connect to our camera using Qt's multimedia module in the previous section. Now, we will try and capture a still image from the camera and save it into a JPEG file. It's actually very very simple with Qt.

First, open mainwindow.h and add the following variable:

private:
Ui::MainWindow *ui;
QCamera* camera;
QCameraViewfinder* viewfinder;
QCameraImageCapture* imageCapture;
bool connected;

Then, right-click on the Capture button in mainwindow.ui and select Go to slot.... Then, select clicked() and press OK. Now, a new slot function will be created for you in mainwindow.cpp. Add the following code to capture an image from the camera:

void MainWindow::on_captureButton_clicked()
{
if (connected)
{
imageCapture = new QCameraImageCapture(camera);
camera->setCaptureMode(QCamera::CaptureStillImage);
camera->searchAndLock();
imageCapture->capture(qApp->applicationDirPath());
camera->unlock();
}
}

What we did in the preceding code is basically create a new QCameraImageCapture object and set its media object as the active camera. Then, set its capture mode as a still image. Before we ask the QCameraImageCapture object to capture an image, we must lock the camera so that the settings remain unchanged during the process of capturing the image. You may unlock it by calling camera->unlock() after you have successfully captured the image.

We used qApp->applicationDirPath() to get the application directory so that the image will be saved alongside the executable file. You can change this to whatever directory you want. You can also put your desired filename behind the directory path; otherwise, it will save the images sequentially using the default filename format starting with IMG_00000001.jpg, IMG_00000002.jpg, and so on.

Recording a camera video to file

After we have learned how to capture a still image from our camera, let's proceed to learn how to record videos as well. First, open mainwindow.h and add the following variables:

private:
Ui::MainWindow *ui;
QCamera* camera;
QCameraViewfinder* viewfinder;
QCameraImageCapture* imageCapture;
QMediaRecorder* recorder;

bool connected;
bool recording;

Next, open mainwindow.ui again and right-click on the Record button. Choose Go to slot... from the menu and select the clicked() option, then, click the OK button. A slot function will be created for you; then proceed to add the following code into the slot function:

void MainWindow::on_recordButton_clicked()
{
if (connected)
{
if (!recording)
{
recorder = new QMediaRecorder(camera);
camera->setCaptureMode(QCamera::CaptureVideo);
recorder->setOutputLocation(QUrl(qApp-
>applicationDirPath()));
recorder->record();
recording = true;
}
else
{
recorder->stop();
recording = false;
}
}
}

This time, we use a QMediaRecorder for recording video instead. We must also set the camera's capture mode to QCamera::CaptureVideo before calling recorder->record().

To check the error message produced by the media recorder during the recording stage, you may connect the error() signal of the media recorder to a slot function like this:

void MainWindow::on_recordButton_clicked()
{
if (connected)
{
if (!recording)
{
recorder = new QMediaRecorder(camera);
connect(recorder, SIGNAL(error(QMediaRecorder::Error)),
this, SLOT(recordError(QMediaRecorder::Error)));
camera->setCaptureMode(QCamera::CaptureVideo);
recorder->setOutputLocation(QUrl(qApp-
>applicationDirPath()));
recorder->record();
recording = true;
}
else
{
recorder->stop();
recording = false;
}
}
}

Then, simply display the error message in the slot function:

void MainWindow::recordError(QMediaRecorder::Error error)
{
qDebug() << errorString();
}

Do note that, at the time of writing this chapter, the QMediaRecorder class only supports video recording on macOS, Linux, mobile platforms and Windows XP. It doesn't work on Windows 8 and Windows 10 at the moment, but it will be ported over in one of the upcoming versions. The main reason is that Qt is using Microsoft's DirectShow API to record video on the Windows platform, but it has since been deprecated from the Windows operating system. Hopefully, by the time you're reading this book, this feature has been completely implemented in Qt for Windows 8 and 10.

If it hasn't, you may use third-party plugins that use OpenCV API for recording video, such as the Qt Media Encoding Library (QtMEL) API, as a temporary solutions. Do note that the code used in QtMEL is completely different than the one we're showing here in this chapter.

For more information about QtMEL, please check out the following link:
http://kibsoft.ru.

Summary

In this chapter, we have learned how to connect to our camera using Qt. We have also learned how to capture an image or record a video from the camera. In the next chapter, we will learn about the networking module and try and make an instant messenger using Qt!

Instant Messaging

One important feature of corporate software is the ability to communicate with staff. Thus, an internal instant messaging system is a crucial part of the software. By incorporating the networking module in Qt, we can easily create a chat system out of it.

In this chapter, we will cover the following topics:

- Qt networking module

- Creating an instant messaging server

- Creating an instant messaging client

Creating an instant messaging system using Qt is a lot easier than you think. Let's get started!

The Qt networking module

In the following section, we will learn about the Qt networking module and how it can help us to achieve server-client communication via the TCP or UDP connection protocols.

Connection protocols

The networking module in Qt is the module that offers both low-level networking functionality, such as TCP and UDP sockets, as well as high-level networking classes for web integration and network communication.

In this chapter, we will use the TCP (Transmission Control Protocol) internet protocol for our program instead of the UDP (User Datagram Protocol) protocol. The main difference is that TCP is a connection-oriented protocol that requires all clients to establish a connection to the server before they are able to communicate with each other.

UDP on the other hand is a connectionless protocol that does not require a connection. The client will just send whatever data it needs to send to the destination, without checking if the data has been received by the other end. There are pros and cons for both protocols, but TCP is much more suitable for our sample project. We want to make sure every chat message is being received by the recipient, don't we?

The differences between both protocols are as follows:

- TCP:

- Connection-oriented protocol

- Suitable for applications that require high reliability, and it is less critical toward its data transmission time

- The speed for TCP is slower than UDP

- Requires acknowledgment of receipt from the receiving client before sending the next data

- There is an absolute guarantee that the data transferred remains intact and arrives in the same order in which it was sent

- UDP:

- Connectionless protocol

- Suitable for applications that need fast, efficient transmission, such as games and VOIP

- UDP is lightweight and faster than TCP because error recovery is not attempted

- Also suitable for servers that answer small queries from huge numbers of clients

- There is no guarantee that the data sent reaches its destination at all as there is no tracking connections and no need for any acknowledgment from the receiving client

Since we are not going for the peer-to-peer connection approach, our chat system will require two different pieces of software—the server program and the client program. The server program will act as the middleman (just like a postman) who receives all the messages from all the users and sends them to the targeted recipients accordingly. The server program will be locked away from the normal users in one of the computers in the server room.

The client program, on the other hand, is the instant messaging software that is used by all the users. This program is the one that is being installed on the users' computers. Users can send their messages using this client program and see the messages sent by others as well. The overall architecture of our messaging system looks something like this:

Let's proceed to setting up our project and enabling Qt's networking module! For this project, we will start on the server program before working on the client program.

Setting up a new project

First, create a new Qt Console Application project. Then, open up the project file (.pro) and add in the following module:

QT += core network
Qt -= gui

You should have noticed that this project doesn't have any gui module (we make sure it's explicitly removed) as we don't need any user interface for the server program. That is also the reason why we chose Qt Console Application instead of the usual Qt Widgets Application.

Actually, that's it—you have successfully added the networking module to your project. In the next section, we will learn how to create the server program for our chat system.

Creating an instant messaging server

In the following section, we will learn how to create an instant messaging server that receives messages sent by the users and redistributes them to their respective recipients.

Creating TCP Server

In this section, we will learn how to create a TCP server that constantly listens to a specific port for incoming messages. For the sake of simplicity, we will just create a global chat room in which every user can see the messages sent by each and every user within the chat room, instead of a one-to-one messaging system with a friend list. You can easily improvise this system to the latter once you have understood how a chat system functions.

First, go to File | New File or Project and choose C++ Class under the C++ category. Then, name the class as server and select QObject as the base class. Make sure the Include QObject option is ticked before proceeding to create the custom class. You should have also noticed the absence of mainwindow.ui, mainwindow.h, and mainwindow.cpp. This is because there is no user interface in a console application project.

Once the server class has been created, let's open up server.h and add in the following headers, variables and functions:

#ifndef SERVER_H
#define SERVER_H

#include <QObject>
#include <QTcpServer>
#include <QTcpSocket>
#include <QDebug>
#include <QVector>

private:
QTcpServer* chatServer;
QVector<QTcpSocket*>* allClients;

public:
explicit server(QObject *parent = nullptr);
void startServer();
void sendMessageToClients(QString message);

public slots:
void newClientConnection();
void socketDisconnected();
void socketReadyRead();
void socketStateChanged(QAbstractSocket::SocketState state);

Next, create a function called startServer() and add the following code to the function definition in server.cpp:

void server::startServer()
{
allClients = new QVector<QTcpSocket*>;

chatServer = new QTcpServer();
chatServer->setMaxPendingConnections(10);
connect(chatServer, SIGNAL(newConnection()), this,
SLOT(newClientConnection()));

if (chatServer->listen(QHostAddress::Any, 8001))
{
qDebug() << "Server has started. Listening to port 8001.";
}
else
{
qDebug() << "Server failed to start. Error: " + chatServer-
>errorString();
}
}

We created a QTcpServer object called chatServer and made it constantly listen to port 8001. You can choose any unused port number ranging from 1024 to 49151. Other numbers outside of this range are usually reserved for common systems, such as HTTP or FTP services, so we better not use them to avoid conflicts. We also created a QVector array called allClients to store all the connected clients so that we can make use of it later to redirect incoming messages to all users.

We also used the setMaxPendingConnections() function to limit the maximum pending connections to 10 clients. You can use this method to keep the number of active clients to a specific amount so that your server's bandwidth is always within its limit. This can ensure good service quality and maintain a positive user experience.

Listening to clients

The chatServer will trigger the newConnection() signal whenever a client has connected to the server, so we connect that signal to our custom slot function called newClientConnection(). The slot function looks like this:

void server::newClientConnection()
{
QTcpSocket* client = chatServer->nextPendingConnection();
QString ipAddress = client->peerAddress().toString();
int port = client->peerPort();

connect(client, &QTcpSocket::disconnected, this, &server::socketDisconnected);
connect(client, &QTcpSocket::readyRead, this, &server::socketReadyRead);
connect(client, &QTcpSocket::stateChanged, this, &server::socketStateChanged);

allClients->push_back(client);

qDebug() << "Socket connected from " + ipAddress + ":" + QString::number(port);
}

Every new client connected to the server is a QTcpSocket object, which can be obtained from the QTcpServer object by calling nextPendingConnection(). You can obtain information about the client such as its IP address and port number by calling peerAddress() and peerPort(), respectively. We then store each new client into the allClients array for future use. We also connect the client's disconnected(), readyRead() and stateChanged() signals to its respective slot function.

When a client is disconnected from the server, the disconnected() signal will be triggered, and subsequently the socketDisconnected(), slot function will be called. What we are doing in this function is just displaying the message on the server console whenever it happens, and nothing more. You can do anything you like here such as saving the user's offline state to the database and so on. For the sake of simplicity, we will just print out the message on the console window:

void server::socketDisconnected()
{
QTcpSocket* client = qobject_cast<QTcpSocket*>(QObject::sender());
QString socketIpAddress = client->peerAddress().toString();
int port = client->peerPort();

qDebug() << "Socket disconnected from " + socketIpAddress + ":" +
QString::number(port);
}

Next, whenever a client is sending in a message to the server, the readyRead() signal will be triggered. We have connected the signal to a slot function called socketReadyRead() and it looks something like this:

void server::socketReadyRead()
{
QTcpSocket* client = qobject_cast<QTcpSocket*>(QObject::sender());
QString socketIpAddress = client->peerAddress().toString();
int port = client->peerPort();

QString data = QString(client->readAll());

qDebug() << "Message: " + data + " (" + socketIpAddress + ":" +
QString::number(port) + ")";

sendMessageToClients(data);
}

In the preceding code, we simply redirect the message to a custom function called sendMessageToClients(), which handles passing the message to all connected clients. We will look at how this function works in a minute. We use QObject::sender() to get the pointer of the object that emitted the readyRead signal and convert it to the QTcpSocket class so that we can access its readAll() function.

After that, we also connected another signal called stateChanged() to the socketStateChanged() slot function. The slow function looks like this:

void server::socketStateChanged(QAbstractSocket::SocketState state)
{
QTcpSocket* client = qobject_cast<QTcpSocket*>(QObject::sender());
QString socketIpAddress = client->peerAddress().toString();
int port = client->peerPort();

QString desc;

if (state == QAbstractSocket::UnconnectedState)
desc = "The socket is not connected.";
else if (state == QAbstractSocket::HostLookupState)
desc = "The socket is performing a host name lookup.";
else if (state == QAbstractSocket::ConnectingState)
desc = "The socket has started establishing a connection.";
else if (state == QAbstractSocket::ConnectedState)
desc = "A connection is established.";
else if (state == QAbstractSocket::BoundState)
desc = "The socket is bound to an address and port.";
else if (state == QAbstractSocket::ClosingState)
desc = "The socket is about to close (data may still be
waiting to be written).";
else if (state == QAbstractSocket::ListeningState)
desc = "For internal use only.";

qDebug() << "Socket state changed (" + socketIpAddress + ":" +
QString::number(port) + "): " + desc;
}

This function gets triggered whenever a client's network state has changed, such as connected, disconnected, listening, and so on. We will simply print out a relevant message according to its new state so that we can debug our program more easily.

Now, let's look at what the sendMessageToClients() function looks like:

void server::sendMessageToClients(QString message)
{
if (allClients->size() > 0)
{
for (int i = 0; i < allClients->size(); i++)
{
if (allClients->at(i)->isOpen() && allClients->at(i)-
>isWritable())
{
allClients->at(i)->write(message.toUtf8());
}
}
}
}

In the preceding code, we simply loop through the allClients array and pass the message data to all the connected clients.

Lastly, open up main.cpp and add the following code to start our server:

#include <QCoreApplication>
#include "server.h"

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

server* myServer = new server();
myServer->startServer();

return a.exec();
}

Build and run the program now, and you should see something like this:

It doesn't look like anything is happening except showing that the server is listening to port 8001. Don't worry, because we haven't created the client program yet. Let's proceed!

Creating an instant messaging client

In the following section, we will proceed to create our instant messaging client, which the users will be using to send and receive messages.

Designing the user interface

In this section, we will learn how to design the user interface for the instant messaging client and create functionality for it:

1. First, create another Qt project by going to File | New File or Project. Then select Qt Widget Application under the Application category.

2. After the project has been created, open up mainwindow.ui and drag a Line Edit and Text Browser to the window canvas. Then, select the central widget and click the Lay Out Vertically button, located on the widget bar above, to apply the vertical layout effect to the widgets:

3. After that, place a Horizontal Layout at the bottom and put the Line Edit into the layout. Then, pull a Push Button from the widget box into the Horizontal Layout and name it as sendButton; we also set its label as Send, like this:

4. Once you're finished, drag and drop another Horizontal Layout and place it on top of the text browser. After that, place a Label, Line Edit, and a Push Button into the horizontal layout, like this:

We call the line edit widget nameInput and set a default text for it as John Doe, just so the user has a default name. Then, we call the push button connectButton and change its label to Connect.

We have completed the user interface design for a very simple instant messaging program, which will do the following tasks:

1. Connect to a server

2. Let a user set their name

3. Can see messages sent by all users

4. A user can type and send their messages for all to see

Compile and run the project now, you should see your program looking something like this:

Do note that I also changed the window title to Chat Client so that it looks slightly more professional. You can do so by selecting the MainWindow object at the hierarchy window and change its windowTitle property.

In the next section, we will start working on the programming part and implement the features mentioned in the list above.

Implementing chat features

Before we start writing any code, we must first enable the networking module by opening our project file (.pro) and add the network keyword there:

QT += core gui network

Next, open up mainwindow.h and add the following headers and variables:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QDebug>
#include <QTcpSocket>

private:
Ui::MainWindow *ui;
bool connectedToHost;
QTcpSocket* socket;

We set the connectedToHost variable to false by default in mainwindow.cpp:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
connectedToHost = false;
}

Once this is done, the first feature we need to implement is the server connection. Open up mainwindow.ui, right-click on the Connect button, then choose Go to slot..., and pick clicked(). After that, a slot function will be created for you automatically. Add in the following code to the SLOT function:

void MainWindow::on_connectButton_clicked()
{
if (!connectedToHost)
{
socket = new QTcpSocket();

connect(socket, SIGNAL(connected()), this,
SLOT(socketConnected()));
connect(socket, SIGNAL(disconnected()), this,
SLOT(socketDisconnected()));
connect(socket, SIGNAL(readyRead()), this,
SLOT(socketReadyRead()));

socket->connectToHost("127.0.0.1", 8001);
}
else
{
QString name = ui->nameInput->text();
socket->write("<font color="Orange">" + name.toUtf8() + " has
left the chat room.</font>");

socket->disconnectFromHost();
}
}

What we did in the preceding code was basically check for the connectedToHost variable. If the variable is false (meaning the client is not connected to the server), create a QTcpSocket object called socket and make it connect to a host at 127.0.0.1 on port 8801. The IP address 127.0.0.1 stands for a localhost. Since this is only for testing purposes, we will connect the client to our test server, which is located on the same computer. If you're running the server on another computer, you may change the IP address to a LAN or WAN address, depending on your need.

We also connected the socket object to its respective slot functions when connected(), disconnected(), and readReady() signals were triggered. This is exactly the same as the server code, which we did previously. If the client is already connected to the server and the Connect (now labeled Disconnect) button is clicked, then send a disconnection message to the server and terminate the connection.

Next, we will look at the slot functions, which we connected to the socket object in the previous step. The first one is the socketConnected() function, which will be called when the client has successfully connected to the server:

void MainWindow::socketConnected()
{
qDebug() << "Connected to server.";

printMessage("<font color="Green">Connected to server.</font>");

QString name = ui->nameInput->text();
socket->write("<font color="Purple">" + name.toUtf8() + " has joined
the chat room.</font>");

ui->connectButton->setText("Disconnect");
connectedToHost = true;
}

First, the client will display a Connected to server. message on both the application output and the text browser widget. We will see what the printMessage() function looks like in a minute. Then, we take the user's name from the input field and incorporate it into a text message and send it to the server so that all users are being notified. Finally, set the Connect button's label to Disconnect, and set the connectedToHost variable to true.

After this, let's look at socketDisconnected(), which as its name implies, will be called whenever the client is disconnected from the server:

void MainWindow::socketDisconnected()
{
qDebug() << "Disconnected from server.";

printMessage("<font color="Red">Disconnected from server.</font>");

ui->connectButton->setText("Connect");
connectedToHost = false;
}

The preceding code is quite straightforward. All it does is show disconnected messages on both the application output and text browser widget, then sets the Disconnect button's label to Connect and the connectedToHost variable to false. Do note that since this function will only be called after the client has been disconnected from the server, we can no longer send any message to the server at that point to notify it of the disconnection. You should check for the disconnection at the server side and notify all users accordingly.

Then, there is the socketReadyRead() function,  which will be triggered whenever the server is sending data to the client. This function is even simpler than the previous ones, as all it does is pass the incoming data to the printMessage() function and nothing else:

void MainWindow::socketReadyRead()
{
ui->chatDisplay->append(socket->readAll());
}

Finally, let's look at what the printMessage() function looks like. Actually, it is just as simple. All it does is to append the message to the text browser and it is done:

void MainWindow::printMessage(QString message)
{
ui->chatDisplay->append(message);
}

Last but not least, let's check out how to implement the function for sending messages to the server. Open up mainwindow.ui, right-click on the Send button, select Go to slot..., and choose the clicked() option. Once the slot function has been created for you, add the following code to the function:

void MainWindow::on_sendButton_clicked()
{
QString name = ui->nameInput->text();
QString message = ui->messageInput->text();
socket->write("<font color="Blue">" + name.toUtf8() + "</font>: " +
message.toUtf8());

ui->messageInput->clear();
}

First, we take the user's name and combine it with the message. Then, we set the name to a blue color before sending the entire thing to the server by calling write(). After that, clear the message input field, and we're done. Since the text browser accepts rich text by default, we can use that to color our text by placing the text within the <font> tags.

Compile and run the project now; you should be able to chat among yourselves on different clients! Don't forget to turn on the server before connecting the clients. If everything goes right, you should see something like this:

Meanwhile, you should also see all the activities on the server side as well:

That's it! We have successfully created a simple chat system using Qt. You are welcome to improvise on this and create a fully fledged messaging system!

Summary

In this chapter, we learned how to create an instant messaging system using Qt's networking module. In the following chapter, we will dive into the wonders of graphics rendering using Qt.

Implementing a Graphics Editor

Qt provides us with low-level graphics rendering using the QPainter class. Qt is capable of rendering both bitmap and vector images. In this chapter, we will learn how to draw shapes using Qt, and finally, create a paint program of our own.

In this chapter, we will cover the following topics:

- Drawing vector shapes

- Saving vector images to an SVG file

- Creating a paint program

Are you ready? Let's get started!

Drawing vector shapes

In the following section, we will learn how to render vector graphics on our Qt application using the QPainter class.

Vector versus bitmap

There are two types of format in computer graphics—bitmap and vector. Bitmap images (also known as raster images) are images that are stored as a series of tiny dots called pixels. Each pixel will be assigned a color and gets displayed on screen exactly how it's stored—a one-to-one correspondence between the pixels and what is displayed on the screen.

On the other hand, vector images are not based on bitmap patterns but rather use mathematical formulas to represent lines and curves that can be combined to create geometrical shapes.

The main characteristics of both formats are listed here:

- Bitmap:

- Usually a larger file size

- Cannot be enlarged into a higher resolution as the image quality will be affected

- Used to display complex images with many colors, such as photographs

- Vector:

- Very small in file size

- Graphics can be resized without affecting the image quality

- Only a limited amount of color can be applied to each shape (single color, gradient, or pattern)

- Complex shapes require high-processing power to be generated

The diagram here compares bitmap and vector graphics:

We will focus on learning how to draw vector graphics using Qt in this section, but we will also cover bitmap graphics later in this chapter.

Drawing vector shapes using QPainter

First, create another Qt project by going to File | New File or Project. Then select Qt Widget Application under the Application category. After the project has been created, open up mainwindow.h and add in the QPainter header:

#include <QMainWindow>
#include <QPainter>

After that, we also declare a virtual function called paintEvent(), which is a standard event handler in Qt that gets called whenever there is something that needs to be painted, be it a GUI update, a window resize, or when the update() function is being called manually:

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
virtual void paintEvent(QPaintEvent *event);

Then, open up mainwindow.cpp and add the paintEvent() function:

void MainWindow::paintEvent(QPaintEvent *event)
{
QPainter painter;
painter.begin(this);

// Draw Line
painter.drawLine(QPoint(50, 60), QPoint(100, 100));

// Draw Rectangle
painter.setBrush(Qt::BDiagPattern);
painter.drawRect(QRect(40, 120, 80, 30));

// Draw Ellipse
QPen ellipsePen;
ellipsePen.setColor(Qt::red);
ellipsePen.setStyle(Qt::DashDotLine);
painter.setPen(ellipsePen);
painter.drawEllipse(QPoint(80, 200), 50, 20);

// Draw Rectangle
QPainterPath rectPath;
rectPath.addRect(QRect(150, 20, 100, 50));
painter.setPen(QPen(Qt::red, 1, Qt::DashDotLine, Qt::FlatCap,
Qt::MiterJoin));
painter.setBrush(Qt::yellow);
painter.drawPath(rectPath);

// Draw Ellipse
QPainterPath ellipsePath;
ellipsePath.addEllipse(QPoint(200, 120), 50, 20);
painter.setPen(QPen(QColor(79, 106, 25), 5, Qt::SolidLine,
Qt::FlatCap, Qt::MiterJoin));
painter.setBrush(QColor(122, 163, 39));
painter.drawPath(ellipsePath);

painter.end();
}

If you build the program now, you should see the following:

The preceding code is really long. Let's break it down, so it's easier for you to understand. Whenever the paintEvent() is called (usually it will be called once when the window needs to be drawn), we call QPainter::begin() to tell Qt we're about to draw something, and we call QPainter::end() when we're done. Therefore, the code that draws graphics will be contained within QPainter::begin() and QPainter::end().

Let's look at the following steps:

1. The first thing that we drew was a straight line, which is quite simple – just call QPainter::drawLine() and insert the start point and end point values to the function. Do note that the coordinate system used by Qt is in pixel format. Its origin starts from the top-left corner of the application window and increases to the right and bottom directions, depending on the x and y values. The increment of the x value moves the position to the right direction, while the increment of the y value moves the position to the bottom direction.

2. Next, draw a rectangle that has a hatching pattern within the shape. This time, we called QPainter::setBrush() to set the pattern, before calling drawRect().

3. After that, we drew an elliptical shape with a dash-dot outline and hatching pattern within the shape. Since we have already set the pattern in the previous step, we don't have to do it again. Instead, we use the QPen class to set the outline style before calling drawEllipse(). Just remember that in Qt's terms, a brush is used to define the inner color or pattern of a shape, while a pen is used to define the outline.

4. The next two shapes are basically similar to the previous ones; we only changed different colors and patterns so that you can see the distinctions between them and the previous examples.

Drawing text

Additionally, you can also draw text using the QPainterclass. All you need to do is to call QPainter::setFont() to set the font properties before calling QPainter::drawText(), like so:

QPainter painter;
painter.begin(this);

// Draw Text
painter.setFont(QFont("Times", 14, QFont::Bold));
painter.drawText(QPoint(20, 30), "Testing");

// Draw Line
painter.drawLine(QPoint(50, 60), QPoint(100, 100))

The setFont() function is optional as you will get a default font if you don't specify it. Once you're done, build and run the program. You should see the word Hello World! displayed in the window:

As you can see here, the vector shapes are basically generated by Qt in real time, which looks perfectly fine regardless of how you rescale the window and change its aspect ratio. If you're rendering a bitmap image instead, its visual quality may get degraded when its rescaled along with the window or changed in its aspect ratio.

Saving vector images to an SVG File

Beside drawing vector graphics, Qt allows us to save these graphics into a vector image file, called the SVG (Scalable Vector Graphics) file format. The SVG format is an open format used by a lot of software, including web browsers to display vector graphics. In fact, Qt can also read SVG files and render them on screen, but we'll skip that for now. Let's check out how we can save our vector graphics to an SVG file!

This example continues from where we left it in the previous section. Therefore, we don't have to create a new Qt project and can just stick to the previous one.

First, let's add a menu bar to our main window if it doesn't already have one. Then, open mainwindow.ui, and in the form editor, right-click on the MainWindow object on the hierarchy window and select Create Menu Bar:

Once you're done, add File to the menu bar, followed by Save as SVG underneath it:

Then, go to the Action Editor at the bottom and right-click on the menu option we just added and select Go to slot...:

A window will pop up and ask you to pick a signal. Choose triggered() and click OK. A new slot function will be created for you in mainwindow.cpp. Before we open up mainwindow.cpp, let's open up our project file(.pro) and add the following svg module:

QT += core gui svg

The svg keyword tells Qt to add relevant classes to your project that can help you to handle the SVG file format. Then, we also need to add two more headers to our mainwindow.h:

#include <QtSvg/QSvgGenerator>
#include <QFileDialog>

After that, open up mainwindow.cpp and add the following code to the slot function we just added in the previous step:

void MainWindow::on_actionSave_as_SVG_triggered()
{
QString filePath = QFileDialog::getSaveFileName(this, "Save SVG", "", "SVG files (*.svg)");

if (filePath == "")
return;

QSvgGenerator generator;
generator.setFileName(filePath);
generator.setSize(QSize(this->width(), this->height()));
generator.setViewBox(QRect(0, 0, this->width(), this->height()));
generator.setTitle("SVG Example");
generator.setDescription("This SVG file is generated by Qt.");

paintAll(&generator);
}

In the preceding code, we used QFileDialog to let the users choose where they want to save their SVG file. Then, we used the QSvgGenerator class to export the graphics into an SVG file. Finally, we called the paintAll() function, which is a custom function we are going to define in the next step.

Actually, we need to modify the existing paintAll() method and put our rendering code into it. Then, pass the QSvgGenerator object into the function input as the paint device:

void MainWindow::paintAll(QSvgGenerator *generator)
{
QPainter painter;

if (generator)
painter.begin(generator);
else
painter.begin(this);

// Draw Text
painter.setFont(QFont("Times", 14, QFont::Bold));
painter.drawText(QPoint(20, 30), "Hello World!");

Therefore, our paintEvent() now simply looks like this in mainwindow.cpp:

void MainWindow::paintEvent(QPaintEvent *event)
{
paintAll();
}

The procedure here might seem a little confusing, but what it does is basically call the paintAll() function to draw all the graphics once when the window is being created, and then you call paintAll() again when you want to save the graphics to an SVG file.

The only difference is the paint device – one is the main window itself, which we use as the drawing canvas, and for the latter one we will pass the QSvgGenerator object as the paint device, which will save the graphics into an SVG file instead.

Build and run the program now, click File | Save SVG File, you should be able to save the graphics into an SVG file. Try and open up the file with the web browser and see what it looks like:

It seems like my web browser (Firefox) does not support the hatching pattern, but other things turn out to be fine. Since vector graphics are generated by the program and the shapes are not stored in the SVG file (only the mathematical formula and its variables are stored), you may need to make sure the features that you use are supported by the user's platform.

In the next section, we will learn how to create our own paint program and draw bitmap images using it!

Creating a paint program

In the following section, we will move over to the realm of pixels and learn how to create a paint program using Qt. Users will be able to express their creativity by using different sizes and colors of the brush to draw pixel images!

Setting up a user interface

Again, for this example, we will create a new Qt Widget Application. After that, open up mainwindow.ui and add a menu bar to the main window. Then, add the following options to the menu bar:

We have three menu items on the menu bar—File, Brush Size, and Brush Color. Under the File menu are functions for saving the canvas into a bitmap file, as well as clearing the entire canvas. The Brush Size category contains different options for the brush size; last but not least, the Brush Color category contains several options for setting the brush color.

You can go for something more paint-like or Photoshop-like for the GUI design, but we will use this for now for the sake of simplicity.

Once you're done with all that, open up mainwindow.h and add the following headers on top:

#include <QMainWindow>
#include <QPainter>
#include <QMouseEvent>
#include <QFileDialog>

After that, we also declare a few virtual functions, like so:

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
virtual void mousePressEvent(QMouseEvent *event);
virtual void mouseMoveEvent(QMouseEvent *event);
virtual void mouseReleaseEvent(QMouseEvent *event);
virtual void paintEvent(QPaintEvent *event);
virtual void resizeEvent(QResizeEvent *event);

Besides the paintEvent() function which we used in the previous example, we can also add a few more for handling mouse events and window resize events. Then, we also add the following variables to our MainWindow class:

private:
Ui::MainWindow *ui;
QImage image;
bool drawing;
QPoint lastPoint;
int brushSize;
QColor brushColor;

After that, let's open up mainwindow.cpp and  start with the class constructor:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

image = QImage(this->size(), QImage::Format_RGB32);
image.fill(Qt::white);

drawing = false;
brushColor = Qt::black;
brushSize = 2;
}

We need to first create a QImage object, which acts as the canvas, and set its size to match our window size. Then, we set the default brush color to black and its default size to 2. After that, we will look at each of the event handlers and how they work.

First, let's take a look at the paintEvent() function, which we also used in the vector graphics, example. This time, all it does is call QPainter::drawImage() and render the QImage object (our image buffer) on top of our main window:

void MainWindow::paintEvent(QPaintEvent *event)
{
QPainter canvasPainter(this);
canvasPainter.drawImage(this->rect(), image, image.rect());
}

Next, we will look at the resizeEvent() function, which gets triggered whenever the main window is being resized by the user. To avoid image stretching, we must resize our image buffer to match the new window size. This can be achieved by creating a new QImage object and setting its size the same as the resized main window, then copying the previous QImage's pixel information and placing it at the exact same spot on the new image buffer.

This means that your image will be cropped if the window size is smaller than the drawing, but at least the canvas will not be stretched and distort the image when the window is resized. Let's take a look at the code:

void MainWindow::resizeEvent(QResizeEvent *event)
{
QImage newImage(event->size(), QImage::Format_RGB32);
newImage.fill(qRgb(255, 255, 255));

QPainter painter(&newImage);
painter.drawImage(QPoint(0, 0), image);
image = newImage;
}

Next, we will look at the mouse event handlers, which we use to apply colors on the canvas. First, the mousePressEvent() function, which will be triggered when we start pressing our mouse button (left mouse button in this case). We are still not drawing anything at this point, but set the drawing Boolean to true and save our cursor position to the lastPoint variable:

void MainWindow::mousePressEvent(QMouseEvent *event)
{
if (event->button() == Qt::LeftButton)
{
drawing = true;
lastPoint = event->pos();
}
}

Then, here is the mouseMoveEvent() function, which will be called when the mouse cursor is moved:

void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
if ((event->buttons() & Qt::LeftButton) && drawing)
{
QPainter painter(&image);
painter.setPen(QPen(brushColor, brushSize, Qt::SolidLine,
Qt::RoundCap, Qt::RoundJoin));
painter.drawLine(lastPoint, event->pos());

lastPoint = event->pos();
this->update();
}
}

In the preceding code, we check if indeed we are moving the mouse while holding the left mouse button. If we are, then we draw a line from the previous cursor position to our current cursor position. Then, we save the current cursor position to the lastPoint variable and call update() to notify Qt to trigger the paintEvent() function.

Finally, when we release the left mouse button, the mouseReleaseEvent() will be called. We simply set the drawing variable to false, and we're done:

void MainWindow::mouseReleaseEvent(QMouseEvent *event)
{
if (event->button() == Qt::LeftButton)
{
drawing = false;
}
}

If we build the program and run it now, we should be able to start drawing something on our little paint program:

Even though we can draw something now, it's all the same brush size and with the same color all the time. That's a little boring! Let's right-click on each of the options on the Brush Size category in the main menu and select Go to slot..., then pick the triggered() option and then press OK. Qt will then create the slot functions accordingly for us, and what we need to do within these functions is basically change the brushSize variable, like so:

void MainWindow::on_action2px_triggered()
{
brushSize = 2;
}

void MainWindow::on_action5px_triggered()
{
brushSize = 5;
}

void MainWindow::on_action10px_triggered()
{
brushSize = 10;
}

The same goes for all the options under the Brush Color category. This time, we set the brushColor variable accordingly:

void MainWindow::on_actionBlack_triggered()
{
brushColor = Qt::black;
}

void MainWindow::on_actionWhite_triggered()
{
brushColor = Qt::white;
}

void MainWindow::on_actionRed_triggered()
{
brushColor = Qt::red;
}

void MainWindow::on_actionGreen_triggered()
{
brushColor = Qt::green;
}

void MainWindow::on_actionBlue_triggered()
{
brushColor = Qt::blue;
}

If you build and run the program again, you will be able to draw your images with a variety of settings for your brush:

Other than that, we can also add an existing bitmap image to our canvas so that we can draw on top of it. Let's say I have a penguin image in the form of a PNG image (called tux.png), we can then add the following code to the class constructor:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

image = QImage(this->size(), QImage::Format_RGB32);
image.fill(Qt::white);

QImage tux;
tux.load(qApp->applicationDirPath() + "/tux.png");
QPainter painter(&image);
painter.drawImage(QPoint(100, 100), tux);

drawing = false;
brushColor = Qt::black;
brushSize = 2;
}

The preceding code basically opens up the image file and moves it to position 100 x 100 before drawing the image onto our image buffer. Now we can see a penguin on the canvas whenever we start the program:

Next, we will look at the Clear option under File. When the user clicks on this option on the menu bar, we use the following code to clear the entire canvas (including the penguin) and start all over again:

void MainWindow::on_actionClear_triggered()
{
image.fill(Qt::white);
this->update();
}

Finally, when the user clicks on the Save option under File, we open up a file dialog and let the users save their artwork into a bitmap file. In the following code, we filter out the image formats and only allow the users to save PNG and JPEG formats:

void MainWindow::on_actionSave_triggered()
{
QString filePath = QFileDialog::getSaveFileName(this, "Save Image", "", "PNG (*.png);;JPEG (*.jpg *.jpeg);;All files (*.*)");

if (filePath == "")
return;

image.save(filePath);
}

That's it, we have successfully created a simple paint program from scratch using Qt! You may even combine the knowledge learned from this chapter with the previous chapter to create an online collaborative whiteboard! The only limitation is your creativity. Lastly, I would like to say thank you to all the readers for creating the following masterpiece, using our newly created paint program:

Summary

In this chapter, we have learned how to draw vector and bitmap graphics, and subsequently we created our very own paint program using Qt. In the following chapter, we will look into the aspects of creating a program that transfers and stores our data on to the cloud.

Cloud Storage

In the previous chapter, we learned how to draw images on-screen using Qt. In this chapter, however, we are going learn something totally different, which is setting up our own file server and linking it to our Qt application.

In this chapter, we will cover the following topics:

- Setting up the FTP server

- Displaying the file list on the list view

- Uploading files to the FTP server

- Downloading files from the FTP server

Let's get started!

Setting up the FTP server

In the following section, we will learn how to set up an FTP server, which stores all the files uploaded by a user and allows them to download them at any time. This section is not related to Qt, so if you already have a running FTP server, please skip this part and proceed to the next section of this chapter.

Introducing FTP

FTP is an acronym for File Transfer Protocol. FTP is used to transfer files from one computer to another on a network, usually over the internet. FTP is just one of the many different forms of cloud storage technology, but it is also a simple one that you can easily set up on your own computer.

There are many different FTP servers that have been developed by different groups of people for a specific operating system. In this section of the chapter, we will learn how to set up a FileZilla server, which runs on the Windows operating system. If you're running other operating systems such as GNU, Linux, or macOS, there are many other FTP server programs that you can use, such as VSFTP and Pure-FTPd.

On Debian, Ubuntu, or other similar variants of Linux, running sudo apt-get install vsftpd on the Terminal will install and configure an FTP server. On macOS, open System Preferences from the Apple menu and select Sharing. Then, click on the Service tab and select FTP access. Finally, click the Start button to start running the FTP server.

If you already have a running FTP server, please skip to the next section, in which we'll start learning about C++ programming.

Downloading FileZilla

FileZilla is really easy to set up and configure. It provides a fully functional and easy-to-use user interface and doesn't require any prior experience to operate it. The first thing we need to do is download FileZilla. We will do it as follows:

1. Open up your browser and hop over to https://filezilla-project.org. You will see two download buttons located at the Home page.

2. Click on Download FileZilla Server and it will bring us to the download page:

3. Once you're at the download page, click on the Download FileZilla Server button and start downloading the software. We're not going to use the FileZilla Client, so you don't have to download that. Once everything is ready, let's proceed to install the software.

4. Like most Windows software, the installation process is very straightforward. Keep everything as default and click Next all the way until the installation process begins. It will take a couple of minutes at most for the installation to complete.

5. Once it's completed, click on the Close button and we're done!:

Setting up FileZilla

Once you have installed FileZilla, the control panel will most likely open by itself.

1. Since this is the first time you have launched FileZilla, it will ask you to set up the server. Keep the server IP address as 127.0.0.1 (which means localhost) and the admin port to 14147.

2. Key in your desired password for administrating the server and check on the Always connect to this server option. Press Connect and the FTP server will now start up! This is shown in the following screenshot:

3. After the FTP server has started running, we need to create a user account. Click on the fourth icon from the left to open up the Users dialog:

4. Then, under the General page, click on the Add button located at the right side of the window. Create an account by setting a username and press OK.

5. We don't have to set the user to any group for now, as user groups are only useful when you have many users that have the same privilege settings since it is easier to change all users settings at once or move users to different groups. Once you have created the user, check on the Password option and key in your desired password. It is always a good practice to put the password on your FTP account:

6. After that, we will proceed to the Shared folders page and add a shared directory for our newly created user.

7. Make sure the Delete and Append options are checked so that files that have the same name can be replaced. We will be using that for updating our file list in a moment:

8. If you click on the third icon from the left, the FileZilla Server options dialog will appear. You can basically configure everything here to suit your needs. For instance, if you don't want to use the default port number 21, you can simply change it on the options window, under the General settings page:

9. You can also set the speed limit for all users or a specific user under the Speed Limits page. This can prevent your server from becoming low performance when many users are downloading huge files at the same time:

Next, let's proceed to create our Qt project!

Displaying the file list on the list view

In the previous section, we successfully set up a FTP server and kept it running. In the following section, we will learn how to create an FTP client program that displays the file list, uploads files to the FTP server, and finally downloads files from it.

Setting up a project

As usual, let's create a new project using Qt Creator. The following steps will help:

1. We can create a new project by going to File | New File or Project and selecting Qt Widgets Application.

2. Once your project has been created, open your project (.pro) file and add the network keyword so that Qt knows that you need the Networking module in your project:

QT += core gui network

Setting up user interface

After that, open up mainwindow.ui and perform the following steps to design the upper part of our user interface for uploading files:

1. Place a Label that says Upload File: on top of every other widget.

2. Put a horizontal layout and two Push Buttons alongside it that say Open and Upload, under the Label respectively.

3. Place a Progress Bar under the Horizontal Layout.

4. Put a Horizontal Line followed by a Vertical Spacer at the bottom:

Next, we're going to construct the bottom part of the user interface that is used for downloading files:

This time, our user interface is very similar to the upper part, except we have added a List View before the second Progress Bar for displaying the file list. We put everything on the same page for this example program so that it's simpler and less confusing to explain.

Displaying the file list

Next, we will learn how to save and display the file list on the FTP server. Actually, the FTP server does provide the file list by default, and Qt was able to display it using the qtftp module back in older versions. However, since Version 5, Qt has completely dropped the qtftp module and this feature no longer exists.

If you're still interested in the old qtftp module, you can still obtain its source code on GitHub by visiting the following link: https://github.com/qt/qtftp

In Qt, we use the QNetworkAccessManager class to communicate with our FTP server so features that are specifically designed for FTP no longer work. But, don't worry, we will look into some other alternative methods to achieve the same result.

The best method, in my opinion, is using an online database to store the file list and its information (file size, format, status, and so on). If you're interested in learning how to connect your Qt application to a database, please refer to Chapter 3, Database Connection. However, for the sake of simplicity, we will use another method that works just fine but is less secure—by saving the file names directly on a text file and storing it on the FTP server.

If you're doing a serious project for your client or company, please do not use this method. Check out Chapter 3, Database Connection, and learn to use an actual database instead.

Alright, just assume that there is no other way but to use the text file; how are we going to do that? It's very simple: create a text file called files.txt and place it into the FTP directory we just created at the very beginning of this chapter.

Writing the code

Next, open up mainwindow.h and add the following headers:

#include <QMainWindow>
#include <QDebug>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QFile>
#include <QFileInfo>
#include <QFileDialog>
#include <QListWidgetItem>
#include <QMessageBox>

After that, add in the following variables and functions:

private:
Ui::MainWindow *ui;
QNetworkAccessManager* manager;

QString ftpAddress;
int ftpPort;
QString username;
QString password;

QNetworkReply* downloadFileListReply;
QNetworkReply* uploadFileListReply;

QNetworkReply* uploadFileReply;
QNetworkReply* downloadFileReply;

QStringList fileList;
QString uploadFileName;
QString downloadFileName;

public:
void getFileList();

Once you are done with the previous step, open up mainwindow.cpp and add the following code to the class constructor:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

manager = new QNetworkAccessManager(this);

ftpAddress = "ftp://127.0.0.1/";
ftpPort = 21;
username = "tester"; // Put your FTP user name here
password = "123456"; // Put your FTP user password here

getFileList();
}

What we did was basically initialize the QNetworkAccessManager object and set up the variables that store our FTP server's information, since we will be repeated using it many times in later steps. After that, we will call the getFileList() function to start downloading files.txt from our FTP server. The getFileList() function looks like the following:

void MainWindow::getFileList()
{
QUrl ftpPath;
ftpPath.setUrl(ftpAddress + "files.txt");
ftpPath.setUserName(username);
ftpPath.setPassword(password);
ftpPath.setPort(ftpPort);

QNetworkRequest request;
request.setUrl(ftpPath);

downloadFileListReply = manager->get(request);
connect(downloadFileListReply, &QNetworkReply::finished, this,
&MainWindow::downloadFileListFinished);
}

We used a QUrl object to store the information about our server and the location of the file we're trying to download, and then fed it to a QNetworkRequest object before sending it off by calling QNetworkAccessManager::get(). Since we have no idea when all the files will get downloaded completely, we make use of Qt's SIGNAL and SLOT mechanisms.

We connected the finished() signal that comes from our downloadFileListReply pointer (which points to a QNetworkReply object in mainwindow.h) and linked it to the slot function downloadFileListFinished(), which we defined as follows:

void MainWindow::downloadFileListFinished()
{
if(downloadFileListReply->error() != QNetworkReply::NoError)
{
QMessageBox::warning(this, "Failed", "Failed to load file
list: " + downloadFileListReply->errorString());
}
else
{
QByteArray responseData;
if (downloadFileListReply->isReadable())
{
responseData = downloadFileListReply->readAll();
}

// Display file list
ui->fileList->clear();
fileList = QString(responseData).split(",");

if (fileList.size() > 0)
{
for (int i = 0; i < fileList.size(); i++)
{
if (fileList.at(i) != "")
{
ui->fileList->addItem(fileList.at(i));
}
}
}
}
}

The code is a bit long, so I have broken down the function into the following steps:

1. If any problems occur during the download, display a message box that tells us the nature of the problem.

2. If everything went nicely and the download has completed, we will try and read the data by calling downloadFileListReply | readAll().

3. Then, clear the List Widget and start parsing the content of the text file. The format we used here is very simple; we only used a comma symbol to separate each file name: filename1,filename2,filename,... It is important that we do not do this in the actual project.

4. Once we have called split(",") to split the string into a string list, do a for loop and display each file name on the List Widget.

To test whether the preceding code works or not, create a text file called files.txt and add the following text to the file:

filename1,filename2,filename3

Then, place the text file to your FTP directory and run your project. You should be able to see it appear like this on the application:

Once it is working, we can clear away the content of the text file and proceed to our next section.

Uploading files to the FTP server

Since we don't have any files in our FTP directory yet (except the file list), let's write the code to allow us to upload our first file.

1. First, open mainwindow.ui and right click on the Open button. Then, select Go to slot and select the clicked() option:

2. A slot function will be automatically created for you. Then, add the following code to the function to open up the file selector window for our users to select their desired file for upload:

void MainWindow::on_openButton_clicked()
{
QString fileName = QFileDialog::getOpenFileName(this, "Select
File", qApp->applicationDirPath());
ui->uploadFileInput->setText(fileName);
}

3. After that, repeat this step and do the same for the Upload button. This time, the code for its slot function looks something like the following:

void MainWindow::on_uploadButton_clicked()
{
QFile* file = new QFile(ui->uploadFileInput->text());
QFileInfo fileInfo(*file);
uploadFileName = fileInfo.fileName();

QUrl ftpPath;
ftpPath.setUrl(ftpAddress + uploadFileName);
ftpPath.setUserName(username);
ftpPath.setPassword(password);
ftpPath.setPort(ftpPort);

if (file->open(QIODevice::ReadOnly))
{
ui->uploadProgress->setEnabled(true);
ui->uploadProgress->setValue(0);

QNetworkRequest request;
request.setUrl(ftpPath);

uploadFileReply = manager->put(request, file);
connect(uploadFileReply,
SIGNAL(uploadProgress(qint64,qint64)), this,
SLOT(uploadFileProgress(qint64,qint64)));
connect(uploadFileReply, SIGNAL(finished()), this,
SLOT(uploadFileFinished()));
}
else
{
QMessageBox::warning(this, "Invalid File", "Failed to open
file for upload.");
}
}

The code looks a bit long, so let's break it down:

1. We used the QFile class for opening the file that we want to upload (the file path is taken from ui->uploadFileInput->text()). If the file doesn't exist, display a message box to inform the user.

2. Then, we fill in the information of our FTP server and the upload destination into a QUrl object before feeding it to a QNetworkRequest object.

3. After that, we start reading the content of our file and provide it to the QNetworkAccessManager::put() function.

4. Since we have no idea when the file will get uploaded completely, we used the SIGNAL and SLOT mechanisms provided by Qt. We linked the uploadProgress() and finished() signals to our two custom slot function called uploadFileProgress() and uploadFileFinised(), respectively.

The slot function uploadFileProgress() will tell us the current progress of our upload, and therefore we can use it to set the progress bar:

void MainWindow::uploadFileProgress(qint64 bytesSent, qint64 bytesTotal)
{
qint64 percentage = 100 * bytesSent / bytesTotal;
ui->uploadProgress->setValue((int) percentage);
}

Meanwhile, the uploadFileFinished() function will be triggered when the file has been completely uploaded:

void MainWindow::uploadFileFinished()
{
if(uploadFileReply->error() != QNetworkReply::NoError)
{
QMessageBox::warning(this, "Failed", "Failed to upload file: "
+ uploadFileReply->errorString());
}
else
{
QMessageBox::information(this, "Success", "File successfully
uploaded.");
}
}

We are not done with the preceding function yet. Since a new file has been added to the FTP server, we must update the existing file list and replace the files.txt file stored within the FTP directory. Since the code is slightly longer, we will break the code into several parts, which all are occurring before showing the File successfully uploaded message box.

1. First, let's check whether the newly uploaded file has already existed within our file list (replacing an old file on the FTP server). If it does, then we can skip the entire thing; otherwise, append the filename to our fileList string list, as shown in the following code:

// Add new file to file list array if not exist yet
bool exists = false;
if (fileList.size() > 0)
{
for (int i = 0; i < fileList.size(); i++)
{
if (fileList.at(i) == uploadFileName)
{
exists = true;
}
}
}

if (!exists)
{
fileList.append(uploadFileName);
}

2. After that, create a temporary text file (files.txt) in our application's directory and save the new file list in the text file:

// Create new files.txt
QString fileName = "files.txt";
QFile* file = new QFile(qApp->applicationDirPath() + "/" + fileName);
file->open(QIODevice::ReadWrite);
if (fileList.size() > 0)
{
for (int j = 0; j < fileList.size(); j++)
{
if (fileList.at(j) != "")
{
file->write(QString(fileList.at(j) + ",").toUtf8());
}
}
}
file->close();

3. Finally, we use the QFile class to open the text file we just created, and we upload it again to the FTP server to replace the old file list:

// Re-open the file
QFile* newFile = new QFile(qApp->applicationDirPath() + "/" + fileName);
if (newFile->open(QIODevice::ReadOnly))
{
// Update file list to server
QUrl ftpPath;
ftpPath.setUrl(ftpAddress + fileName);
ftpPath.setUserName(username);
ftpPath.setPassword(password);
ftpPath.setPort(ftpPort);

QNetworkRequest request;
request.setUrl(ftpPath);
uploadFileListReply = manager->put(request, newFile);
connect(uploadFileListReply, SIGNAL(finished()), this, SLOT(uploadFileListFinished()));
file->close();
}

4. Again, we use the SIGNAL and SLOT mechanisms so that we are notified when the file list has been uploaded. The slot function uploadFileListFinished() looks something like the following:

void MainWindow::uploadFileListFinished()
{
if(uploadFileListReply->error() != QNetworkReply::NoError)
{
QMessageBox::warning(this, "Failed", "Failed to update file list: " + uploadFileListReply->errorString());
}
else
{
getFileList();
}
}

5. We basically just call getFileList() again after we have updated the file list into the FTP server. If you build and run the project now, you should be able to upload your first file to your local FTP server, hooray!

Downloading files from the FTP server

Now that we have successfully uploaded our first file to the FTP server, let's create the feature for downloading the file back onto our computer!

1. First, open mainwindow.ui again and right-click on the Set Folder button. Select Go to slot... and pick the clicked() signal to create a slot function. The slot function is very simple; it will just open up a file selection dialog, but this time it will only let the user select a folder instead since we provided it with a QFileDialog::ShowDirsOnly flag:

void MainWindow::on_setFolderButton_clicked()
{
QString folder = QFileDialog::getExistingDirectory(this, tr("Open Directory"), qApp->applicationDirPath(), QFileDialog::ShowDirsOnly);
ui->downloadPath->setText(folder);
}

2. Then, right click on the List Widget and select Go to slot... This time around, we will pick the itemDoubleClicked(QListWidgetItem*) option instead:

3. When the user double-clicks on an item in the List Widget, the following function will be triggered, which kick-starts the download. The file name can be obtained from the QListWidgetItem object by calling item->text():

void MainWindow::on_fileList_itemDoubleClicked(QListWidgetItem *item)
{
downloadFileName = item->text();

// Check folder
QString folder = ui->downloadPath->text();
if (folder != "" && QDir(folder).exists())
{
QUrl ftpPath;
ftpPath.setUrl(ftpAddress + downloadFileName);
ftpPath.setUserName(username);
ftpPath.setPassword(password);
ftpPath.setPort(ftpPort);

QNetworkRequest request;
request.setUrl(ftpPath);

downloadFileReply = manager->get(request);
connect(downloadFileReply,
SIGNAL(downloadProgress(qint64,qint64)), this,
SLOT(downloadFileProgress(qint64,qint64)));
connect(downloadFileReply, SIGNAL(finished()), this,
SLOT(downloadFileFinished()));
}
else
{
QMessageBox::warning(this, "Invalid Path", "Please set the
download path before download.");
}
}

4. Just like what we did in the upload function, we also used the SIGNAL and SLOT mechanisms here to obtain the progression of the download process as well as the completed signal. The slot function downloadFileProgress() will be called during the download process, and we used that to set the value of our second progress bar:

void MainWindow::downloadFileProgress(qint64 byteReceived,qint64 bytesTotal)
{
qint64 percentage = 100 * byteReceived / bytesTotal;
ui->downloadProgress->setValue((int) percentage);
}

5. Then, the slot function downloadFileFinished() will be called when the file has been completely downloaded. What we're going to do after that is read all the data of the file and save it to our desired directory:

void MainWindow::downloadFileFinished()
{
if(downloadFileReply->error() != QNetworkReply::NoError)
{
QMessageBox::warning(this, "Failed", "Failed to download
file: " + downloadFileReply->errorString());
}
else
{
QByteArray responseData;
if (downloadFileReply->isReadable())
{
responseData = downloadFileReply->readAll();
}

if (!responseData.isEmpty())
{
// Download finished
QString folder = ui->downloadPath->text();
QFile file(folder + "/" + downloadFileName);
file.open(QIODevice::WriteOnly);
file.write((responseData));
file.close();

QMessageBox::information(this, "Success", "File
successfully downloaded.");
}
}
}

6. Build the program now and you should be able to download any files listed on the file list!:

Summary

In this chapter, we learned how to create our own cloud storage client using Qt's Networking module. In the following chapter, we will learn more about the multimedia module and create our own multimedia player from scratch using Qt.

Multimedia Viewers

In the previous chapter, we learned how to upload and download files through cloud storage. Now, in this chapter, we are going to learn how to open these files, specifically media files such as images, music, and videos, using Qt's multimedia module.

In this chapter, we will cover the following topics:

- Revisiting the multimedia module

- The image viewer

- The music player

- The video player

Let's get started!

Revisiting the multimedia module

In this chapter, we will be using the multimedia module again, which we covered previously in Chapter 9, The Camera Module. However, this time we will be using some other parts of the module, so I thought it would be a good idea to dissect the module and see what's inside it.

Dissecting the module

The multimedia module is a really large module that consists of many different parts, that provide very different features and functionality. The main categories are as follows:

- Audio

- Video

- Camera

- Radio

Do note that classes that handle image formats, such as QImage, QPixmap, and so on, are not a part of the multimedia module but rather the GUI module. This is because they are an important part of the GUI that cannot be separated. Despite this, we will still cover the QImage class within this chapter.

Under each category are subcategories that look something like the following:

- Audio:

- Audio output

- Audio recorder

- Video:

- Video recorder

- Video player

- Video playlist

- Camera:

- Camera viewfinder

- Camera image capture

- Camera video recorder

- Radio:

- Radio tuner (for devices that support analog radio)

Each of the classes is designed to fulfill a different purpose. For example, the QSoundEffect is used for playing low latency audio files (such as WAV files). QAudioOutput, on the other hand, outputs raw audio data to a specific audio device, which gives you low-level control over your audio output. Finally, the QMediaPlayer is a high-level audio (and video) player that supports many different high-latency audio formats. You must understand the differences between all the classes before choosing the right one for your project.

The multimedia module in Qt is such a huge beast that often confuses newcomers, but can be advantageous if you know which to choose from. Another issue with the multimedia module is that it may or may not work on your target platform. This is because underneath all these classes are native implementations for specific platforms. If a particular platform does not support a feature, or there is not yet an implementation for it, then you won't be able to use those functionalities.

For more information regarding the different classes provided by Qt's multimedia module, please visit the following link:
https://doc.qt.io/qt-5.10/qtmultimedia-index.html

The image viewer

Digital images have become an important aspect of our daily life. Whether it's a selfie, prom night photo, or a funny meme, we spend a lot of our time looking at digital images. In the following section, we will learn how to create our own image viewer using Qt and C++.

Designing a user interface for the image viewer

Let's get started with creating our first multimedia program. In this section, we will create an image viewer, which, as its name implies, opens up an image file and displays it on the window:

1. Let's open up Qt Creator and create a new Qt Widgets Application project.

2. After that, open up mainwindow.ui and add a Label (name it as imageDisplay) to the central widget, which will serve as the canvas for rendering our image. Then, add a layout to the centralWidget by selecting it and pressing Layout Vertically, located on top of the canvas:

3. You can remove the tool bar and status bar to give space to the Label. Also, set the layout margins of the central widget to 0:

4. After that, double-click on the menu bar and add a File action, followed by Open File underneath it:

5. Then, under the Action Editor, right-click on the Open File action and select Go to slot...:

6. A window will pop out and ask you to pick a signal, so choose triggered() and click OK:

A slot function will be created for you automatically, but we will keep that for the next section. We are done with the user interface, and it is really that simple. Next, let's move on and start writing our code!

Writing C++ code for image viewers

Let's get started by using the following steps:

1. First, open up mainwindow.h and add the following headers:

#include <QMainWindow>
#include <QFileDialog>
#include <QPixmap>
#include <QPainter>

2. Then, add the following variable, called imageBuffer, which will serve as the pointer that points to the actual image data before rescaling. Then, add the functions as well:

private:
Ui::MainWindow *ui;
QPixmap* imageBuffer;

public:
void resizeImage();
void paintEvent(QPaintEvent *event);

public slots:
void on_actionOpen_triggered();

3. Next, open up mainwindow.cpp and initialize the imageBuffer variable in the class constructor:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
imageBuffer = nullptr;
}

4. After that, add the following code to the slot function Qt created for us in the previous section:

void MainWindow::on_actionOpen_triggered()
{
QString fileName = QFileDialog::getOpenFileName(this, "Open Image File", qApp->applicationDirPath(), "JPG (*.jpg *.jpeg);;PNG (*.png)");

if (!fileName.isEmpty())
{
imageBuffer = new QPixmap(fileName);
resizeImage();
}
}

5. The preceding code basically opens up the file-selection dialog, and it creates a QPixmap object with the selected image file. After all that is done, it will call the resizeImage() function, which looks like the following code:

void MainWindow::resizeImage()
{
if (imageBuffer != nullptr)
{
QSize size = ui->imageDisplay->size();
QPixmap pixmap = imageBuffer->scaled(size,
Qt::KeepAspectRatio);

// Adjust the position of the image to the center
QRect rect = ui->imageDisplay->rect();
rect.setX((this->size().width() - pixmap.width()) / 2);
rect.setY((this->size().height() - pixmap.height()) / 2);

QPainter painter;
painter.begin(this);
painter.drawPixmap(rect, pixmap, ui->imageDisplay->rect());
painter.end();
}
}

What the resizeImage() function does is simply copy the image data from the imageBuffer variable and resize the image to fit the window size before displaying it on the window's canvas. You could be opening an image that is way larger than your screen resolution, and we don't want the image to get cropped when opening such a large image file.

The reason why we use the imageBuffer variable is so that we can keep a copy of the original data and not affect the image quality by resizing it many times.

Lastly, we also call this resizeImage() function within the paintEvent() function. Whenever the main window is being resized or restored from a minimized state, paintEvent() will automatically get called, and so will the resizeImage() function, shown as follows:

void MainWindow::paintEvent(QPaintEvent *event)
{
resizeImage();
}

That's it. If you build and run the project now, you should get a pretty neat image viewer that looks like the following:

The music player

In the following section, we will learn how to build our own custom music player using Qt and C++.

Designing a user interface for music players

Let's move on to the next project. In this project, we will be building an audio player using Qt. Perform the following steps:

1. As with the previous project, we will be creating a Qt Widgets Application project.

2. Open up the project file (.pro), and add in the multimedia module:

QT += core gui multimedia

3. We added the multimedia text so that Qt includes classes related to the multimedia module in our project. Next, open up mainwindow.ui, and refer to the following screenshot to construct the user interface:

We basically added a Label at the top, followed by a Horizontal Slider and another Label to show the current time of the audio. After that, we added three Push Buttons at the bottom for the Play button, Pause button, and Stop button. Located at the right-hand side of these buttons is another Horizontal Layout that controls the audio volume.

As you can see, all the Push Buttons have no icon for now, and it's very confusing which button is for what purpose.

1. To add icons to the buttons, let's go to File | New File or Project and select Qt Resource File under the Qt category. Then, create a prefix called icons, and add the icon images to the prefix:

2. After that, add those icons to the Push Button by setting its icon property and selecting Choose Resource.... Then, set the pixmap property of the label, located beside the volume slider, as the volume icon:

3. After you have added the icons to the Push Button and Label, the user interface should look a lot better:

We're done with the user interface; let's move on to the programming part!

Writing C++ code for music players

To write the C++ code for music players, perform the following steps:

1. First and foremost, open up mainwindow.h and add the following headers:

#include <QMainWindow>
#include <QDebug>
#include <QFileDialog>
#include <QMediaPlayer>
#include <QMediaMetaData>
#include <QTime>

2. After that, add the player variable, which is a QMediaPlayer pointer. Then, declare the functions that we're going to define later:

private:
Ui::MainWindow *ui;
QMediaPlayer* player;

public:
void stateChanged(QMediaPlayer::State state);
void positionChanged(qint64 position);

3. Next, open up mainwindow.cpp and initialize the player variable:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

player = new QMediaPlayer(this);
player->setVolume(ui->volume->value());
connect(player, &QMediaPlayer::stateChanged, this, &MainWindow::stateChanged);
connect(player, &QMediaPlayer::positionChanged, this, &MainWindow::positionChanged);
}

The QMediaPlayer class is the main class that is used by our application to play any audio file loaded by it. Thus, we need to know the state of the audio playing and its current position. We can get this information by connecting its stateChanged() and positionChanged() signals to our custom slot functions.

4. The stateChanged() signal allows us to obtain information about the current state of the audio playing. Then, we enable and disable the Push Button accordingly:

void MainWindow::stateChanged(QMediaPlayer::State state)
{
if (state == QMediaPlayer::PlayingState)
{
ui->playButton->setEnabled(false);
ui->pauseButton->setEnabled(true);
ui->stopButton->setEnabled(true);
}
else if (state == QMediaPlayer::PausedState)
{
ui->playButton->setEnabled(true);
ui->pauseButton->setEnabled(false);
ui->stopButton->setEnabled(true);
}
else if (state == QMediaPlayer::StoppedState)
{
ui->playButton->setEnabled(true);
ui->pauseButton->setEnabled(false);
ui->stopButton->setEnabled(false);
}
}

5. As for the positionChanged() and slot functions, we use them to set the timeline slider, as well as the timer display:

void MainWindow::positionChanged(qint64 position)
{
if (ui->progressbar->maximum() != player->duration())
ui->progressbar->setMaximum(player->duration());

ui->progressbar->setValue(position);

int seconds = (position/1000) % 60;
int minutes = (position/60000) % 60;
int hours = (position/3600000) % 24;
QTime time(hours, minutes,seconds);
ui->durationDisplay->setText(time.toString());
}

6. Once you're done, open up mainwindow.ui and right-click on each of the Push Buttons, and select Go to slot... followed by selecting the clicked() signal. This will generate a slot function for each of the Push Buttons. The code for these slot functions is very simple:

void MainWindow::on_playButton_clicked()
{
player->play();
}

void MainWindow::on_pauseButton_clicked()
{
player->pause();
}

void MainWindow::on_stopButton_clicked()
{
player->stop();
}

7. After that, right-click on both of the Horizontal Sliders, and select Go to slot... followed by choosing the sliderMoved() signal, and click OK:

8. The sliderMoved() signal will be called whenever the user drags the slider to change its position. We need to send this position to the media player and tell it to adjust the audio volume or change the current audio position. Do be cautious not to set the default position of your volume slider to zero. Consider the following code:

void MainWindow::on_volume_sliderMoved(int position)
{
player->setVolume(position);
}

void MainWindow::on_progressbar_sliderMoved(int position)
{
player->setPosition(position);
}

9. Then, we need to add File and Open File actions to the menu bar, just like we did in the previous example project.

10. Then, right-click on the Open File action in the Action Editor and select Go to slot... After that, select triggered(), and let Qt generate a slot function for you. Add the following code to the slot function for audio file selection:

void MainWindow::on_actionOpen_File_triggered()
{
QString fileName = QFileDialog::getOpenFileName(this,
"Select Audio File", qApp->applicationDirPath(),
"MP3 (*.mp3);;WAV (*.wav)");
QFileInfo fileInfo(fileName);

player->setMedia(QUrl::fromLocalFile(fileName));

if (player->isMetaDataAvailable())
{
QString albumTitle = player-
>metaData(QMediaMetaData::AlbumTitle).toString();
ui->songNameDisplay->setText("Playing " + albumTitle);
}
else
{
ui->songNameDisplay->setText("Playing " +
fileInfo.fileName());
}

ui->playButton->setEnabled(true);
ui->playButton->click();
}

The preceding simply opens up a file-selection dialog that only accepts MP3 and WAV files. You can add other formats too if you wish, but the supported formats vary between platforms; therefore, you should test it to make sure the format you want to use is supported.

After that, it will send the selected audio file to the media player for preloading. Then, we try to get the music's title from the metadata and display it on the Labelwidget. However, this feature (obtaining metadata) may or may not be supported on your platform, so just in case it won't show up, we replace it with the audio file name. Lastly, we enable the play button and automatically start playing the music.

That's it. If you build and run the project now, you should be able to get a simple but fully functional music player!

The video player

In the previous section, we have learned how to create an audio player. In this chapter, we will further improvise our program and create a video player using Qt and C++.

Designing a user interface for video players

The next example is that of the video player. Since QMediaPlayer also supports video output, we can use the same user interface and C++ code from the previous audio player example, and just make some minor changes to it.

1. First, open project file (.pro) and add in another keyword, called multimediawidgets:

QT += core gui multimedia multimediawidgets

2. Then, open up mainwindow.ui and add a Horizontal Layout (name it as movieLayout) above the timeline slider. After that, right-click on the layout and select Morph into | QFrame. We then set its sizePolicy property to Expanding, Expanding:

3. After that, we set the QFrame's background to black color by setting its styleSheet property, like so:

background-color: rgb(0, 0, 0);

4. The user interface should look something like the following, and we're done:

Writing C++ code for video players

To write the C++ code for video players, we perform the following steps:

1. For mainwindow.h, there aren't many changes to it. All we need to do is to include QVideoWidget in the header:

#include <QMainWindow>
#include <QDebug>
#include <QFileDialog>
#include <QMediaPlayer>
#include <QMediaMetaData>
#include <QTime>
#include <QVideoWidget>

2. Then, open mainwindow.cpp. We must define a QVideoWidget object and set it as the video output target, before adding it to the layout of the QFrame object we just added in the previous step:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

player = new QMediaPlayer(this);

QVideoWidget* videoWidget = new QVideoWidget(this);
player->setVideoOutput(videoWidget);
ui->movieLayout->addWidget(videoWidget);

player->setVolume(ui->volume->value());
connect(player, &QMediaPlayer::stateChanged, this, &MainWindow::stateChanged);
connect(player, &QMediaPlayer::positionChanged, this, &MainWindow::positionChanged);
}

3. In the slot function thatgets called when the Open File action has been triggered, we simply change the file-selection dialog to accept only MP4 and MOV formats. You can add in other video formats too if you wish:

QString fileName = QFileDialog::getOpenFileName(this, "Select Movie File", qApp->applicationDirPath(), "MP4 (*.mp4);;MOV (*.mov)");

That's it. The rest of the code is literally the same as the audio player example. The main difference with this example is that we defined the video output widget, and Qt will handle the rest for us.

If we build and run the project now, we should be getting a really slick video player, like what you see here:

On a windows system, there was a case where the video player would throw an error. This problem is similar to the one reported here: https://stackoverflow.com/questions/32436138/video-play-returns-directshowplayerservicedoseturlsource-unresolved-error-cod
To resolve this error, simply download and install the K-Lite_Codec_Pack which you can find here: https://www.codecguide.com/download_k-lite_codec_pack_basic.htm. After this, the video should play like a charm!

Summary

In this chapter, we have learned how to create our own multimedia players using Qt. What comes next is something quite different from our usual topics. In the following chapter, we will learn how to use QtQuick and QML to create touchscreen-friendly, mobile-friendly, and graphics-oriented applications.

Qt Quick and QML

In this chapter, we are going to learn something very different from the rest of the chapters in this book. Qt consists of two different methods for developing an application. The first method is Qt Widgets and C++, which we have covered in all previous chapters. The second method is using Qt Quick controls and the QML scripting language, which we will be covering in this chapter.

In this chapter, we will cover the following topics :

- Introduction to Qt Quick and QML

- Qt Quick Widgets and Controls

- Qt Quick Designer

- Qt Quick Layouts

- Basic QML Scripting

Are you ready? Let's get started!

Introduction to Qt Quick and QML

In the following section, we will learn what Qt Quick and QML are, and how we can make use of them to develop Qt applications without the need for writing C++ code.

Introducing Qt Quick

Qt Quick is a module in Qt that provides a whole set of user-interface engines and language infrastructure for developing touch-oriented and visual-oriented applications. Instead of using the usual Qt Widgets for user-interface design, developers who choose Qt Quick will be using the Qt Quick objects and controls instead.

Furthermore, developers will be writing their code using the QML language, which has similar syntax to JavaScript, rather than writing in C++ code. You can, however, use the C++ API provided by Qt to extend the QML application by cross-calling each language's functions (calling C++ functions in QML, and vice versa).

Developers can choose which method they prefer for developing their applications by choosing the right option when creating the project. Instead of choosing the usual Qt Widgets Application option, developers can pick Qt Quick Application, which tells Qt Creator to create different starting files and settings for your project that empowers the Qt Quick modules:

When you're creating a Qt Quick Application project, Qt Creator will ask you to choose the Minimal required Qt version for your project:

Once you have selected a Qt version, Qt Quick Designer will determine which features to enable and which widgets will appear on the QML Types window. We will talk more about those in later sections.

Introducing QML

QML (Qt Modeling Language) is a user-interface markup language for designing touch-friendly user interfaces, similar to how CSS works on HTML. Unlike C++ or JavaScript, which are both imperative languages, QML is a declarative language. In declarative programming, you only express the logic in your script without describing its control flow. It simply tells the computer what to do, instead of how to do it. Imperative programing, however, requires statements to specify actions.

When you open up your newly created Qt Quick project, you will see main.qml and MainForm.ui.qml in your project, instead of the usual mainwindow.h and mainwindow.cpp files. You can see this in the project directory in the following screenshot:

This is because the entire project will be mainly running on QML instead of C++. The only C++ file you will see is main.cpp, and all that does is load the main.qml file during the application startup. The code that does this in main.cpp is shown in the following code:

int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;

return app.exec();
}

You should have realized that there are two types of QML files, one with the extension .qml, and another with extension .ui.qml. Even though they are both running on the same syntax and so forth, they serve a very different purpose in your project.

First, the .ui.qml file (with an extra .ui at the beginning) serves as the declarative file for Qt Quick-based user interface design. You can edit a .ui.qml file, using the Qt Quick Designer visual editor, and easily design your application's GUI. You can also add your own code to the file, but there are some limitations on what code they can contain, especially those related to logic code. When you run your Qt Quick application, the Qt Quick engine will read through all the information stored in the .ui.qml file and construct the user interface accordingly, which is very similar to the .ui file used in Qt Widgets applications.

Then, we have another file with only the .qml extension. This file is only used for constructing the logic and functionality in your Qt Quick application, much like the .h and .cpp files used in the Qt Widget application. These two different formats separate the visual definitions of your application from its logic blocks. This allows the developer to apply the same logic code to different user interface templates. You cannot open a .qml file using Qt Quick Designer, since it is not used for GUI declaration. .qml files are written by developers by hand, and they have no restrictions on the QML language features they use.

Let's look at the differences with both of these QML files by first opening up MainForm.ui.qml. By default, Qt Creator will open up the user interface designer (Qt Quick Designer); however, let's move over to code-editing mode by pressing the Edit button on the left panel:

Then, you will be able to see the QML script that forms the user interface you just saw in the design mode. Let's analyze this code to see how QML works compared to C++. The first thing you see in the MainForm.ui.qml is this line of code:

import QtQuick 2.6

This is quite straightforward; we need to import the Qt Quick module with the appropriate version number. Different Qt Quick versions may have different functionalities, and support different widget controls. Sometimes, even the syntax could be slightly different. Please make sure you pick the right version for your project, and that it supports the features you need. If you don't know which version to use, do consider the latest version.

Next, we will see different GUI objects (which we call QML types) being declared between two curly braces. The first one that we see is a Rectangle type:

Rectangle {
property alias mouseArea: mouseArea
property alias textEdit: textEdit

width: 360
height: 360
...

The Rectangle type, in this case, is the window background, much like the central widget used in the Qt Widget Application project. Let's look at the other QML types that are under the Rectangle:

MouseArea {
id: mouseArea
anchors.fill: parent
}

TextEdit {
id: textEdit
text: qsTr("Enter some text...")
verticalAlignment: Text.AlignVCenter
anchors.top: parent.top
anchors.horizontalCenter: parent.horizontalCenter
anchors.topMargin: 20
Rectangle {
anchors.fill: parent
anchors.margins: -10
color: "transparent"
border.width: 1
}
}

The MousArea type, as its name implies, is an invincible shape that detects mouse clicks and touch events. You can basically turn anything into a button by placing a MouseArea on top of it. After that, we also have a TextEdit type, which acts exactly like a Line Edit widget in a Qt Widget Application.

You may have noticed that there are two properties in the Rectangle declaration that carry the alias keyword. These two properties expose the MouseArea and TextEdit types, and allow other QML scripts to interact with them, which we will learn how to do next.

Now, open up main.qml and look at its code:

import QtQuick 2.6
import QtQuick.Window 2.2

Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")

MainForm {
anchors.fill: parent
mouseArea.onClicked: {
console.log(qsTr('Clicked on background. Text: "' +
textEdit.text + '"'))
}
}
}

In the code above, there is a Window type that is only available by importing the QtQuick.Window module. After setting the properties of the Window type, the MainForm type is declared. This MainForm type is actually the entire user interface  we saw previously in MainForm.ui.qml. Since the MouseArea and TextEdit types have been exposed in MainForm.ui.qml, we can now access and make use of them in main.qml.

QML also uses the signal-and-slot mechanism provided by Qt, but in a slightly different form of writing, since we're no longer writing C++ code. For example, we can see onClicked being used in the code above, which is a built-in signal equivalent to clicked() in a Qt Widgets Application. Since the .qml file is the place where we define the application logic, we can define what happens when onClicked is being called. On the other hand, we cannot do the same in .ui.qml since only visual-related code is allowed in it. You will get warnings from Qt Creator if you try to write logic-related code in a .ui.qml file.

Just like the Qt Widgets Application, you can also build and run the project the same way as before. The default example application looks something like this:

You might realize that the build process is pretty fast. This is because QML code doesn't get compiled into binary by default. QML is an interpreted language, just like JavaScript, and thus it doesn't need to be compiled in order for it to be executed. All the QML files will just get packed into your application's resource system during the build process. Then, the QML files will be loaded and interpreted by the Qt Quick engine once the application is started.

However, you can still choose to compile your QML scripts into binary, using the Qt Quick Compiler program included in Qt, to make the code execution slightly faster than usual. It is an optional step that is not required unless you are trying to run your application on an embedded system that has very limited resources.

Now that we have understood what Qt Quick and QML language are, let's take a look at all the different QML types provided by Qt.

Qt Quick widgets and controls

In Qt Quick's realm, widgets and controls are known as QML types. By default, Qt Quick Designer provides us with a set of basic QML types. You can also import additional QML types that come with different modules. Furthermore, you can even create your own custom QML types if none of the existing ones fit, your needs.

Let's take a look at what QML types come with Qt Quick Designer by default. First off, here are the QML types under the Basic category:

Let's have a look at the different options:

- Border Image: Border Image is a QML type that is designed to create scalable rectangular shapes that can maintain their corner shapes and borders.

- Flickable: Flickable is a QML type that contains all its children types, and, displays them within its clipping area. Flickable has also been extended and used by the ListView and GridView types for scrolling long content. It can also be moved by a touchscreen flick gesture.

- Focus Scope: Focus Scope is a low-level QML type that is used to facilitate the construction of other QML types that can acquire keyboard focus when being pressed or released. We usually don't directly use this QML type, but rather use other types that are directly inherited from it, such as GroupBox, ScrollView, StatusBar, and so on.

- Image: The Image type is pretty much self-explanatory. It loads an image either locally or from a network.

- Item: The Item type is the most basic QML type for all visual items in Qt Quick. All the visual items in Qt Quick inherit from this Item type.

- MouseArea: We have seen the example usage of the MouseArea type in the default Qt Quick Application project. It detects mouse clicks and touch events within a predefined area, and calls the clicked signal whenever it detects one.

- Rectangle: A Rectangle QML type is pretty similar to the Item type, except it has a background that can be filled with solid color or a gradient. Optionally, you can also add a border to it with its own color and thickness.

- Text: The Text QML type is also pretty self-explanatory. It simply displays a line of text on the window. You can use it to display both plain and rich text with a specific font family and font size.

- Text Edit: The Text Edit QML type is equivalent to the Text Edit widget in Qt Widgets Application. It allows the user to key in the text when being focused. It can display both plain and formatted text, which is very different from the Text Input type.

- Text Input: The Text Input QML type is equivalent to the Line Edit widget in Qt Widgets Application, in that it can only display a single line of editable plain text, which is different from the Text Edit type. You can also apply an input constraint to it through a validator or input mask. It can also be used for password input fields by setting the echoMode to Password or PasswordEchoOnEdit.

The QML types that we have discussed here are the most basic ones that come with Qt Quick Designer by default. These are also the basic building blocks used for constructing some other more complex QML types. There are many additional modules that come with Qt Quick that we can import into our project, for example, if we add the following line to our MainForm.ui.qml file:

import QtQuick.Controls 2.2

A bunch of additional QML types will then appear on your Qt Quick Designer when you switch over to Design mode:

We won't go through all these QML types one by one, as there are too many of them. If you are interested in learning more about these QML types, please visit the following link: https://doc.qt.io/qt-5.10/qtquick-controls-qmlmodule.html

Qt Quick Designer

Next, we will look at the Qt Quick Designer layout for the Qt Quick Application project. When you open up a .ui.qml file, Qt Quick Designer, the designer tool included in the Qt Creator toolset, will be launched automatically for you.

Those of you who have followed all the example projects since the very first chapter of this book may realize the Qt Quick Designer looks a bit different from the one we have been using all this time. This is because the Qt Quick project is very different from the Qt Widgets project, so naturally the designer tool should also look different to suit its needs.

Let's look at how the Qt Quick Designer looks in the Qt Quick project:

1. Library: The Library window displays all the QML types available for the current project. You can click and drag it to the canvas window to add it to your UI. You can also create your own custom QML type and display it here.

2. Resources: The Resources window displays all the resources in a list, which can then be used in your UI design.

3. Imports: The Imports window allows you to import different Qt Quick modules into your current project.

4. Navigator: The Navigator window displays the items in the current QML file as a tree structure. It's similar to the object operator window in the Qt Widgets Application project.

5. Connections: The Connections window consists of several different tabs: Connections, Bindings, Properties, and Backends. These tabs allow you to add Connections (signal-and-slot), Bindings, and Properties to your QML file, without switching over to Edit mode.

6. State Pane: State pane displays the different states in the QML project that typically describe UI configurations, such as the UI controls, their properties and behavior, and available actions.

7. Canvas: Canvas is the working area where you design your application UI.

8. Properties Pane: Similar to the property editor we used in Qt Widgets Application projects, the Properties pane in the QML designer displays the properties of the selected item. You can immediately see the result in the UI after changing the values here.

Qt Quick layouts

Just like the Qt Widget applications, a layout system also exists in Qt Quick applications. The only difference is it's called the Positioners in Qt Quick:

The most noticeable similarity is the Column and Row positioners. These two are exactly the same as the Vertical Layout and Horizontal Layout in Qt Widgets Application. Besides that, the Grid positioner is also the same as the Grid Layout.

The only extra thing in Qt Quick is the Flow positioner. The items contained within the Flow positioner arrange themselves like words on a page, with items arranged in lines along one axis, and lines of items placed next to each other along another axis.

Basic QML scripting

In the following section, we will learn how to create our very first Qt Quick application using Qt Quick Designer and QML!

Setting up the project

Without further ado, let's put our hands on QML and create a Qt Quick application ourselves! For this example project, we are going to create a dummy login screen using Qt Quick Designer and a QML script. First, let's open up Qt Creator and create a new project by going to File | New File or Project...

After that, select Qt Quick Application and press Choose.... After that, press Next all the way until the project is created. We are just going to use all the default settings for this example project, including the Minimal required Qt version:

Once the project has been created, we need to add a few image files to our project, so that we can use them later:

You can get the source files (including these images) at our GitHub page: http://github.com/PacktPublishing/Hands-On-GUI-Programming-with-C-QT5

We can add these images to our project by right-clicking on the qml.qrc file in the Project pane and selecting Open in Editor. Add a new prefix called images, and add all the image files into that prefix:

After that, open up MainForm.ui.qml, and delete everything in the QML file. We start all over by adding an Item type to the canvas, set its size to 400 x 400, and call it the loginForm. After that, add an Image type underneath it, and call it background. We then apply the background image to the Image type, and the canvas now looks like this:

Then, add a Rectangle type under the Image type (background), and open up the Layout tab in the Properties pane. Enable both the vertical and horizontal anchor options. After that, set the width to 402, the height to 210, and the vertical anchor margin to 50:

Following that, we set the rectangle's color to #fcf9f4 and border color to #efedeb, then set the border value to 1. The user interface so far looks something like this:

Next, add an Image QML type under the rectangle, and set its anchor settings to top anchor and horizontal anchor. We then set its top anchor margin to -110 and apply the logo image to its image source property. You can turn the QML type's bounding rectangle and stripes on and off by clicking on the little button located on top of your canvas, so that it's easier to look at the result, especially when your canvas is full of stuff:

Then, we add three Rectangle types to the canvas under the loginRect rectangle, and call them emailRect, passwordRect, and loginButton. The anchor settings for the rectangles are shown as follows:

Then, we set the border value of both the emailRect and passwordRect to 1, the color to #ffffff and the bordercolor to #efedeb. As for the loginButton, we set the border to 0, the radius to 2 and the color to #27ae61. The login screen now looks like the following:

Looks good so far. Next, we're going to add a TextInput, Image, MouseArea, and a Text QML type to both the emailRect and the passwordRect. Since there are many QML types here, I will list the properties that need to be set:

- TextInput:

- Selection color set to #4f0080

- Enable left anchor, right anchor, and vertical anchor

- Left anchor margin 20, right anchor margin 40 and vertical margin 3

- Set echoMode to Password for password input only

- Image:

- Enable right anchor and vertical anchor

- Right anchor margin set to 10

- Set image source to email icon or password icon respectively

- Set image fill mode to PreserveAspectFit

- MouseArea:

- Enable fill parent item

- Text:

- Set the text property to E-Mail and Password respectively

- Text color set to #cbbdbd

- Text alignment set to Left and Top

- Enable left anchor, right anchor, and vertical anchor

- Left anchor margin 20, right anchor margin 40, and vertical margin -1

Once you're done, add a MouseArea and Text to the loginButton as well. Enable fill parent item for the MouseArea, and enable both vertical and horizontal anchors for the Text QML type. Then, set its text property to LOGIN.

You don't have to follow all my steps by 100%, they are just a guideline for you to achieve a similar result as the screenshot above. However, it's better for you to apply your own design and create something unique!

Phew! After the long process above, our login screen should now look something like this:

One last thing we need to do before moving on to main.qml is to expose some of the QML types in our login screen, so that we can link it to our main.qml file for logic programming. We can, in fact, do this directly on the designer tool. All you need to do is to click on the small rectangle icon located next to the object name, and make sure the three lines on the icon are penetrating the rectangular box, like this:

The QML types that we need to expose/export are emailInput (TextInput), emailTouch (MouseArea), emailDisplay (Text), passwordInput (TextInput), passwordTouch (MouseArea), passwordDisplay (Text), and loginMouseArea (MouseArea). Once you have done all that, let's open up main.qml.

At first, our main.qml should look something like this, which will just open an empty window:

import QtQuick 2.6
import QtQuick.Window 2.2

Window {
id: window
visible: true
width: 800
height: 600
title: qsTr("My App")
}

After that, add in the MainForm object, and set its anchor setting to anchors.fill: parent. Then, print out a line of text, Login pressed, on the console window when the loginButton is clicked (or touched, if running on the touch device):

Window {
id: window
visible: true
width: 800
height: 600
title: qsTr("My App")

MainForm
{
anchors.fill: parent

loginMouseArea.onClicked:
{
console.log("Login pressed");
}
}
}

After that, we are going to program the behavior when the MouseArea on the email input is clicked/touched. Since we are manually creating our own text field, instead of using the TextField QML type provided by the QtQuick.Controls module, we must manually hide and show the E-Mail and Password text displays, as well as changing the input focus when the user  clicks/touches down on the MouseArea.

The reason why I chose not to use the TextField type is that I can hardly customize the TextField's visual presentation, so why don't I create my own? The code for doing manual focus for the email input looks like the following:

emailTouch.onClicked:
{
emailDisplay.visible = false;      // Hide emailDisplay
emailInput.forceActiveFocus();     // Focus emailInput
Qt.inputMethod.show();       // Activate virtual keyboard
}

emailInput.onFocusChanged:
{
if (emailInput.focus == false && emailInput.text == "")
{
emailDisplay.visible = true;   // Show emailDisplay if
emailInput is empty when loses focus
}
}

After that, do the same for the password field:

passwordTouch.onClicked:
{
passwordDisplay.visible = false;   // Hide passwordDisplay
passwordInput.forceActiveFocus();  // Focus passwordInput
Qt.inputMethod.show();       // Activate virtual keyboard
}

passwordInput.onFocusChanged:
{
if (passwordInput.focus == false && passwordInput.text == "")
{
passwordDisplay.visible = true;      // Show passwordDisplay if
passwordInput is empty when loses focus
}
}

That's it; we're done! You can now compile and run the program. You should get something like this:

If you're not seeing the images, and are getting error messages that say Qt is unable to open the images, please go back to your MainForm.ui.qml and add in the prefix image/ to the front of the source property. This is because Qt Quick Designer loads the images without the prefix, while your final program needs the prefix. After you have added the prefix, you may realize you no longer see the images getting displayed on Qt Quick Designer, but it will work just fine in your final program.

I'm not sure if this is a bug or if they intended it like that. Hopefully, Qt's developers can get it fixed, and we won't have to do that extra step anymore. That's it; hopefully, you have understood the similarities and differences between Qt Widgets Application and Qt Quick Application. You can now pick the best option from the two to fit your project's needs!

Summary

In this chapter, we have learned what Qt Quick is and how to create a program using the QML language. In the following chapter, we are going to learn how to export our Qt project to different platforms without much hassle. Let's go!

Cross-Platform Development

Qt has been known for its cross-platform capability since its first release. It was also one of the main goals of the founders when they decided to create this framework, long before it was taken over by Nokia, and later The Qt Company.

In this chapter, we will cover the following topics:

- Compilers

- Build settings

- Deploying to PC platforms

- Deploying to mobile platforms

Let's get started.

Understanding compilers

In this chapter, we will learn about the process of generating an executable file from a Qt project. This process is what we call compile or build. The tool that is used for this purpose is called a compiler. In the following section, we will learn what a compiler is and how to use it to generate an executable file for our Qt project.

What is a compiler?

When we develop an application, either using Qt or any other software development kit, we often have to compile our project into an executable, but what is actually going on when we're compiling our project?

A compiler is a piece of software that transforms computer code written in a high-level programming language or computer instructions into a machine code or lower-level form that can be read and executed by a computer. This low-level machine code is very different depending on the operating system and computer processor you're running, but you don't have to worry about it as the compiler will convert it for you.

That means all you need to worry about is writing your logic code in a human-readable programming language, and let the compiler do the job for you. By using different compilers, theoretically, you should be able to compile your code into executable programs that can be run on different operating systems and hardware. I'm using the word theoretically here because in practice it's actually much more difficult than just using different compilers, you may also need to implement libraries that support the target platform. However, Qt has already handled all this for you, so you don't have to do the extra work.

In the current version, Qt supports the following compilers:

- GNU Compiler Collection (GCC): GCC is a compiler for Linux and macOS

- MinGW (Minimalist GNU for Windows): MinGW is a native software port of GCC and GNU Binutils (binary utilities) for developing applications on Windows

- Microsoft Visual C++ (MSVC): Qt supports MSVC 2013, 2015, and 2017 for building Windows applications

- XCode: XCode is the primary compiler used by developers who develop applications for macOS and iOS

- Linux ICC (Intel C++ Compiler): Linux ICC is a set of compilers of C and C++ compilers developed by Intel for Linux application development

- Clang: Clang is a C, C++, Objective C, and Objective C++ frontend for the LLVM compiler for Windows, Linux, and macOS

- Nim: Nim is the Nim compiler for Windows, Linux, and macOS

- QCC: QCC is the interface for compiling C++ applications for the QNX operating system

Build automation with Make

In software development, Make is a build automation tool that automatically builds executable programs and libraries from source code by reading configuration files called Makefiles that specify how to derive the target platform. In a nutshell, a Make program generates build configuration files and uses them to tell the compiler what to do before generating the final executable program.

Qt supports two types of Make programs:

- qmake: It is the native Make program developed by the Qt team. It works best on Qt Creator, and I strongly recommend using it for all Qt projects.

- CMake: On the other hand, although this is a very powerful build system, it doesn't do all the things that qmake does specifically for a Qt project, such as:

- Running the Meta Object Compiler (MOC)

- Telling the compiler where to look for Qt headers

- Telling the linker where to look for Qt libraries

You have to do the preceding steps manually on CMake in order to successfully compile a Qt project. You should use CMake only if:

- You're working on a non-Qt project but wish to use Qt Creator for writing the code

- You're dealing with a massive project that requires complex configurations, which qmake simply cannot handle

- You really love to use CMake and you know exactly what you're doing

Qt is really flexible when it comes to choosing the right tools for your project. It doesn't stick to just its own build system and compiler. It gives the developers freedom to choose what suits best for their projects.

Build settings

Before a project is compiled or built, the compiler needs to know several details before proceeding. These details are known as the build settings, which are a very important aspect of the compilation process. In the following section, we will learn what the build settings are and how we can configure them in an accurate manner.

Qt Project (.pro) File

I'm sure you already know about the Qt Project File since we have mentioned it countless times throughout the book. A .pro file is actually the project file used by qmake to build your application, library, or plugin. It contains all the information, such as links to the headers and source files, libraries required by the project, custom-build processes for different platforms/environments, and so on. A simple project file could look something like this:

QT += core gui widgets

TARGET = MyApp
TEMPLATE = app

SOURCES +=
main.cpp
mainwindow.cpp

HEADERS +=
mainwindow.h

FORMS +=
mainwindow.ui

RESOURCES +=
resource.qrc

It simply tells qmake which Qt modules should be included in the project, what the name of the executable program is, what's the type of the application, and finally the links to the header files, source files, form declaration files, and resource files that need to be included in the project. All of this information is crucial in order for qmake to generate the configuration files and successfully build the application. For a more complex project, you may want to configure your project differently for different operating systems. This can also be done easily in the Qt Project File.

To learn more about how you can configure your project differently for different operating systems, please refer to the following link: http://doc.qt.io/qt-5/qmake-language.html#scopes-and-conditions.

Comment

You can add your own comments in the project file to remind yourself of the purpose of adding a specific line of configuration, so that you won't forget why you added a line after not touching it for a while. A comment starts with the hash symbol (#) after which you can write anything since the build system will simply ignore the entire line of text. For example:

# The following define makes your compiler emit warnings if you use
# any feature of Qt which has been marked as deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

You can also add dash lines or use spaces to make your comment stand out from others:

#-------------------------------------------------
#
# Project created by QtCreator 2018-02-18T01:59:44
#
#-------------------------------------------------

Modules, configurations, and definitions

You can add different Qt modules, configuration options, and definitions to your project. Let's take a look at how we can achieve these. To add additional modules, you simply add the module keyword behind QT +=, like so:

QT += core gui sql printsupport charts multimedia

Or you can also add in a condition in front to determine when to add a specific module to your project:

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

You can also add configuration settings to your project. For example, we want to specifically ask the compiler to follow the 2011 version of the C++ specifications (known as C++11) when compiling our project, as well as making it a multithreaded application:

CONFIG += qt c++11 thread

You must use +=, not =, or qmake will not be able to use Qt's configuration to determine the settings needed for your project. Alternatively, you can also use -= to remove a module, configuration, and definition from your project.

As for adding definitions (or variables) to our compiler, we use the DEFINES keyword, like so:

DEFINES += QT_DEPRECATED_WARNINGS

Qmake adds the values of this variable as a compiler C preprocessor macro (-D option) before compiling your project. The earlier definition tells the Qt compiler to emit warnings if you have used any feature of Qt that has been marked as deprecated.

Platform-specific settings

You can set different configurations or settings for different platforms, since not every setting can fit all use cases. For example, if we want to include different header paths for different operating systems, we can do the following:

win32:INCLUDEPATH += "C:/mylibs/extra headers"
unix:INCLUDEPATH += "/home/user/extra headers"

Alternatively, you can also put your settings in curly braces which behave like the if statements in a programming language:

win32 {
SOURCES += extra_code.cpp
}

You can check out all the settings you can use in your project file by visiting the following link: http://doc.qt.io/qt-5/qmake-variable-reference.html.

Deploying to PC platforms

Let's move on to learn how to deploy our applications on platforms such as Windows, Linux, and macOS.

Windows

In this section, we will learn how to deploy our application to different operating systems. Even though Qt supports all major platforms out of the box, there might be some configurations which you need to set in order to make your application easily deployable to all platforms.

The first operating system we're going to cover is the most common one, Microsoft Windows.

Starting from Qt 5.6, Windows XP is no longer supported by Qt.

There could be certain plugins that may not work properly on the Windows version you're trying to deploy, so do check out the documentation before you decide to work on your project. However, it's safe to say most of the features will work out of the box on Qt.

By default, the MinGW 32-bit compiler comes together with Qt when you're installing it to your Windows PC. Unfortunately, it doesn't support 64-bit by default unless you compile Qt from source. If you need to build 64-bit applications, you can consider installing the MSVC version of Qt alongside the Microsoft Visual Studio. Microsoft Visual Studio can be obtained for free from the following link: https://www.visualstudio.com/vs.

You can set up your compiler settings in Qt Creator by going to Tools | Options, then go to the Build & Run category and select the Kits tab:

As you can see, there are multiple kits that run on different compilers, in which you can configure. By default, Qt already comes with five kits—one for Android, one for MinGW, and three for MSVC (version 2013, 2015, and 2017). Qt will automatically detect the existence of these compilers and configure these settings for you accordingly.

If you have not installed Visual Studio or Android SDK, there will be a red icon with an exclamation mark appearing in front of the kit option. After you have installed the compiler you need, try restarting Qt Creator. It will now detect the newly installed compilers. You should have no problem compiling for the Windows platform as Qt will handle the rest for you. We will talk more about the Android platform in another section.

Once you have compiled your application, open up the folder in which you installed Qt. Copy the relevant DLL files to your application folder, and pack it together before distributing it to your users. Without these DLL files, your users may not be able to run the Qt application.

For more information, please visit the following link: http://doc.qt.io/qt-5/windows-deployment.html.

As for setting a custom icon for your application, you must add the following code to your project (.pro) file:

win32:RC_ICONS = myappico.ico

The preceding code only works on Windows platforms, which is why we have to add the win32 keyword before it.

Linux

Linux (or GNU/Linux) in general is considered a major operating system that dominates the cloud/server market. Since Linux is not a single operating system (Linux is offered by different vendors in the form of different Linux distributions that are not entirely compatible) like Windows or macOS, it is very hard for developers to build their applications and expect them to run flawlessly on different Linux distributions (distros). However, if you develop your Linux application on Qt, there is a high chance that it will work on most distributions, if not on all of the major distros out there, as long as the Qt library exists on the target system.

The default kit selection on Linux is much simpler than Windows. Since a 64-bit application has been mainstream and standard on most Linux distros for some time now, we only need to include the GCC 64-bit compiler when installing Qt. There is also an option for Android, but we will talk more about it later:

If you are compiling your Linux application on Qt Creator for the first time, I'm pretty sure you will get the following error:

This is because you have not installed the relevant tools required to build Linux applications, such as Make, GCC, and other programs.

Different Linux distros have a slightly different method to install programs, but I won't be explaining every single one of them here. In my case, I'm using an Ubuntu distro, so I did was first opened up the terminal and typed the following command to install the build-essential package which includes Make and GCC:

sudo apt-get install build-essential

The preceding command only works on distros that inherit from Debian and Ubuntu, and it may not work on other distributions such as Fedora, Gentoo, Slackware, and so on. You should search for the appropriate command used by your Linux distro to install these packages, as shown in the following screenshot:

Once you have installed the appropriate packages, restart Qt Creator and go to Tools | Options. Then, go to the Build & Run category and open up the Kits tab. You should now be able to select the compilers for both C and C++ options for your Desktop kit:

However, you might get another error that says cannot find -lGL when trying to compile again:

This is because Qt is trying to look for the OpenGL libraries, and it can't find them on your system. This can be easily fixed by installing the Mesa development library package with the following command:

sudo apt-get install libgl1-mesa-dev

Again, the preceding command only works on Debian and Ubuntu variants. Please look for the appropriate command for your Linux distro if you're not running one of the Debian or Ubuntu forks:

Once the package has been installed, you should be able to compile and run your Qt application without any problem:

As for using one of the other compilers that are less popular, such as Linux ICC, Nim, or QCC, you must set it manually by clicking on the Add button located on the right-hand side of the Kits interface, then key in all the appropriate settings to get it to work. Most people do not use these compilers, so we'll just skip them for now.

When it comes to distributing Linux applications, it's a lot more complicated than Windows or macOS. This is owing to the fact that Linux is not a single operating system, but rather a bunch of different distros with their own dependencies and configurations, which makes distributing programs very difficult.

The safest way is to compile your program statically, which has its own pros and cons. Your program will become really huge in size, and that makes updating software a great burden to users who have slow internet connections. Other than that, the Qt license also forbids you from building statically if you're not doing an open source project and do not have a Qt commercial license. To learn more about Qt's licensing options, please visit the following link: https://www1.qt.io/licensing-compariso.n.

Another method is to ask your users to install the right version of Qt before running your application, but that will yield a ton of problems on the user side since not every user is very tech savvy and has the patience to go through all those hassles to avoid the dependency hell.

Therefore, the best way is to distribute the Qt library alongside your application, just like we did on the Windows platform. The library might not work on some of the Linux distros (rarely the case, but there is a slight possibility), but that can be easily overcome by creating a different installer for different distros, and everyone's happy now.

However, due to security reasons, a Linux application doesn't usually look for its dependencies in its local directory by default. You must use the $ORIGIN keyword in the executable's rpath setting in your qmake project (.pro) file:

unix:!mac{
QMAKE_LFLAGS += -Wl,--rpath=$$ORIGIN
QMAKE_RPATH=
}

Setting the QMAKE_RPATH clears the default rpath setting for the Qt libraries. This allows for bundling the Qt libraries with the application. If you want the rpath to include the path to the Qt libraries, don't set QMAKE_RPATH.

After that, just copy all the library files from the Qt installation folder to your application's folder and remove its minor version numbers from the filename. For example, rename libQtCore.so.5.8.1 to libQtCore.so.5 and now it should be able to get detected by your Linux application.

As for application icons, you can't apply any icon to Linux applications by default as it is not supported. Even though some desktop environments such as KDE and GNOME do support application icons, the icon has to be installed and configured manually, which is not very convenient to the users. It may not even work on some user's PC since every distro works a little bit differently than the others. The best way to set icons for your application is to create a desktop shortcut (symlink) during installation and apply the icon to the shortcut.

macOS

In my opinion, macOS is the single most centralized operating system in the software world. Not only is it designed to run only on the Macintosh machines, you are also required to download or buy software only from the Apple App Store.

No doubt this has caused an uneasy feeling for some people who care about freedom of choice, but on the other hand it also means that developers have less problems to deal with when it comes to application building and distribution.

Other than that, macOS applications behave pretty much similar to a ZIP archive, where each and every application has its own directory that carries the appropriate libraries with it. Therefore, there is no need for the users to install the Qt libraries on their operating system beforehand and everything just works out of the box.

As for the Kit Selection, Qt for macOS supports kits for Android, clang 64-bit, iOS, and iOS Simulator:

As of Qt 5.10 and above, Qt no longer supports 32-bit builds for macOS. Also, Qt does not support OS X on PowerPC; and since Qt uses Cocoa internally, building for Carbon is also not possible, please be aware of that.

Before compiling your macOS applications, please install Xcode from the App Store before proceeding. Xcode is an integrated development environment for macOS, containing a suite of software development tools developed by Apple for developing software for macOS and iOS. Once you have installed Xcode, Qt Creator will detect its existence and automatically set the compiler settings for you, which is great:

Once you have compiled your project, the resulting executable program is a single app bundle that can be easily distributed to your users. Since all the library files are packed within the application bundle, it should work out of the box on the user's PC.

Setting application icons for Mac is quite a simple task. Just add the following line of code to your project (.pro) file and we're good to go:

ICON = myapp.icns

Do note that the icon format is .icns, instead of .ico, which we usually use for Windows.

Deploying to mobile platforms

Apart from platforms such as Windows, Linux, and macOS, mobile platforms do hold equal importance. There are many developers who would like to deploy their applications to mobile platforms. Let's see how that's done. We will cover two major platforms, they are, iOS and Android.

iOS

Deploying Qt applications on iOS is really simple and easy. Just like we did previously for macOS, you need to first install Xcode on your development PC:

Then, restart Qt Creator. It should now detect the existence of Xcode, and it will then automatically set the compiler settings for you:

After that, just plug in your iPhone and hit the Run button!

Building iOS applications on Qt is really that easy. However, distributing them is not. This is because iOS is a very closed ecosystem, just like a walled garden. You are not only required to register as an app developer with Apple, you also need to code sign your iOS applications before you're able to distribute it to your users. There is no way you can avoid these steps if you want to build your apps for the iOS.

You can learn more about these by visiting the following link: https://developer.apple.com/app-store/submissions.

Android

Even though Android is a Linux-based operating system, it is very different when comparing it  to the Linux platforms that you run on your PC. To build Android applications on Qt, you must first install Android SDK, Android NDK, and Apache ANT to your development PC, regardless of whether you're running Windows, Linux, or macOS:

These three packages are essential when it comes to building Android applications on Qt. Once all of them have been installed, restart Qt Creator, and voilà, it should have now detected their existence and the build settings will now have been set automatically:

Lastly, you can configure your Android app by opening the AndroidManifect.xml file with Qt Creator:

You can set everything here, such as the package name, version code, SDK version, application icon, permissions, and so on.

Android is an open system compared to iOS, so there is no need for you to do anything before you're able to distribute your applications to your users. You can, however, choose to register as a Google Play developer if you want to distribute your apps on the Google Play Store.

Summary

In this chapter, we have learned how to compile and distribute our Qt applications for different platforms, such as Windows, Linux, macOS, Android, and iOS. In the next chapter, we will learn different debugging methods that could save development time. Let's check it out!

Testing and Debugging

We often see the word debug when reading tutorials or articles related to programming. But do you know what debugging means? A bug in programming terms means an error or defect within a computer program that prevents the software from operating correctly, which often leads to incorrect output or even a crash.

In this chapter, we will cover the following topics and learn how to debug our Qt project:

- Debugging techniques

- Debuggers supported by Qt

- Unit testing

Let's get started.

Debugging techniques

Technical issues occur all the time during the development process. To tackle these problems, we need to find out all these issues and solve them before releasing our application to the users, so as not to affect the company/team's reputation. The method used to look for technical issues is called debugging. In this section, we will look at the common debugging techniques used by professionals to ensure their program is reliable and of a high quality.

Identifying the problem

The most important thing when it comes to debugging your program, regardless of programming language or platform, is to know which part of your code is causing the problem. There are several ways you can identify your problematic code:

- Ask the user at which point the bug happened; for example, which button was pressed, what were the steps leading to the crash, and so on.

- Comment away part of your code, then build and run the program again to check whether the problem still occurs or not. If it still does, continue to comment out more code until you find the problematic line of code.

- Use the built-in debugger to check for the variable changes within your targeted function by setting a data breakpoint. You can easily spot if one of your variables has changed to an unexpected value or an object pointer has become an undefined pointer.

- Make sure all the libraries that you included in the installer for your users have matching version numbers with the ones used in your project.

Print variables using QDebug

You can also print out the value of a variable to the application output window, using the QDebug class. QDebug is quite similar to std::cout in the standard library, but the advantage of using QDebug is that since it is part of Qt, it supports Qt classes out of the box, and it is able to output its value without the need for any conversion.

To enable QDebug, we must first include its header:

#include <QDebug>

After that, we can call qDebug() to print out variables to the application output window:

int amount = 100;
qDebug() << "You have obtained" << amount << "apples!";

The result will look like this:

By using QDebug, we will be able to check if our function is running correctly. You can just comment out the particular line of code that contains qDebug() after you have finished checking for the problem.

Setting breakpoints

Setting a breakpoint is another good way to debug your program. When you right-click on the line number of your script in Qt Creator, you will get a pop-up menu with three options, which you can see in the following screenshot:

The first option is called Set Breakpoint at Line..., which lets you set a breakpoint at a specific line on your script. A red dot icon will appear beside the line number once you have created a breakpoint:

The second option is called Set Message Tracepoint at Line..., which prints a message when the program reaches this particular line of code. An eye icon will appear beside the line number once you have created a breakpoint:

The third option is Toggle Bookmark, which lets you set a bookmark for your own reference. Let's create a function called test() to try out the breakpoint:

void MainWindow::test()
{
int amount = 100;
amount -= 10;
qDebug() << "You have obtained" << amount << "apples!";
}

After that, we call the test() function at the MainWindow constructor:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
test();
}

Then, press the start debug button located at the bottom left of your Qt Creator window:

You may get an error message that looks like this:

In this case, make sure your project kit has a debugger linked to it. If this error still occurs, close your Qt Creator, go to your project folder and delete the .pro.user file. After that, open up your project with Qt Creator. Qt Creator will reconfigure your project again, and the debug mode should work by now.

Let's add two breakpoints to our code and run it. Once our program has been started, we will see a yellow arrow appearing on top of the first red dot:

This means that the debugger has stopped at the first breakpoint. The Locals and Expression window, which is located on the right-hand side of your Qt Creator, will now display the variable along with its value and type here:

In the preceding image, you can see the value is still at 100 because at this point the minus operation has not  yet been run. The next thing we need to do is to click on the Step Into button on top of the Stack window located at the bottom of your Qt Creator:

After that, the debugger will move on to the next breakpoint, and here we can see the value has decreased to 90 as expected:

You can use this method to easily examine your application. To delete a breakpoint, you just have to click on the red dot icon again.

Do note that you must run this in the debug mode. This is because when compiling in debug mode, additional debugging symbols will be embedded into your application or library that allow your debugger to gain access to information from the source code of the binary, such as the name of identifiers, variables, and routines. This is also the reason why your application or library will be much bigger in file size if compiled in debug mode.

Debuggers supported by Qt

There are different types of debuggers that are supported by Qt. Depending on the platform and compiler you're running for your project, the debugger used will also be different. The following is the list of debuggers commonly supported by Qt:

- Windows (MinGW): GDB (GNU Debugger)

- Windows (MSVC): CDB (Debugging Tools for Windows)

- macOS: LLDB (LLVM Debugger), FSF GDB (Experimental)

- Linux: GDB, LLDB (Experimental)

- Unix (FreeBSD, OpenBSD, etc.): GDB

- Android: GDB

- iOS: LLDB

Debugging for PC

With GDB (GNU Debugger), there is no need for any manual setup if you're using MinGW compiler on Windows, as it usually comes together with your Qt installation. If you're running other operating systems such as Linux, you may need to install it manually before linking it up with your Qt Creator. Qt Creator detects the existence of GDB and links it with your project automatically. If it doesn't, you can easily find the GDB executable located in your Qt directory and link it by yourself.

CDB (Debugging Tools for Windows) on the other hand, needs to be installed manually on your Windows machine. Do note that Qt doesn't support the built-in debugger of Visual Studio. Therefore, you need to install the CDB debugger separately by selecting an optional component called debugging tools for windows while installing the Windows SDK. Qt Creator also normally would recognize the existence of CDB and put it on the debugger list under the Debuggers Options page. You can go to Tools | Options | Build and Run | Debuggers to look for the settings as seen in the following screenshot:

Debugging for Android devices

Debugging for Android devices is slightly more complicated than for a PC. You must install all the necessary packages for Android development, such as JDK (version 6 or later), Android SDK, and Android NDK. Then you also need the Android Debug Bridge (ADB) driver on the Windows platform to enable USB debugging, since the default USB driver on Windows does not allow for debugging.

Debugging for macOS and iOS

As for macOS and iOS, the debugger used is LLDB (LLVM Debugger), which comes with Xcode by default. Qt Creator will also recognize its existence and link it with your project automatically.

Every debugger is a little different from another and may behave differently on Qt Creator. You can also run the non-GDB debuggers on their respective IDE (Visual Studio, XCode, and so on), if you are familiar with those tools and know what you're doing.

If you need to add other debuggers to your project, you can go over to Tools | Options | Build and Run | Kits and click Clone to copy an existing kit. Then, under the Debuggers tab, click on the Add button to add a new debugger selection:

In the Name field, type in the descriptive name for the debugger so you can easily remember its purpose. Then, specify the path to the debugger binary in the Path field so that Qt Creator knows which executable to run when you start the debugging process. Other than that, the Type and Version fields are used by Qt Creator to identify the types of version of the debugger. In addition, Qt Creator shows the ABI version that will be used on embedded devices in the ABIs field.

To learn more about the in-depth information on how to set up different debuggers in Qt, please visit the following link:
http://doc.qt.io/qtcreator/creator-debugger-engines.html.

Unit testing

Unit testing is an automated process for testing an individual module, class, or method in your application. Unit testing finds problems early in the development cycle. This includes both bugs in the programmer's implementation and flaws or missing parts of the specification for the unit.

Unit testing in Qt

Qt comes with a built-in unit testing module, which we can use by adding the testlib keyword to our project file (.pro):

QT += core gui testlib

After that, add the following header to our source code:

#include <QtTest/QtTest>

Then, we can start testing our code. We must declare our test functions as private slots. Other than that, the class must also inherit from the QOBject class. For example, I created two text functions called testString() and testGui(), like so:

private slots:
void testString();
void testGui();

The function definitions look something like this:

void MainWindow::testString()
{
QString text = "Testing";
QVERIFY(text.toUpper() == "TESTING");
}

void MainWindow::testGui()
{
QTest::keyClicks(ui->lineEdit, "testing gui");
QCOMPARE(ui->lineEdit->text(), QString("testing gui"));
}

We used some of the macros provided by the QTest class, such as QVERIFY, QCOMPARE, and so on, to evaluate the expression passed as its argument. If the expression evaluates to true, the execution of the test function continues. Otherwise, a message describing the failure is appended to the test log, and the test function stops executing.

We also used QTest::keyClicks() to simulate mouse clicking in our application. In the earlier example, we simulate clicking on the line edit widget on our main window widget. Then, we input a line of text to the line edit and use QCOMPARE macro to test if the text has been correctly inserting into the line edit widget. If anything wrong happened, Qt will show us the problem in the application output window.

After that, comment out our main() function and use the QTEST_MAIN() function instead to start testing our MainWindow class:

/*int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();

return a.exec();
}*/
QTEST_MAIN(MainWindow)

If we build and run our project now, we should be getting similar results as follows:

********* Start testing of MainWindow *********
Config: Using QtTest library 5.9.1, Qt 5.9.1 (i386-little_endian-ilp32 shared (dynamic) debug build; by GCC 5.3.0)
PASS   : MainWindow::initTestCase()
PASS   : MainWindow::_q_showIfNotHidden()
PASS   : MainWindow::testString()
PASS   : MainWindow::testGui()
PASS   : MainWindow::cleanupTestCase()
Totals: 5 passed, 0 failed, 0 skipped, 0 blacklisted, 880ms
********* Finished testing of MainWindow *********

There are many more macros that you can use to test your application.

For more information, please visit the following link:
http://doc.qt.io/qt-5/qtest.html#macros

Summary

In this chapter, we have learned how to identify technical issues in our Qt project by using multiple debugging techniques. Other than that, we have also learned about different debuggers that are supported by Qt on different operating systems. Finally, we also learned how to automate some of the debugging steps through unit testing.

That's it! We have reached the end of this book. Hopefully, you have found this book useful on learning how to build your own applications from scratch using Qt. You can look for all the source code on GitHub. I wish you all the best!

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