The signals and slots mechanism in Qt 5 is one of its most important features. It's a method that allows communication between objects, which is a crucial part of a program's graphical user interface. A signal can be emitted from any QObject objects or its subclasses, which will then trigger any slot functions of any objects that are connected to the signal.

Compared to callbacks (which Qt 5 also support), the signals and slots mechanism is comparably more fluid and flexible for the programmer to use. The signals and slots mechanism is both type safe and not strongly coupled to the processing function, which makes it better than the callback's implementation.

The technical requirements for this chapter include Qt 5.11.2 MinGW 32 bit, Qt Creator 4.8.2, and Windows 10.

All the code used in this chapter can be downloaded from the following GitHub link: https://github.com/PacktPublishing/Qt5-CPP-GUI-Programming-Cookbook-Second-Edition/tree/master/Chapter02.

Check out the following video to see the code in action: http://bit.ly/2FozGKA

The signals and slots mechanism has gone through some changes in the recent version of Qt, most notably its coding syntax. Qt 5 continues to support the older syntax for future releases, but there is no mention of how long it will be before it will be dropped completely. Therefore, it's better for us to start learning the new syntax before that day comes.

Let's get started by following these steps:

1. Let's create a Qt Widgets Application project and open up mainwindow.ui.
2. Drag and drop a PushButton from the Widget Box to the UI canvas:

3. Right-click on the PushButton and select Go to slot. A window will appear:

4. You will see a list of built-in slot functions available...

In the previous example, we demonstrated the use of signals and slots on a push button. Now, let's explore the signals and slots that are available in other common widget types.

To learn how to use signals and slots with UI events, follow these steps:

1. Let's create a new Qt Widgets Application project.
2. Drag and drop a Push Button, Combo Box, Line Edit, Spin Box, and Slider from the Widget Box into your UI canvas:

3. Then, right-click on the push button and select clicked() and press the OK button to proceed. A slot function will be created for you by Qt Creator:

4. Repeat the previous step, but this time, select the next selection until every function in QAbstractButton...

Since the signals and slots mechanism is asynchronous in nature, we can make use of it for things other than user interfaces. In programming terms, an asynchronous operation is a process that works independently, allowing the program to continue its operation without waiting for the process to complete, which may stall the whole program.

Qt 5 allows you to make use of its signals and slots mechanism to easily achieve asynchronous processes without much effort. This is even more true after Qt 5 introduced the new syntax for signals and slots, which allows a signal to trigger a normal function instead of a slot function from a QObject.

In the following example, we will further explore this opportunity and learn how we can improve our program's efficiency by using asynchronous operations through the signals and slots mechanism that's...

Even though Qt 5 supports the signals and slots mechanism, some of the features in Qt 5 still use function callbacks, such as keyboard input, window resize, graphics painting, and others. Since these events only need to be implemented once, there is no need to use the signals and slots mechanism.

Let's get started by following this example:

1. Create a Qt Widgets Application project, open up mainwindow.h, and add the following headers:

#include <QDebug>
#include <QResizeEvent>
#include <QKeyEvent>
#include <QMouseEvent>

2. Then, declare these functions in mainwindow.h:

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    void resizeEvent(QResizeEvent...