The signals and slots mechanism in Qt 6 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 anyQObject object or its subclasses, which will then trigger any slot functions of any objects that are connected tothe signal.

In this chapter, we’re going to cover the followingmain topics:

• Signals and slots ina nutshell
• UI events with signalsand slots
• Asynchronous programmingmade easier
• Function callbacks

The technical requirements for this chapter includeQt 6.6.1 MinGW 64-bit andQt Creator 12.0.2. All the code used in this chapter can be downloaded from the following GitHubrepository:https://github.com/PacktPublishing/QT6-C-GUI-Programming-Cookbook---Third-Edition-/tree/main/Chapter02.

Compared tocallbacks (which Qt 6 also supports), thesignals and slots mechanism is comparably more fluid and flexible for theprogrammer to use. It is both type-safe and not strongly coupled to the processing function, which makes it better thancallback implementation.

How to do it…

Let’s get started by followingthese steps:

1. Let’s create aQt Widgets Application project and openupmainwindow.ui.
2. Drag and drop aPushButton widget from theWidget Box to theUI canvas:

Figure 2.1 – Dragging and dropping a push button to the UI canvas

3. Right-click on thePushButton widgetand selectGo to slot. A windowwill appear:

Figure 2.2 – Selecting the clicked() signal and pressing OK

4. You will see a list of built-in slot functions available for the push button. Let’s select theclicked() option and pressOK. A slot function...

In the previous recipe, wedemonstrated the use of signals and slots on apush button. Now, let’s explore the signals and slots that are available in other commonwidget types.

How to do it…

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

1. Let’s create a newQt WidgetsApplication project.
2. Drag and drop aPushButton,Combo Box,Line Edit,Spin Box, andSlider widget from theWidget Box into yourUI canvas:

Figure 2.6 – Placing several widgets on the UI canvas

3. Then, right-click on the push button, selectclicked(), and press theOK button to proceed. A slot function will be created for you byQt Creator:

Figure 2.7 – Selecting the clicked() signal and pressing OK

4. Repeat the previous step, but this time, select the next selection until every function inQAbstractButton has been added to thesource...

Since the signals and slots mechanism isasynchronous in nature, we can make use of it for things other than user interfaces. In programming terms, anasynchronous operation is a process that works independently, allowing the program to continue its operation without waiting for theprocess to complete, which may stall the whole program. Qt 6 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 6 enforced the new syntax for signals and slots, which allows a signal to trigger a normal function instead of a slot function from aQobject object.

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 provided byQt 6.

How to do it…

To learn how to achieve asynchronous operations using...

Even though Qt 6 supports the signals and slots mechanism, some of the features in Qt 6 still usefunction callbacks, such as keyboard input, window resizing, graphics painting, and others. Since these events only need to be implemented once, there is no need to use the signals andslots mechanism.

How to do it…

Let’s get started by followingthis example:

1. Create aQt Widgets Application project, open upmainwindow.h, and add thefollowing headers:

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

2. Then, declare these functionsinmainwindow.h:

   public:    explicit MainWindow(QWidget *parent = 0);    ~MainWindow();    void resizeEvent(QResizeEvent *event);    void keyPressEvent(QKeyEvent *event);    void keyReleaseEvent(QKeyEvent *event);    void mouseMoveEvent...