Qt 5 signals and slots mechanism. How signals and slots in Qt differ from the callback architecture in other widget toolkits. A Qt basics tutorial. How to add signals and slots in Qt Creator.

1. Qt Signal Slot Array Antenna
2. Qt Signal Slot Array C++

Part 9 of the Qt Creator C++ Tutorial

What are Qt 5 Signals and Slots?

One key and distinctive feature of Qt framework is the use of signals and slots to connect widgets and related actions. But as powerful the feature is, it may look compelling to a lot of developers not used to such a model, and it may take some time at the beginning to get used to understand how to use signals and slots properly. Further properties of signal/slots. Qt provides the QObject::sender function, which returns a pointer to the object that sent the signal Note: if the slot was not activated by a signal, the return is undefined. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. What are Qt 5 Signals and Slots? Very basically, signals and slots in Qt allow communication between objects. In Qt, a signal is emitted when an event occurs. A slot is a function that is called when a signal is emitted. For example, a push button emits a clicked signal when clicked by a user. A slot that is attached to that signal is called. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget) connect (sender, SIGNAL (valueChanged (QString, QString)), receiver, SLOT (updateValue (QString))); New: connecting to QObject member.

Very basically, signals and slots in Qt allow communication between objects.

In Qt, a signal is emitted when an event occurs. A slot is a function that is called when a signal is emitted. For example, a push button emits a clicked signal when clicked by a user. A slot that is attached to that signal is called when the clicked signal is emitted.

Multiple signals can be connected to any slot. Signals can be connected to any number of slots.

Most of the details of signals and slots are hidden in their implementation in Qt. At this stage of the tutorial series we do not look in depth at signals and slots.

Qt Signal Slot Array Antenna

Using Signals and Slots in Qt Creator

There are several ways to use signals and slots in Qt Creator projects. This includes manually adding them in code. Here we briefly look at the easier ways to use signals and slots to respond to events. Events are generated by users interacting with widgets in an application. These events cause signals to be emitted. Corresponding slots, or functions then run.

Qt 5 Signals and Slots Demonstration

The following image shows the application built in this section using Qt Creator. It demonstrates some methods of using signals and slots.

Each section below shows a method of adding signals and slots to a Qt Creator program. Watch the video embedded near the top of this page for details.

Add a Slot to a Button for the Clicked Signal

Place a push button on the main window. Right click the push button and select Go to slot… to add code for the clicked signal.

Connect a Slider to a Progress Bar Visually

Qt Signal Slot Array C++

Place a Horizontal Slider and a Progress Bar on the main window.

Press F4 on the keyboard. This toggles to Edit Signals/Slots mode.

Drag to connect the slider to the progress bar.

Press F3 to change back to Edit Widgets mode.

Connect a Slider to a Progress Bar with Code

Place a second Horizontal Slider and a Progress Bar on the main window.

Right-click the Horizontal Slider. In the menu that pops up, click Go to slot…

In the dialog box that pops up, select sliderMoved(int). Click the OK button.

Add code for the sliderMoved signal.

Menu Bar Item with Action Editor

Add a File menu with Open, Close and Quit menu items.

Qt Creator must be in Design mode. Make sure that the Action Editor and Signal and Slots Editor are visible. Do this from the top menu as follows. Select Window → Views and then click the check box next to each of the desired editors.

Add slots for the triggered() signal for the Open and Close menu items. Do this in the Action Editor as follows. Right click a menu item. Click Go to slot… on the menu that pops up. Click triggered() in the dialog box that pops up and then click the OK button.

Add code in the slot function.

Menu Bar Item with Signals and Slots Editor

In Design mode, select the Signals and Slots tab. Click the big green + sign to add an item. Change the following for the new item.

• Sender : actionQuit
• Signal : triggered()
• Receiver : MainWindow
• Slot : close()

Code Listing

Below is the code listing for mainwindow.cpp for the example project. Follow the video embedded near the top of this page to add the code.

mainwindow.cpp

Quite a frequent problem when working with signals with slots in Qt5, according to my observations on the forum, is the connection of slots in the syntax on the pointers to signals having an overload of the signature. The same applies to slots that have an overload.

Let's take a test class that has overloaded signals.

Here there is a signal, with an overload of the signature. Connect this signal will also be to the slots that are declared in the Widget class, and which also have an overload of the signature.

How it was in Qt4

Within Qt4, everything was solved quite simply by specifying the signature of the signal and the slot in the SIGNAL and SLOT macros.

How it became in Qt5

But in Qt5, when writing in the new syntax of signals and slots, there are some problems. Because you need to make the static_cast of the method signature.

By the way, the new syntax also allows you to connect signals to slots with a smaller signature, as it was in Qt4.

Advantages of the new syntax

And now a stumbling block. Why use the new syntax of signals and slots? I still hear this question from time to time. Especially when people see such terrible castes of signatures.

1. Therefore, I will list potential advantages:The ability to track errors in the connection of signals and slots at the compilation stage, rather than in the runtime
2. Reducing compilation time by excluding macros from the code
3. The ability to connect lambda functions, it's quite an important bun
4. We protect ourselves from errors when we try to connect from the outside to a private slot. Yes!! Yes!! The SIGNAL and SLOT macros ignore the access levels of methods, violating OOP.

In general, for me this is enough, but for you?