last modified July 16, 2020

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 5 Documentation — QMainWindow Signals As you can see, alongside the two QMainWindow signals, there are 4 signals inherited from QWidget and 2 signals inherited from Object. If you click through to the QWidget signal documentation you can see a.windowTitleChanged signal implemented here. PyQt5 signals and slots Graphical applications (GUI) are event-driven, unlike console or terminal applications. A users action like clicks a button or selecting an item in a list is called an event. If an event takes place, each PyQt5 widget can emit a signal.

In this part of the Qt5 C++ programming tutorial we talk about events and signals.

Events are an important part in any GUI program. All GUI applications are event-driven. An application reacts to different event types which are generated during its life. Events are generated mainly by the user of an application. But they can be generated by other means as well, e.g. Internet connection, window manager, or a timer. In the event model, there are three participants:

• event source
• event object
• event target

The event source is the object whose state changes. It generates Events. The event object (Event) encapsulates the state changes in the event source.The event target is the object that wants to be notified. Event source object delegates the task of handling an event to the event target.

When we call the application's exec() method, the applicationenters the main loop. The main loop fetches events and sends them to the objects. Qt has a unique signal and slot mechanism. This signal and slot mechanism is an extension to the C++ programming language.

Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot is a normal C++ method;it is called when a signal connected to it is emitted.

Click

The first example shows a very simple event handling example. We have one push button. By clicking on the push button, we terminate the application.

This is the header file.

We display a QPushButton on the window.

The connect() method connects a signal to the slot. When we click on the Quit button, the clicked signal is generated. The qApp is a global pointer to the application object. It is defined in the <QApplication> header file. The quit() method is called when the clicked signal is emitted.

This is the main file.

KeyPress

In the following example, we react to a key press.

This is the keypress.h header file.

The application terminates if we press the Escape key.

One of the ways of working with events in Qt5 is to reimplement an event handler. The QKeyEvent is an event object, which holds information about what has happened. In our case, we use the event object to determine which key was actually pressed.

This is the main file.

QMoveEvent

The QMoveEvent class contains event parameters for move events.Move events are sent to widgets that have been moved.

This is the move.h header file.

In our code programming example, we react to a move event. We determine the current x, y coordinates of the upper left corner of the client area of the window and set those values to the title of the window.

We use the QMoveEvent object to determine the x, y values.

We convert the integer values to strings.

The setWindowTitle() method sets the text to the title of the window.

This is the main file.

Disconnecting a signal

A signal can be disconnected from the slot. The next example shows how we can accomplish this.

In the header file, we have declared two slots. The slots is not a C++ keyword, it is a Qt5 extension. These extensions are handled by the preprocessor, before the code is compiled. When we use signals and slots in our classes, we must provide a Q_OBJECT macro at the beginning of the class definition. Otherwise, the preprocessor would complain.

In our example, we have a button and a check box. The check box connects and disconnects a slot from the buttons clicked signal. This example must be executed from the command line.

Here we connect signals to our user defined slots.

If we click on the Click button, we send the 'Button clicked' text to the terminal window.

Inside the onCheck() slot, we connect or disconnect the onClick() slot from the Click button, depending on the received state.

This is the main file.

Timer

A timer is used to implement single shot or repetitive tasks. A good example where we use a timer is a clock; each second we must update our label displaying the current time.

This is the header file.

In our example, we display a current local time on the window.

To display a time, we use a label widget.

Here we determine the current local time. We set it to the label widget.

We start the timer. Every 1000 ms a timer event is generated.

Qt5 Signal Slot Example

To work with timer events, we must reimplement the timerEvent() method.

This is the main file.

This chapter was dedicated to events and signals in Qt5.

This article is the most comprehensive description of signals and slots in QML compared to all previous articles on this site.

In this article, I will try to explain the following when working with Qt/QML + Qt/C++:

• ways to declare signals and slots, also called methods in the C ++ class, which will be registered in the QML layer
• ways to connect to signals of classes declared in C ++ as context
• work with Q_PROPERTY, which also requires signals and slots
• ways to connect signals and slots in QML
• etc.

Signals and slots from the C++ class

Let's create our first class that will work with signals and slots in QML. This is one of the very first examples that I have already shown, but I will repeat this example so that the article is as complete as possible.

In this example, I want to create an application that has one button and by pressing this button increases the counter that is inside the C++ class. This C++ class will be registered as a context property in the QML engine of our application.

App appearance will be next

AppCore.h

Declaring signals and slots in C ++ code will not differ much from the classical Qt/C++.

AppCore.cpp

As well as the implementation of the methods themselves.

main.cpp

main.qml

And now the most interesting. How to use an object loaded in a QML context and how to connect to its signals.

As you remember, we loaded the object into the context QML under the name appCore , we will use this object to access it. But to connect to the signal, we will need to use the QML type Connections .

Thus, you can access the object that was loaded into the context of the QML engine, call its slot, and process the signal from this object.

It is also not necessary to declare receiveFromQml() as a slot in this case. This method can also be declared as Q_INVOKABLE method.

Using Q_PROPERTY

Qt5 Signal Slot Tutorial Android Studio

The next option is to use the Q_PROPERTY macro. A classic property in Qt might look like this for our task

This property has the following components:

• type of property, as well as its name: int counter , which are bound to the variable int m_counter inside the class, this is the logic of code generation in Qt
• name of the method to read, matches the name of the property: counter
• method name for setting the value: setCounter
• signal that reports property changes: counterChanged

You can also pass additional parameters to this macro, but this is beyond the scope of this article. And also the property can be read only, that is, without a setter.

Now look at the full code using Q_PROPERTY

AppCore.h

AppCore.cpp

main.qml

Here you will see that connecting the property and accessing it has become easier thanks to the declarative style of QML code. Of course, you cannot always use properties, sometimes you just need to use signals, slots, and Q_INVOKABLE methods. But for variables like counter, properties are likely to be much more convenient.

Connecting signals inside QML files

Now consider the option of connecting signals and slots (functions) inside QML files. There will no longer be any C ++ code.

Among other things, you can use and disable signals from slots

Connect a signal to a signal

Also in QML there is still the ability to connect a signal to a signal, as in Qt/C++. Look at the following artificial example.

Qt5 Signal Slot Tutorial For Beginners

In this case, the counter will continue to increase when the button is pressed. But the button press signal is not connected directly to the counter increase function, but is forwarded through the signal.

Using Variables in Signals

QML also has the ability to use variables in signals.

Conclusion

For the most part, this entire article fits into several points:

• In C ++, to interact with the QML layer, you can use signals, slots, Q_INVOKABLE methods, as well as create properties using the Q_PROPERTY macro
• In order to respond to signals from objects, you can use the QML type Connections
• Q_PROPERTY obeys the declarative style of QML and, when a property is changed, it can automatically set new values, if the property has been added to any object in QML. In this case, the signal slot connections are set automatically.
• In QML, you can connect and disconnect signal / slot connections using the following syntax:
  • object1.signal.connect (object2.slot)
  • object1.signal.disconnect (object2.slot)
• Signals in QML can also be connected to other signals, as is done in Qt / C ++
• Signals in QML may also have arguments