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.

. The games are intended for a mature audience. The games do not offer 'real Qt5 Signal Slots money gambling' or an opportunity to win real money or prizes. Past success at social casino gaming has no relationship to future success at 'real Qt5 Signal Slots money gambling.”. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function. In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques − QtCore.QObject.connect(widget, QtCore.SIGNAL(‘signalname’), slot.

If an event takes place, each PyQt5 widget can emit a signal. A signal does not execute any action, that is done by a slot.

Related course:
Create GUI Apps with PyQt5

Signals and slot introduction
Consider this example:

The button click (signal) is connected to the action (slot). In this example, the method slot_method will be called if the signal emits.

This principle of connecting slots methods or function to a widget, applies to all widgets,

or we can explicitly define the signal:

PyQt supports many type of signals, not just clicks.

Example
We can create a method (slot) that is connected to a widget. A slot is any callable function or method.

On running the application, we can click the button to execute the action (slot).

If you are new to programming Python PyQt, I highly recommend this book.

Introduction

Signal And Slot Qt C++

Remarks

Official documentation on this topic can be found here.

Signals And Slots Qt5

A Small Example

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.

The minimal example requires a class with one signal, one slot and one connection:

counter.h

The main sets a new value. We can check how the slot is called, printing the value.

Signals And Slots Qt5 C++

Finally, our project file:

Connecting overloaded signals/slots

While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use static_casts to member function pointers, or (starting in Qt 5.7) qOverload and friends:

Multi window signal slot connection

A simple multiwindow example using signals and slots.

There is a MainWindow class that controls the Main Window view. A second window controlled by Website class.

The two classes are connected so that when you click a button on the Website window something happens in the MainWindow (a text label is changed).

I made a simple example that is also on GitHub:

mainwindow.h

mainwindow.cpp

website.h

website.cpp

Project composition:

Consider the Uis to be composed:

• Main Window: a label called 'text' and a button called 'openButton'
• Website Window: a button called 'changeButton'

So the keypoints are the connections between signals and slots and the management of windows pointers or references.

The new Qt5 connection syntax

The conventional connect syntax that uses SIGNAL and SLOT macros works entirely at runtime, which has two drawbacks: it has some runtime overhead (resulting also in binary size overhead), and there's no compile-time correctness checking. The new syntax addresses both issues. Before checking the syntax in an example, we'd better know what happens in particular.

Signals And Slots Qt

Let's say we are building a house and we want to connect the cables. This is exactly what connect function does. Signals and slots are the ones needing this connection. The point is if you do one connection, you need to be careful about the further overlaping connections. Whenever you connect a signal to a slot, you are trying to tell the compiler that whenever the signal was emitted, simply invoke the slot function. This is what exactly happens.

Here's a sample main.cpp:

Hint: the old syntax (SIGNAL/SLOT macros) requires that the Qt metacompiler (MOC) is run for any class that has either slots or signals. From the coding standpoint that means that such classes need to have the Q_OBJECT macro (which indicates the necessity to run MOC on this class).

The new syntax, on the other hand, still requires MOC for signals to work, but not for slots. If a class only has slots and no signals, it need not have the Q_OBJECT macro and hence may not invoke the MOC, which not only reduces the final binary size but also reduces compilation time (no MOC call and no subsequent compiler call for the generated *_moc.cpp file).