Qt Signal Slot Class
Loosely coupled: a class which emits a signal neither knows nor cares which slots receive the signal. 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. Each PyQt widget, which is derived from QObject class, is designed to emit ‘ signal ’ in response to one or more events. 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.
- Qt Signal Slot Abstract Class
- Qt Connect Signal Slot Different Class
- Qt Signal Slot Between Classes
- How Qt Signal And Slots Works
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.
Part 9 of the Qt Creator C++ Tutorial
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 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.
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
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.
Qt Signal Slot Abstract Class
mainwindow.cpp
This page describes the use of signals and slots in Qt for Python.The emphasis is on illustrating the use of so-called new-style signals and slots, although the traditional syntax is also given as a reference.
The main goal of this new-style is to provide a more Pythonic syntax to Python programmers.
- 2New syntax: Signal() and Slot()
Traditional syntax: SIGNAL () and SLOT()
QtCore.SIGNAL() and QtCore.SLOT() macros allow Python to interface with Qt signal and slot delivery mechanisms.This is the old way of using signals and slots.
The example below uses the well known clicked signal from a QPushButton.The connect method has a non python-friendly syntax.It is necessary to inform the object, its signal (via macro) and a slot to be connected to.
New syntax: Signal() and Slot()
The new-style uses a different syntax to create and to connect signals and slots.The previous example could be rewritten as:
Using QtCore.Signal()
Signals can be defined using the QtCore.Signal() class.Python types and C types can be passed as parameters to it.If you need to overload it just pass the types as tuples or lists.
In addition to that, it can receive also a named argument name that defines the signal name.If nothing is passed as name then the new signal will have the same name as the variable that it is being assigned to.
The Examples section below has a collection of examples on the use of QtCore.Signal().
Note: Signals should be defined only within classes inheriting from QObject.This way the signal information is added to the class QMetaObject structure.
Using QtCore.Slot()
Slots are assigned and overloaded using the decorator QtCore.Slot().Again, to define a signature just pass the types like the QtCore.Signal() class.Unlike the Signal() class, to overload a function, you don't pass every variation as tuple or list.Instead, you have to define a new decorator for every different signature.The examples section below will make it clearer.
Another difference is about its keywords.Slot() accepts a name and a result.The result keyword defines the type that will be returned and can be a C or Python type.name behaves the same way as in Signal().If nothing is passed as name then the new slot will have the same name as the function that is being decorated.
Examples
Qt Connect Signal Slot Different Class
The examples below illustrate how to define and connect signals and slots in PySide2.Both basic connections and more complex examples are given.
- Hello World example: the basic example, showing how to connect a signal to a slot without any parameters.
- Next, some arguments are added. This is a modified Hello World version. Some arguments are added to the slot and a new signal is created.
- Add some overloads. A small modification of the previous example, now with overloaded decorators.
- An example with slot overloads and more complicated signal connections and emissions (note that when passing arguments to a signal you use '[]'):
- An example of an object method emitting a signal:
Qt Signal Slot Between Classes
- An example of a signal emitted from another QThread:
- Signals are runtime objects owned by instances, they are not class attributes: