Signal And Slots Qt5
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.
Signals are protected in Qt4 but are public in Qt5, thus the contradictory information. Slots are functions and public/protected/private is honored when calling them as such, when connecting to a signal, the metaobject system ignores it though. As signals is defined as public:, prepending them with e.g. Private leads; to. 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. Qt5 Tutorial Signals and Slots - 2020, Introduction#. Signals and slots are. This is a continued tutorial from the previous one, Qt 5 QTcpSocket. We're going to use Signal and Slot mechanism instead of calling functions manually(?). Note: Qt5 document. The QTcpSocket class provides a TCP socket. TCP (Transmission Control Protocol) is a reliable, stream-oriented, connection-oriented transport protocol. A slot remains an ordinary function, so you can call it yourself. A single signal can be linked to different slots. A single slot can be called by different linked signals. A connection can be made between a signal and a slot from different objects, and even between objects living inside different threads!
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:
Signals And Slots Qt
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.
- PyQt5 Tutorial
- PyQt5 Useful Resources
- Selected Reading
Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.
Widgets used to build the GUI interface act as the source of such events. 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.
Using Qt Designer's Signal/Slot Editor
First design a simple form with a LineEdit control and a PushButton.
It is desired that if button is pressed, contents of text box should be erased. The QLineEdit widget has a clear() method for this purpose. Hence, the button’s clicked signal is to be connected to clear() method of the text box.
To start with, choose Edit signals/slots from Edit menu (or press F4). Then highlight the button with mouse and drag the cursor towards the textbox
As the mouse is released, a dialog showing signals of button and methods of slot will be displayed. Select clicked signal and clear() method
The Signal/Slot Editor window at bottom right will show the result −
Signal And Slot Qt C++
Save ui and Build and Python code from ui file as shown in the below code −
Generated Python code will have the connection between signal and slot by the following statement −
Run signalslot.py and enter some text in the LineEdit. The text will be cleared if the button is pressed.
Building Signal-slot Connection
Instead of using Designer, you can directly establish signal-slot connection by following syntax −
Signals And Slots Qt5 C++
Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following technique −
Example
In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.
When b1 is clicked, the clicked() signal is connected to b1_clicked() function −
When b2 is clicked, the clicked() signal is connected to b2_clicked() function.
Signals And Slots Qt5
The above code produces the following output −