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Signals, Slots & Events

Build complex application behaviours using signals and slots, and override widget event handling with custom events.

As already described, every interaction the user has with a Qt application causes an Event. There are multiple types of event, each representing a difference type of interaction — e.g. mouse or keyboard events.

Events that occur are passed to the event-specific handler on the widget where the interaction occurred. For example, clicking on a widget will cause a QMouseEvent to be sent to the .mousePressEvent event handler on the widget. This handler can interrogate the event to find out information, such as what triggered the event and where specifically it occurred.

You can intercept events by subclassing and overriding the handler function on the class, as you would for any other function. You can choose to filter, modify, or ignore events, passing them through to the normal handler for the event by calling the parent class function with super().

    class CustomButton(Qbutton):

        def keyPressEvent(self, e):
            # My custom event handling
            super(CustomButton, self).keyPressEvent(e)

However, imagine you want to catch an event on 20 different buttons. Subclassing like this now becomes an incredibly tedious way of catching, interpreting and handling these events.

    class CustomButton99(Qbutton)

        def keyPressEvent(self, e):
            # My custom event handling
            super(CustomButton99, self).keyPressEvent(e)

Thankfully Qt offers a neater approach to receiving notification of things happening in your application: Signals.

Signals

Instead of intercepting raw events, signals allow you to 'listen' for notifications of specific occurrences within your application. While these can be similar to events — a click on a button — they can also be more nuanced — updated text in a box. Data can also be sent alongside a signal - so as well as being notified of the updated text you can also receive it.

The receivers of signals are called Slots in Qt terminology. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself.

Load up a fresh copy of `MyApp_window.py` and save it under a new name for this section.

Basic signals

First, let's look at the signals available for our QMainWindow. You can find this information in the Qt documentation. Scroll down to the Signals section to see the signals implemented for this class.

Qt 5 Documentation — QMainWindow Signals
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. Next we'll demonstrate that signal within our application.

Qt 5 Documentation — Widget Signals
Qt 5 Documentation — Widget Signals

The code below gives a few examples of using the windowTitleChanged signal.

python
class MainWindow(QMainWindow):

    def __init__(self, *args, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)
        
        # SIGNAL: The connected function will be called whenever the window
        # title is changed. The new title will be passed to the function.
        self.windowTitleChanged.connect(self.onWindowTitleChange)

        # SIGNAL: The connected function will be called whenever the window
        # title is changed. The new title is discarded in the lambda and the
        # function is called without parameters.
        self.windowTitleChanged.connect(lambda x: self.my_custom_fn())

        # SIGNAL: The connected function will be called whenever the window
        # title is changed. The new title is passed to the function
        # and replaces the default parameter
        self.windowTitleChanged.connect(lambda x: self.my_custom_fn(x))

        # SIGNAL: The connected function will be called whenever the window
        # title is changed. The new title is passed to the function
        # and replaces the default parameter. Extra data is passed from
        # within the lambda.
        self.windowTitleChanged.connect(lambda x: self.my_custom_fn(x, 25))
        
        # This sets the window title which will trigger all the above signals
        # sending the new title to the attached functions or lambdas as the
        # first parameter.
        self.setWindowTitle("My Awesome App")
        
        label = QLabel("THIS IS AWESOME!!!")
        label.setAlignment(Qt.AlignCenter)

        self.setCentralWidget(label)
        
        
    # SLOT: This accepts a string, e.g. the window title, and prints it
    def onWindowTitleChange(self, s):
        print(s)

    # SLOT: This has default parameters and can be called without a value
    def my_custom_fn(self, a="HELLLO!", b=5):
        print(a, b)

Try commenting out the different signals and seeing the effect on what the slot prints.

We start by creating a function that will behave as a ‘slot’ for our signals.

Then we use .connect on the .windowTitleChanged signal. We pass the function that we want to be called with the signal data. In this case the signal sends a string, containing the new window title.

If we run that, we see that we receive the notification that the window title has changed.

Events

Next, let’s take a quick look at events. Thanks to signals, for most purposes you can happily avoid using events in Qt, but it’s important to understand how they work for when they are necessary.

As an example, we're going to intercept the .contextMenuEvent on QMainWindow. This event is fired whenever a context menu is about to be shown, and is passed a single value event of type QContextMenuEvent.

To intercept the event, we simply override the object method with our new method of the same name. So in this case we can create a method on our MainWindow subclass with the name contextMenuEvent and it will receive all events of this type.

def contextMenuEvent(self, event):
    print("Context menu event!")

If you add the above method to your MainWindow class and run your program you will discover that right-clicking in your window now displays the message in the print statement.

Sometimes you may wish to intercept an event, yet still trigger the default (parent) event handler. You can do this by calling the event handler on the parent class using super as normal for Python class methods.

def contextMenuEvent(self, event):
    print("Context menu event!")
    super(MainWindow, self).contextMenuEvent(event)

This allows you to propagate events up the object hierarchy, handling only those parts of an event handler that you wish.

However, in Qt there is another type of event hierarchy, constructed around the UI relationships. Widgets that are added to a layout, within another widget, may opt to pass their events to their UI parent. In complex widgets with multiple sub-elements this can allow for delegation of event handling to the containing widget for certain events.

However, if you have dealt with an event and do not want it to propagate in this way you can flag this by calling .accept() on the event.

class CustomButton(Qbutton):

    def event(self, e):
        e.accept()

Alternatively, if you do want it to propagate calling .ignore() will achieve this.

class CustomButton(Qbutton):
    def event(self, e):
        e.ignore()

In this section we've covered signals, slots and events. We've demonstrated some simple signals, including how to pass less and more data using lambdas. We've created custom signals, and shown how to intercept events, pass on event handling and use .accept() and .ignore() to hide/show events to the UI-parent widget. In the next section we will go on to take a look at two common features of the GUI — toolbars and menus.

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