Get properties of QgsVertexMarker upon mouse click

Question

I'm looking for a way to trigger an event upon clicking on a QgsVertexMarker object which is located on a mapCanvas. In the official docs, there is a mousePressEvent method which I actually re-implement in my custom QgsVertexMarker class:

class CustomMarker(QgsVertexMarker):
...
    def mousePressEvent(self, event):
        print("---MOUSE EVENT---")
        if event.button() == Qt.LeftButton:
            print("Left click")
            self.getMarkerProperties()
...

Now, the problem is that I cannot figure out how to make it work. I read about instantiating a QGraphicsView object and eventFilters but I'm not sure if this is the right path to follow.

NOTE: The default implementation of setToolTip method already works without any tinkering so I'm wondering why mousePressEvent doesn't work out of the box.

Answer 1

The map canvas of QGIS is a subclass of QGraphicsView. For events to be propagated to the actual canvas items it is required to call the default implementations of overridden mouse event methods (ref). QGIS does not do that! It only passes the events to the active map tool, not to the canvas items.

To intercept the events anyway you can use an event filter. Which in this specific case is also not straightforward, because QgsVertexMarker (and all subclasses of QGraphicsItem) don't inherit QObject therefore they don't qualify as valid filter items. Instead you need to define a custom class which can receive the events and then forward them to your actual canvas item.

Consider the following example, which can be pasted in the console to try out:

class CustomMarker(QgsVertexMarker):
    
    def __init__(self, canvas):
        super(CustomMarker, self).__init__(canvas)
        self.canvas = canvas
        self.event_filter = CanvasEventFilter(self)
        self.canvas.viewport().installEventFilter(self.event_filter)

    def mousePressEvent(self, event):
        print("---MOUSE EVENT---")
        if event.button() == Qt.LeftButton:
            print("Left click")
            
        super().mousePressEvent(event)


class CanvasEventFilter(QObject):
    
    def __init__(self, marker):
        self.marker = marker
        super(CanvasEventFilter, self).__init__()
        
    def eventFilter(self, obj, event):
        if event.type() == QEvent.MouseButtonPress:
            # forward event if marker is under current mouse position 
            if self.marker.isUnderMouse():
                self.marker.mousePressEvent(event)
            
        return False
        

m = CustomMarker(iface.mapCanvas())

m.setCenter(QgsPointXY(0, 0))
m.setColor(QColor(0, 0, 255))
m.setPenWidth(5)
m.show()