Qt -- pass events to multiple objects?

Question

I basically have 3 layers (Window > Scene > View) that each need to handle a mouseMove event without blocking the others. It seems only the youngest child is getting the event though. I was hoping I could process the event and then call event->ignore() to pass the event back up the stack, but it doesn't seem to be working.

Some relevant code if you need it:

void EditorWindow::createScene() {
 m_scene = new EditorScene(this);
 m_view = new EditorView(m_scene);
 // ...
}

void EditorScene::mouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent) {
 printf("B\n");
 // ...
}

void EditorView::mouseMoveEvent(QMouseEvent* event) {
 printf("C\n");
 event->ignore();
}

Only "C" is being printed. Note that EditorScene and EditorView receive different types of mouse events so it's not completely trivial to pass them around.

The EditorWindow also needs the mouse coordinates; currently I'm sending a signal from one of the children which is caught by the window... but it shouldn't really be necessary to relay it that way, should it?

---

Found this nice article. Calling ignore() tells Qt to find another receiver. Sounds like it should work, but perhaps it means an unrelated receiver. The proper way to propagate it is actually to call BaseClass::Event like so:

void EditorView::mouseMoveEvent(QMouseEvent* event) {
    QGraphicsView::mouseMoveEvent(event); // propogate to parent widget
    printf("C\n");   
}

Now it's printing BCBCBC... which is great, but I can't seem to nudge it up one more level...

---

Another edit: It was being propogated up properly, I just didn't have setMouseTracking enabled.

Answer 1

QGraphicsView::mouseMoveEvent(event);

Doesn't propagate up to the parent -- it actually propagates down to the scene.

Here is what's happens -- QGraphicsView receives QMouseEvent, translates it into QGraphicsSceneMouseEvent and passes it to the scene. Scene then passes it to appropriate item or, in your case, prints "B". Event handler then returns back to EditorView and prints "C".

Then, if you explicitly ignore event (mouse move is accepted by default), Qt event handler will pass the event to parent of EditorView. So try ignoring after you print "C".

Another thing about mouse move is this:

If mouse tracking is switched off, mouse move events only occur if a mouse button is pressed while the mouse is being moved. If mouse tracking is switched on, mouse move events occur even if no mouse button is pressed.

So make sure you have tracking enabled on parent of EditorView (or that you press buttons :)).

EDIT: BTW, EditorScene is not a parent of EditorView. Well, it is in your code, but only in QObject meaning of parentship (memory management only).

QGraphicsScene and View don't have normal family relationship -- scene can have multiple views and those views are children of unrelated parents.

For window event propagation purposes you must have QWidget based parent. In fact, I'm pretty sure you reparent EditorView to EditorWindow, or one of its children (when you add it into layout).

INSTAEDIT: For coordinates you want View itself to emit a signal. Both for decoupling reasons and because you probably want to show local coordinates of the view, and not of the parent window and not screen coordinates (right?). If you actually want scene coordinates, View is right choice too, because it knows transformation matrix.

Coordinates go like this: Screen -> EditorWindow local -> EditorView local -> Scene transformed -> whatever item local transformed.

Answer 2

QGraphicsView::mousePressEvent( e ) in my mousePressEvent did the trick!