[Qt-interest] QgraphicsScene: best way to manage events

[Ross Bencina]

Oliver Knoll wrote:

> I don't think QGraphicsView/Scene "eats" such events before they deliver 
> it to your method. Unless off course some other widget in your app has 
> focus and already consumes a shortcut, for example, or the aforementioned 
> event filter is installed.
<

I just had a look at my code:

One reason I have not used event filtering is that QGraphicsScene doesn't 
(didn't?) provide a mechanism to intercept all QGraphicsSceneHoverEvents for 
the whole scene -- so I have a convoluted mechanism where the GraphicsItems 
notify my scene subclass of these events, and it then the scene dispatches 
them to active Interactors (this might have been fixed now, QGraphicsScene 
was very new when I started using it) My comment says:
// HACK: it would be really handy to have access to these events

// as they are dispatched so that I didn't have to implement

// the hackneyed dispatch scheme here.


So it wasn't that QGraphicsScene "eats" events -- just that I couldn't work 
out a way to intercept them.

My GraphicsScene subclass also has another mechanism that dispatches 
synthetic mouse move events to my Interactors while drag-and-drop is in 
progress -- this allows me to do things like render QGraphicsItems that 
follow the cursor during an externally initiated drop operation that share 
the same Interactor code as drag-drop within the graphics scene.

That said, I probably wouldn't have used the event filter mechanism anyway. 
I'm not a big fan of subclassing QObject for lightweight classes like 
Interactors -- I have a lot of Interactor subclasses, most of them get 
created when the mouse is clicked and deleted when it's released -- having 
all the QObject overhead is too heavy-weight for my liking.

Ross.