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

[Ross Bencina]

Hi Christian

Sorry to come in late on this...

> I'm trying to implement a kind of drawing tool.
> The user simply select the current tool (select, move, edit; draw
> circle, rectangle, ...) from the main toolbar, and then
> press/move/release the mouse depending on how this tool works.
>
> I have sub-classed the scene, and I have a "Tool" base class.
>
> As each tool will have to manage the mouse events its own way, I'm
> looking at the best way to "forward" the mouse events from the scene
> to the current tool, knowing that I would like later to be able to add
> new tools via plug-ins (so the scene have to know only about the Tool
> base class).
>
> Should I let the tools register/unregister themselves as event filters
> to the scene, or should I forward the events to the current tool in
> the scene's event handler using postEvent()? Or are there any other
> solutions for this typical use-case?

The way I did it was create a subclass of QGraphicsScene that forwards 
events to the registered tool. Actually in my case I implemented the 
Interactor pattern and allow multiple interactors to be registered/active at 
once, which is useful in some cases -- for example you can have an idle 
interactor and spawn different mouse drag Interactors (behaviors) depending 
on what gets clicked on. I think I just created a base Interactor class with 
methods for all the events I was interested in rather than use the standard 
Qt event dispatch mechanism.

I don't remember the exact details but I remember having to do some messing 
around to make sure all the events I wanted got delivered to my Interactors 
correctly since some got "eaten" by the scene by default.

Perhaps using event filters is a good idea.. i'm not sure why I didn't do 
that. Having clearer control over the order of events dispatched to multiple 
interactors might be one reason. Better management of deletion of 
interactors during event dispatch might be another.

Ross.