[Qt-interest] QGraphicsItem ignore scene translation/transform?

[Samuel Rødal]

ext Thomas Fjellstrom wrote:
> On Fri January 22 2010, Samuel Rødal wrote:
>> ext Thomas Fjellstrom wrote:
>>> On Fri January 22 2010, Samuel Rødal wrote:
>>>> Basically the
>>>> top-level item is just an invisible item whose bounding rect is the
>>>> same as the combined bounding rect of the child items (game squares).
>>>> Whenever you earlier scaled / scrolled the view itself you'd now do
>>>> the same with the top level item. You might wish to use
>>>> QGraphicsScene::setSceneRect() to control which part of the scene
>>>> should be mapped to the view at any time.
>>>>
>>>> I don't think this way of doing things should be significantly harder
>>>> than changing the view transformation.
>>> But how to actually handle the events is something else. Right now the
>>> child widgets all accept mouse events, which means the parent widget
>>> doesn't get them, so it can't know when a person is trying to scroll or
>>> zoom.
>>>
>>> I'm not sure how to have both, selecting the game elements, AND letting
>>> the game board accept events to handle scrolling and zooming.
>> Not sure I understand from where you're currently triggering scrolling
>> and zooming. Does each game square currently tell the view it needs to
>> scroll or zoom? If so, can't you keep the current design but instead
>> tell the invisible game board item to scroll / zoom?
> 
> Sorry if I wasn't clear enough before.
> 
> Right now the scrolling happens due to the QGV's own scroll bar. Zooming 
> would have (I haven't implemented it yet) been done with the scroll wheel 
> for now (and touch events in the future).
> 
> Moving to keeping the scene's translation and transformation static would 
> mean disabling the view's scroll bar, and either implementing my own, or 
> scrolling by clicking anywhere on the "game board" and dragging the board 
> around (which is how it _will_ work on touch based devices).

Ah, that will be a problem yes. You'd have to set the scene rect to the 
bounding rect of the game board, but that means whenever graphicsview 
scrolls your non-transformable items will still be transformed. In that 
case I guess you can't avoid having to dynamically change the 
transformation of these items to have them match the current view (and 
you might as well go back to transforming the view directly in this case 
like you're already doing). Maybe there's a way to keep the item 
transformations and the view transformation in sync while avoiding any 
visual lag.

Another option would be to not have these items be graphics items, but 
just draw them at static positions in QGraphicsView::drawForeground(). 
Then override graphics view's event handlers and only pass them on to 
the QGraphicsView base class if the event is not meant for any of the 
foreground "items".