QGraphicsItem::setCacheMode - how does it work?

[wysota]

Subclass QGraphicsRectItem, add some debug messages to this class's paint() method, add an instance of it to your scene and enable caching for it. If paint() gets called after changing the rect of the item, then the blame is in your item code. If it doesn't it's somewhere else (e.g. scene code?).

[d_stranz]

OK, did this, it works, my original class still doesn't. The good news is that when I rubberband over the new QGraphicsRectItem, its paint() method *isn't* being called, which is exactly what I was trying to achieve in my original class and which I had hoped caching would achieve.

Time to dig into the code and figure out what's going wrong.

[wysota]

A shortlist to check (as always with graphics items):
1. Is my bounding rect defined correctly (e.g. is it independent of the placement of the item)?
2. Is my shape defined within the bounding rect?
3. Is my paint() routine painting only within the bounding rect?
4. Am I calling prepareGeometryChange() each time the bounging rect is going to change?

[d_stranz]

1. Is my bounding rect defined correctly (e.g. is it independent of the placement of the item)?

Thank you, thank you. That's where the problem is. My bounding rect was defined in world coordinates, which resulted in a positive width() but negative height(). I have yet to get it right, but at least the item's paint() method is now being called as expected, and caching is doing the right thing when I rubberband.

I owe you a beer or six.

Edit: YES! That was it. Tell me where to send the beer.

[wysota]

Edit: YES! That was it. Tell me where to send the beer.

Qt Developer Days 2012. Hopefully...