I found a strange thing. QGraphicsScene's mousemove event would not be triggered if the scene is empty. Actually if I only add a line item, mousemove event will not be triggered either. Only after a text is added, mousemove event is ok.

It's no big deal because I could add an empty text to get everything work, but I'd like to know what's wrong (perhaps I've misunderstood something?)

I can send you relavent codes if you need, but they're too big to post here.