Weird error with QGraphicsItem sent from one thread to another.

JulienMaille

Hello everyone, I'm facing a strange error.
I have a processing thread that emit a few signal with QGraphicsItem* in parameters.
Those signals are connected to the main thread which then add them to a QGraphicsViews.
It works pretty well, until today. In addition to the ellipses and line I decided to add a QGraphicsTextItem*

It still work, until the application loses focus. At this very specific moment, I get this error:
!http://i.imgur.com/K6arKe3.png!

Why does it crash with text items only? And why when the application loses focus?
I'm lost!

andre

You can't do that. QGraphicsItems are GUI objects, and can only be manipulated from the GUI thread. What you are trying to do is not supported, and can indeed lead to weird crashes and other undefined behaviour.

JulienMaille

But the thread is only responsible for creating the object. All the display is done in the main thread.

andre

AFAIK, that doesn't matter.

giesbert

Each QObject derived class belongs to the thread, that creates it (Thread affinity). So creating UI objects in a non UI thread is a no go.

JulienMaille

But QGraphicsItem does not derive from QObject.

giesbert

But it is a UI object, and it could be that it stores some data...
Maybe some special cases for the text items...