QDrag completion detection

Say class A instance instantiates QDrag and calls exec() on it. There exists also an instance of class B. Is there a way for the instance of class B to find out when the drag finishes (i.e. a drop occurs), even if the 'drop' is invalid (i.e. the mouse button is released when the cursor is a stop sign) or outside of the application. The usual way would be to install an application-wide event filter, but what if the drop is invalid or outside of the application and no QDropEvent is sent?