There is not a clear way to get know if QGraphicsItem's boundingRect changes.

Of course it is possible to reimplement boundingRect() which saves the value from QGraphicsItem::boundingRect() and checks if the QRectF value have been changed, but there should be more clear way to get this information.

One solution would be to add new constant to GraphicsItemChange and call QGraphicsItem::itemChange() with it when QGraphicsItem's boundingRect is updated.