At diagramscene example I see :

Qt Code: 
  1. scene->update();
  2.  view->update();
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I also see :
Qt Code: 
  1. DiagramScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
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The model-view applied to this is some confused.....
It is as the two objects were sharing something ....
Who is the responsible of draw ?
Can I create a mix - object ? (On object with the mixed of both)
Is it possible ? It is a bad - good idea ?

Any explication?
Thanks