Here is a simple re-implementation of mouse events for a QGraphicsView:

Qt Code: 
  1. void kadeView::mousePressEvent (QMouseEvent * event )
  2. {
  3.   if (event->button() == Qt::MidButton) //pan
  4.   {
  5.     isPanning=1;
  6.     saved_cursor=viewport()->cursor();
  7.     setCursor(Qt::ClosedHandCursor);
  8.     mouse_pos=mapToScene(event->pos());
  9.     event->accept();
  10.     return;
  11.   };
  12.   QGraphicsView::mousePressEvent(event);
  13. };
  14.  
  15. void kadeView::mouseMoveEvent ( QMouseEvent * event )
  16. {
  17.   if(isPanning==1)
  18.   {
  19.     QPointF delta(mouse_pos-mapToScene(event->pos()));
  20.     QRectF r1(sceneRect());
  21.     r1.translate(delta.x(),delta.y());
  22.     setSceneRect(r1);
  23.     //translate(delta.x(),delta.y());
  24.     //ensureVisible(pan_rect.x()-line.dx(),pan_rect.y()-line.dy(),pan_rect.width(),pan_rect.height());
  25.     //centerOn(pan_rect.center().x()-line.dx(),pan_rect.center().y()-line.dy());
  26.     event->accept();
  27.     return;
  28.   };
  29.   QGraphicsView::mouseMoveEvent(event);
  30. };
  31.  
  32. void kadeView::mouseReleaseEvent ( QMouseEvent * event )
  33. {
  34.   if(isPanning==1)
  35.   {
  36.     isPanning=0;
  37.     setCursor(saved_cursor);
  38.     event->accept();
  39.     return;
  40.   };
  41.   QGraphicsView::mouseReleaseEvent(event);
  42. };
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It pans the view on middle mouse click and drag.
Can someone explain me why the only command that works is the setSceneRect() in mouseMoveEvent, and not the commented ones?
If I uncomment and replace setSceneRect with any of the translate(), centerOn(), ensureVisible() nothing is happening in the view...

Thank you in advance