in the documentation there's a sample that demonstrates Graphics View Framework. The class DiagramItem inherits QGraphicsPolygonItem but it doesn't override the mouse events, so how can it answer to the mouse events?
here's the code:(copied from the QT Documentation):
Qt Code: 
  1. class DiagramItem : public QGraphicsPolygonItem
  2.  {
  3.  public:
  4.      enum { Type = UserType + 15 };
  5.      enum DiagramType { Step, Conditional, StartEnd, Io };
  6.  
  7.      DiagramItem(DiagramType diagramType, QMenu *contextMenu,
  8.          QGraphicsItem *parent = 0, QGraphicsScene *scene = 0);
  9.  
  10.      void removeArrow(Arrow *arrow);
  11.      void removeArrows();
  12.      DiagramType diagramType() const
  13.          { return myDiagramType; }
  14.      QPolygonF polygon() const
  15.          { return myPolygon; }
  16.      void addArrow(Arrow *arrow);
  17.      QPixmap image() const;
  18.      int type() const
  19.          { return Type;}
  20.  
  21.  protected:
  22.      void contextMenuEvent(QGraphicsSceneContextMenuEvent *event);
  23.      QVariant itemChange(GraphicsItemChange change, const QVariant &value);
  24.  
  25.  private:
  26.      DiagramType myDiagramType;
  27.      QPolygonF myPolygon;
  28.      QMenu *myContextMenu;
  29.      QList<Arrow *> arrows;
  30.  };
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