c++ - Scaling a QGrahicsView when MouseWheenTurned first scrolls, then scales -

i'm trying add zoom in/out functionality graphic i'm drawing in qt. first did extend qgraphicsscene own class graphicsscene , overload wheel event.

class graphicsscene : public qgraphicsscene {     q_object public:     graphicsscene(qobject *parent, bool drawaxes){ /*drawing stuff here.. */}     virtual void wheelevent(qgraphicsscenewheelevent *mouseevent);    signals:     void mousewheelturned(int); };   void graphicsscene::wheelevent(qgraphicsscenewheelevent* mouseevent) {   int numdegrees = mouseevent->delta() / 8;   int numsteps = numdegrees / 15; // see qwheelevent documentation    emit mousewheelturned(numsteps); } 

when wheel turned, event sent view contains scene, , there scale performed:

class graphicsview : public qgraphicsview{     q_object     qreal m_currentscale;  public:  graphicsview(qwidget * parent): qgraphicsview(parent){  m_currentscale = 1.0; } public slots:     void            onmousewheelturned  (int); }; void graphicsview::onmousewheelturned(int steps) {     qreal sign = steps>0?1:-1;     qreal current = sign* pow(0.05, abs(steps));      if(m_currentscale+current > 0){         m_currentscale += current;         qmatrix matrix;         matrix.scale(m_currentscale, m_currentscale);         this->setmatrix(matrix);      } } 

this works, noticed if zoom in lot, example top of graphic, graphic no longer in viewport, , zoom out, program first scrolls botton of graphic. can see vertical scrollbar sliding down. when has reached bottom, start zoom out. problem?

i want zoom in/ out without scroll / down behaviour.

you'd better off handling in scene , sending event view.

simply handle event directly in view qgraphicsview::wheelevent‌​, call scale function.