Qt编程之通过鼠标滚轮事件缩放QGraphicsView里面的Item

首先自己subclass QGraphicsView的一个类，叫DiagramView，然后重新实现它的滚轮事件函数，然后发送一个缩放信号：

 oid DiagramView::wheelEvent(QWheelEvent * event){

     if (event->delta() > )
     {
         emit mouseWheelZoom(true);
     }
     else
     {
         emit mouseWheelZoom(false);
     }

 }

然后用connect把这个信号连接到要实现的槽函数上（ScaleFactor初始化为30）：

 void TopologyEditor::Zoom(bool zoom){

     if (zoom && scaleFactor >= )
     {

         scaleFactor += ;
         QMatrix old_matrix;
         old_matrix = view->matrix();
         view->resetMatrix();
         view->translate(old_matrix.dx(), old_matrix.dy());
         view->scale(scaleFactor/100.0, scaleFactor/100.0);
     }
     else if (!zoom && scaleFactor >= )
     {
         scaleFactor -= ;
         QMatrix old_matrix;
         old_matrix = view->matrix();
         view->resetMatrix();
         view->translate(old_matrix.dx(), old_matrix.dy());

         view->scale( scaleFactor/100.0,  scaleFactor/100.0);

     }

     else if (scaleFactor < ){

         scaleFactor = 0.0;
     }

 }

references：

http://stackoverflow.com/questions/19113532/qgraphicsview-zooming-in-and-out-under-mouse-position-using-mouse-wheel

http://www.qtcentre.org/threads/52603-Zoom-effect-by-mouse-Wheel-in-QGraphicsview