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://*.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