QgraphicsView rubberbanddrag effects zooming behavior

Question

I am stuck with this weird behavior where, if I enable rubberband drag, the wheel event doesn't zoom under mouse anymore. It does zooming but irrespective of mouse position. And if I disable other events, then wheelEvent works properly.

I have a custom class inheriting QGraphicsView as :

class MyGraphics : public QGraphicsView{

public :
    MyGraphics(QWidget *parent = NULL);

public slots:
     void zoomIn() { scale(1.2, 1.2); }
     void zoomOut() { scale(1 / 1.2, 1 / 1.2); }

protected :
   QRubberBand *rubberBand;
   QPoint       origin;
   QPointF      InitialCenterPoint;
   QPointF      CurrentCenterPoint;
   QPoint       rubberBandOrigin;
   bool         rubberBandActive;
   QPoint       LastPanPoint;
   int          _numScheduledScalings;

//if I uncomment these three event handlers, the wheelevent doesnt zoom properly!

 //  virtual void mousePressEvent(QMouseEvent *event);
 //  virtual void mouseMoveEvent(QMouseEvent *event);
 //  virtual void mouseReleaseEvent(QMouseEvent *event);
   virtual void wheelEvent(QWheelEvent *);
   virtual void resizeEvent(QResizeEvent *event);

};

The constructor :

MyGraphics::MyGraphics(QWidget *parent) : QGraphicsView(parent){
    setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);

    this->installEventFilter(this);
    this->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    this->setResizeAnchor( QGraphicsView::AnchorUnderMouse );
    QGraphicsScene *graphScene = new QGraphicsScene(this);

    this->setScene(graphScene);
    QGraphicsItem* pEllipse = graphScene->addEllipse(0,100,50,50);
    QGraphicsItem* pRect = graphScene->addRect(200,100,50,50);

    pEllipse->setFlag(QGraphicsItem::ItemIsSelectable, true);
    pRect->setFlag(QGraphicsItem::ItemIsSelectable, true);

    setSceneRect(0, 0, 1000, 1000);
    rubberBandOrigin = QPoint(0,0);
}

Event handlers :

void MyGraphics::wheelEvent(QWheelEvent *event){
    if(event->delta() > 0){
        //Zoom in
        this->zoomIn();
    } else {
        this->zoomOut();
    }

}

/*
void MyGraphics::mousePressEvent(QMouseEvent *event)
{
        if (event->button() == Qt::MiddleButton) {
        rubberBandOrigin = event->pos();
        rubberBand = new QRubberBand(QRubberBand::Rectangle, this);
        rubberBand->setGeometry(event->x(),event->y(),0, 0);
        rubberBand->show();
        rubberBandActive = true;
    }
if(event->button() == Qt::LeftButton){
    LastPanPoint = event->pos();
}


}

void MyGraphics::mouseMoveEvent(QMouseEvent *event)
{
    if (event->buttons() == Qt::MiddleButton && rubberBandActive == true){
        rubberBand->resize( event->x()-rubberBandOrigin.x(), event->y()-rubberBandOrigin.y() );
    }
    else{
        if(!LastPanPoint.isNull()) {
            //Get how much we panned
            QGraphicsView * view = static_cast<QGraphicsView *>(this);

            QPointF delta = view->mapToScene(LastPanPoint) - view->mapToScene(event->pos());
            LastPanPoint = event->pos();
        }
    }
}

void MyGraphics::mouseReleaseEvent(QMouseEvent *event)
{
   if (event->button() == Qt::MiddleButton){
        QGraphicsView * view = static_cast<QGraphicsView *>(this);

        QPoint rubberBandEnd = event->pos();

        QRectF zoomRectInScene = QRectF(view->mapToScene(rubberBandOrigin), view->mapToScene(rubberBandEnd));
        QPointF center = zoomRectInScene.center();

        view->fitInView(zoomRectInScene, Qt::KeepAspectRatio);
        rubberBandActive =  false;
        delete rubberBand;
    }
    else{
        LastPanPoint = QPoint();
    }
}
*/

Any idea where I am doing wrong or how do I fix it ?

Answer 1

QGraphicsView::scale function's behaviour depends on the mouse position. It's performing automatically and internally by QGraphicsView. Since you don't pass mouse position to the scale function, I think QGraphicsView tracks the mouse and remembers the last position on its own.

By reimplementing mouse event handlers you have taken this ability from it. The view can't determine the mouse position anymore because its original handlers aren't called.

Luckily this issue can be easily fixed. You need to call base class implementation before your own:

void MyGraphics::mousePressEvent(QMouseEvent *event) {
  QGraphicsView::mousePressEvent(event);
  // your implementation goes here
}

It's an example for mousePressEvent but you should add similar statements to all your event handlers unless you need to disable some part of default behavior.