graphicsview.cpp Example File

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 #include "graphicsview.h"

 #include <QScrollBar>
 #include <QTouchEvent>

 GraphicsView::GraphicsView(QGraphicsScene *scene, QWidget *parent)
     : QGraphicsView(scene, parent), totalScaleFactor(1)
 {
     viewport()->setAttribute(Qt::WA_AcceptTouchEvents);
     setDragMode(ScrollHandDrag);
 }

 bool GraphicsView::viewportEvent(QEvent *event)
 {
     switch (event->type()) {
     case QEvent::TouchBegin:
     case QEvent::TouchUpdate:
     case QEvent::TouchEnd:
     {
         QTouchEvent *touchEvent = static_cast<QTouchEvent *>(event);
         QList<QTouchEvent::TouchPoint> touchPoints = touchEvent->touchPoints();
         if (touchPoints.count() == 2) {
             // determine scale factor
             const QTouchEvent::TouchPoint &touchPoint0 = touchPoints.first();
             const QTouchEvent::TouchPoint &touchPoint1 = touchPoints.last();
             qreal currentScaleFactor =
                     QLineF(touchPoint0.pos(), touchPoint1.pos()).length()
                     / QLineF(touchPoint0.startPos(), touchPoint1.startPos()).length();
             if (touchEvent->touchPointStates() & Qt::TouchPointReleased) {
                 // if one of the fingers is released, remember the current scale
                 // factor so that adding another finger later will continue zooming
                 // by adding new scale factor to the existing remembered value.
                 totalScaleFactor *= currentScaleFactor;
                 currentScaleFactor = 1;
             }
             setTransform(QTransform().scale(totalScaleFactor * currentScaleFactor,
                                             totalScaleFactor * currentScaleFactor));
         }
         return true;
     }
     default:
         break;
     }
     return QGraphicsView::viewportEvent(event);
 }