Pinch Zoom Example

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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);
}