qt5base-lts/examples/widgets/touch/knobs/knob.cpp

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#include "knob.h"

#include <QBrush>
#include <QTouchEvent>

Knob::Knob()
    : QGraphicsEllipseItem(-50, -50, 100, 100)
{
    setAcceptTouchEvents(true);
    setBrush(Qt::lightGray);

    QGraphicsEllipseItem *leftItem = new QGraphicsEllipseItem(0, 0, 20, 20, this);
    leftItem->setPos(-40, -10);
    leftItem->setBrush(Qt::darkGreen);

    QGraphicsEllipseItem *rightItem = new QGraphicsEllipseItem(0, 0, 20, 20, this);
    rightItem->setPos(20, -10);
    rightItem->setBrush(Qt::darkRed);
}

bool Knob::sceneEvent(QEvent *event)
{
    switch (event->type()) {
    case QEvent::TouchBegin:
    case QEvent::TouchUpdate:
    case QEvent::TouchEnd:
    {
        QTouchEvent *touchEvent = static_cast<QTouchEvent *>(event);

        if (touchEvent->touchPoints().count() == 2) {
            const QTouchEvent::TouchPoint &touchPoint1 = touchEvent->touchPoints().first();
            const QTouchEvent::TouchPoint &touchPoint2 = touchEvent->touchPoints().last();

            QLineF line1(touchPoint1.lastScenePos(), touchPoint2.lastScenePos());
            QLineF line2(touchPoint1.scenePos(), touchPoint2.scenePos());

            setTransform(QTransform().rotate(line2.angleTo(line1)), true);
        }

        break;
    }

    default:
        return QGraphicsItem::sceneEvent(event);
    }

    return true;
}