Why am I getting invalid data from my QTouchPoint?

I have a QGraphicsScene with QGraphicsItems. I have reimplemented the sceneEvent function and handle multi-touch.

The problem is that randomly I get invalid values out of this section:

    QTouchEvent *touchEvent = static_cast<QTouchEvent *>(event);
    QList<QTouchEvent::TouchPoint> touchPoints = touchEvent->touchPoints();
    const QTouchEvent::TouchPoint &p0 = touchPoints.first();

Gives the following debug information (not all the time):

Debug: TouchUpdate: p0.scenePos(): QPointF(489.76, 160.71) :
Debug: TouchUpdate: p0.startPos(): QPointF(-8.62078e+14, 1.83351e+15)

I have no idea why startPos() should be a random 开发者_Go百科value like that. Any suggestions?

Are you blindly casting events or using a switch-case statement as below ?

    bool MyItem::sceneEvent(QEvent *event)
    {
        case QEvent::TouchBegin:
        case QEvent::TouchUpdate:
        case QEvent::TouchEnd:
        {
            QTouchEvent *touchEvent = static_cast<QTouchEvent *>(event);
            QList<QTouchEvent::TouchPoint> touchPoints = touchEvent->touchPoints();

            //Remember to accept the event too
            event->accept();
            break;
        }
    }