pinchZoom with qgis 2.82 and qt

Question

I am working with qgis 2.8.2 c++ api and qt4.8.5 in my project i had to implement the pinchzoom because i use a 27" touch screen i am looking for information on how to use the qgis lib with the qt qtouchevent of Qt

does anyone have information ?

Answer 1

after a long work i find a solution in order to use the qtouchevent for zooming the map like a tablet you need to catch the viewportevent which is in the qgraphicview qt library

in qgis we use a qgsmapcanvas, and the qgsmapcanvas inherit of the qgraphicview of qt , so it is very easy to implement the touchevent

here is a sample code i use and works

bool CMyQgsMapCanvas::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();

                ZoomFactor((double)currentScaleFactor);
            }
            return true;
        }
        default:
            break;
    }
    return QGraphicsView::viewportEvent(event);
}

when i call the zoomFactor method perhaps you need to adjust the ratio for a more accurate zoom effect on your map.

don't forget to enabled the touchevent you must activate it in the constructor of your overload class of qggsmapcanvas by adding these lines :

setAttribute(Qt::WA_AcceptTouchEvents);
viewport()->setAttribute(Qt::WA_AcceptTouchEvents);
setDragMode(ScrollHandDrag);

that's all.