Unable to get Qt gestures (QPinchGesture) to work - qt

Question

I am experimenting with gestures... and failing.
My code below is trying to get zoom on pinch — but there is no change
#include <QApplication>
#include <QtWidgets>
#include <QGraphicsView>
#include <QPinchGesture>
#include <QGestureEvent>
#include <QGraphicsRectItem>
#include <QMessageBox>
class MyView: public QGraphicsView
{
public:
MyView(QWidget *parent = 0)
{
Q_UNUSED(parent);
setTransformationAnchor(AnchorUnderMouse);
resetTransform();
setDragMode(ScrollHandDrag);
grabGesture(Qt::PinchGesture);
}
protected:
virtual bool event(QEvent *event)
{
if (event->type() == QEvent::Gesture)
return gestureEvent(static_cast<QGestureEvent*>(event));
return QGraphicsView::event(event);
}
private:
bool gestureEvent(QGestureEvent *event)
{
if (QGesture *pinch = event->gesture(Qt::PinchGesture))
pinchTriggered(static_cast<QPinchGesture *>(pinch));
return true;
}
void pinchTriggered(QPinchGesture* event)
{
QMessageBox::information(0, tr("pinch"), tr("triggered."));
// ... rest of the code may or may not work but I can't seem to get here
// so I removed it for now
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QGraphicsScene scene(-20, -20, 150, 150);
MyView view;
view.setScene(&scene);
QGraphicsRectItem* item = new QGraphicsRectItem(QRectF(0,0,100,100));
scene.addItem(item);
view.ensureVisible(scene.sceneRect());
view.show();
return a.exec();
}
I followed examples for Qt - for pinch: Image Gestures Example. I can't see what essential part I am missing - but I see no result.
It is hard to troubleshoot - no breakpoints or qDebug - so I used a QMessageBox, a simple text placed at the start of pinchTriggered() - and it was never shown.
I tried with 2 different devices - a Surface Pro 3, and a custom Android device.
For both, I was able to implement gestures for QTapAndHoldGesture successfully (using the same logic).
So... it seems that
QGesture *pinch = event->gesture(Qt::PinchGesture)
is always NULL...
I hope somebody can find the bugs in my little sample. Did I overlook something ? Or is there some setting needed in the device ?

Answer 1

These changes worked for me (Windows 7, Qt 4.7.1):
In your constructor, replace grabGesture(Qt::PinchGesture) with a similar call on the viewport() instead:
viewport()->grabGesture(Qt::PinchGesture);
Replace your MyView::event() override with a viewportEvent() override like this:
virtual bool viewportEvent(QEvent *event)
{
if (event->type() == QEvent::Gesture)
{
return gestureEvent(static_cast<QGestureEvent*>(event));
}
else if (event->type() == QEvent::TouchBegin)
{
return false;
}
else return QGraphicsView::viewportEvent(event);
}

Answer 2

You probably miss following in the constructor.
setAttribute(Qt::WA_AcceptTouchEvents);