Gestures Programming

Standard Procedure for Event Handling

A QGesture is by default accepted when it arrives at your widget. However, it is good practice to always explicitly accept or reject a gesture. The general rule is that, if you accept a gesture, you are using it. If you are ignoring it you are not interested in it. Ignoring a gesture may mean it gets offered to another target object, or it will get canceled.

Each QGesture has several states it goes through; there is a well defined way to change the state, typically the user input is the cause of state changes (by starting and stopping interaction, for instance) but the widget can also cause state changes.

The first time a particular QGesture is delivered to a widget or graphics item, it will be in the Qt::GestureStarted state. The way you handle the gesture at this point influences whether you can interact with it later.

• Accepting the gesture means the widget acts on the gesture and there will follow gestures with the Qt::GestureUpdatedstate.
• Ignoring the gesture will mean the gesture will never be offered to you again. It will be offered to a parent widget or item as well.
• Calling setGestureCancelPolicy() on the gesture when it is in its starting state, and is also accepted can cause other gestures to be canceled.

Using QGesture::CancelAllInContext to cancel a gesture will cause all gestures, in any state, to be canceled unless they are explicitly accepted. This means that active gestures on children will get canceled. It also means that gestures delivered in the same QGestureEvent will get canceled if the widget ignores them. This can be a useful way to filter out all gestures except the one you are interested in.

 bool ImageWidget::event(QEvent *event)
 {
     if (event->type() == QEvent::Gesture)
         return gestureEvent(static_cast<QGestureEvent*>(event));
     return QWidget::event(event);
 }

 bool ImageWidget::gestureEvent(QGestureEvent *event)
 {
     if (QGesture *swipe = event->gesture(Qt::SwipeGesture))
         swipeTriggered(static_cast<QSwipeGesture *>(swipe));
     else if (QGesture *pan = event->gesture(Qt::PanGesture))
         panTriggered(static_cast<QPanGesture *>(pan));
     if (QGesture *pinch = event->gesture(Qt::PinchGesture))
         pinchTriggered(static_cast<QPinchGesture *>(pinch));
     return true;
 }

 void ImageWidget::swipeTriggered(QSwipeGesture *gesture)
 {
     if (gesture->state() == Qt::GestureFinished) {
         if (gesture->horizontalDirection() == QSwipeGesture::Left
             || gesture->verticalDirection() == QSwipeGesture::Up)
             goPrevImage();
         else
             goNextImage();
         update();
     }
 }