Mouse gestures in Qt apps, to be or not to be.

Thinking of what are the hottest buzz-words in last years "touch screen" and "gestures" come up in mind and many companies work on supporting them in their products. Let's look at "Gestures" - Microsoft has support for touch-screen based gestures in upcoming Windows 7, Opera has mouse gestures for a long time and might have face gestures in future (or not ;) )

Anyway it sounds like Qt should also benefit from having an cross-platform and extensible Gestures API allowing us to use both native (aka "system-generated") gestures (Zoom gesture on Windows and Pinch gesture on Macs) and custom gestures that are defined by the application developer.

We are talking about mouse- and finger-based gestures, i.e. strokes that are drawn with a mouse pointer or with a finger on a touch screen/touchpad (which are translated to mouse events) - from simple double-tapping on a screen (which can also be considered a gesture) to multi-touch gestures (i.e. that are made using several input points - with two fingers).

I've written some mockup code showing how gestures could be used:

void ImageViewWidget::gestureEvent(QGestureEvent *event)

{

    if (event->gesture(Qt::TapGesture)) {

        // we were tapped!

    } else if (const QGesture *g = event->gesture(Qt::DoubleTapGesture)) {

        scaleTo(3.0, 3.0);

        update()

    } else if (const QGesture *g = event->gesture(Qt::PanGesture)) {

        if (g->state() == Qt::GestureStarted)

            setCursor(Qt::SizeAllCursor);

        else

            setCursor(Qt::ArrowCursor);

        panByOffset(g->pos() - g->lastPos());

        update();

    } else if (const QSlideGesture *g = static_cast<QSlideGesture*>(event->gesture(Qt::SlideGesture))) {

        // converted from base class QGesture to QSlide gesture that

        // contains extended info about this particular gesture.

        if (g->state() == Qt::GestureFinished) {

            if (g->direction() == Qt::RightDirection)

                loadNextImage();

            else if (g->direction() == Qt::LeftDirection)

                loadPreviousImage();

            update();

        }

    } else if (const QGesture *g = event->gesture(Qt::TapAndHoldGesture)) {

        if (g->state() == Qt::GestureFinished) {

            QMenu menu;

            menu.addAction(loadNextImageAction);

            menu.addAction(loadPreviousImageAction);

            menu.addAction(toggleZoomAction);

            menu.exec(mapToGlobal(g->hotSpot()));

        }

    }

    event->accept();

}

However there are a lot of unanswered questions - could it be useful? What are the use cases for gestures? What kind of gestures should work out of the box? (ok, ok, I suppose finger scrolling should work where appropriate).

And more technical ones - how should gestures behave regarding mouse/touch input - consider a Tablet PC with a touch screen where the user taps and moves a finger - should mouse events delivery be delayed until we decide whether it is a gesture, or should we deliver mouse down event, several mouse move events and then switch to delivering gesture events instead? What about ambiguous gestures - what should we do if we are not sure whether it is a Tap or Panning gesture, or we should wait until DoubleTap gesture is finished.

Do you have any other interesting use cases?