Change pages in presentation mode by swiping on touch screen
Review Request #125847 - Created Oct. 28, 2015 and updated
This patch adds support for swipe gestures to the presentation mode of okular. Swiping right-to-left goes to the previous page, swiping left-to-right goes to the next page.
1) The swipe gesture used here is a three-finger swipe, which is what the Qt QSwipeGesture class implements. I am not convinced that using three fingers is optimal here, but it is the easiest to implement. Other swiping gestures are possible, but you would need to implement a custom QGestureRecognizer. That would make the patch larger and more error-prone.
2) Swiping from left to right(!) makes the presentation mode go to the next(!) slide. This is the opposite direction compared to what you do when you flip pages in a physical book, so it may feel strange. It is, however, the direction used in the qt5 gesture example at http://doc.qt.io/qt-5/gestures-overview.html Let me know if you want the directions reversed.
3) By default, Qt5 synthesizes a mouse event for each unhandled touch event. In particular, a left mouse click is synthesized for each finger tap to the screen (which usually makes the presentation go to the next slide). This mechanism gets in the way of gesture recognition, because the first finger touch of your swipe gesture will already create a mouse-click and make your presentation go to the next page, irrespective of the swipe direction.
The patch solves this problem but switching off mouse event synthesis in presentation mode. That's the lineQCoreApplication::setAttribute(Qt::AA_SynthesizeMouseForUnhandledTouchEvents,false);
in the constructor of PresentationWidget. This has short-term side effects. For example, with this, you cannot finger-touch on a movie to start it anymore. The fix for this would be to implement proper handling of QTouchEvents, which should not be difficult.
An alternative would be to leave mouse event synthesis turned on, but implement a dummy touch event handler. This will need only a few lines of code, but it will not avoid the side effects mentioned above.
4) This patch applies to the 'frameworks' branch. I failed at backporting it to 'master'. There, the event loop would never receive any touchscreen events at all. This may be a Qt bug, but it may as well be my lack of experience.
Tested on a Thinkpad Yoga running Debian testing.