ipad - ios paper app: swipe from off screen -

I'm studying paper app from fifty three and the way it's used for different gestures is very interesting looks like.

To use the drawing to set the tracks to separate from the pan, you need to swipe to work from screen to screen (like outside the iPad screen) .

How to implement that?

I do not have access to the paper code, but I think they are using a UIPanGestureRecognizer with only sides . Since the recognized "capture", it will not trigger its main drawing system, but will only catch the slide from the offscreen - it can only expand 10-20pt.