Question about TouchEffect xamarin sample

Hi guys, today I copy the implementation of DraggableBoxViewDraggingPage.xaml from https://github.com/xamarin/xamarin-forms-samples/tree/master/Effects/TouchTrackingEffectDemos/TouchTrackingEffectDemos/TouchTrackingEffectDemos . But I want to make a little changes like:

• The dragged box cant be dragged outside, it need to stay completely inside the parent.

Any clue? Im try to do some modifications at the code but Im not understand the value of coordinate used into TouchActionEventArgs

Thx in Advance