protected override void OnTrackedElementTouchUp(object sender, TouchEventArgs e)
{
if (currentEngine == null)
return;
DateTime timestamp = TimestampToDateTime(e.Timestamp);
Point position = e.GetTouchPoint(null).Position;
currentEngine.TrackTouchUp(position, timestamp);
base.OnTrackedElementTouchUp(sender, e);
}
protected override void OnGestureAborted(IGestureEngine engine)