how to judge slide end on screen on ios

i want to realize a game like fruit ninja.but I have no good idea to detect the user slide end and slide multiple objects. there is touchPhase.Stationary and TouchPhase.Moved,but user may be Continuous sliding and finger don’t leave the screen.

var curPos : Vector3 = Vector3(touch.position.x,touch.position.y,0);
	 	var moveRes : float = Vector3.Dot(lastPos,curPos);
	 	if(moveRes > 0.90)
	 		print("----this is one slice");
	 		print("****** change the dir");

I use touch position to detect,but has no good result.I put the code in the function Update().may be should put in other function?

sorry , I think my approach is right


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