iOS programming: How to recognize if a user draws a circle or a cross?
===

Well I'm new to iOS development so I decide to start with a simple game: the tic tac toe

I added some difficulty which is to let the user freely draw their marks: a circle or a cross instead of picking them from, say, a slot containing these marks, just like in the real world we play this game using a pen and a piece of paper.

Now the problem is how can I know if a user draws a circle or a cross on the board? Any advice or reference links are appreciated.