How to draw a free polygon with one finger on the top of mkmapview?

I would like to draw a free polygon with one finger on top of Mkmapview. I know how to draw a free polygon on a UIView, and I know how to call mkmapview.

I don’t know if I can use the overlay function because I need to draw a polygon as my finger moves. I assume this includes the multitouch and drawRect events, but I can't get it to work when I stack them.

Thanks in advance if anyone can help.