How to calculate NSRange of NSString at certain position in UITextView

Question

When I tap on a certain position in a UITextView, I want to retrieve the substring of the NSString which is shown at the line.

I have e.g. a UITextView which displays a string, using 16 lines. When I tap on position (200, 150), I want the substring which is shown by UITextView on that tap.

Any suggestions on how to achieve this?