UIGestureRecognizer and UIMenuController - iOS Programming: The Big Nerd Ranch Guide - page 578

Bronze Challenge: Clearing Lines
Right now, the TouchDrawView clears its lines whenever the user double taps. Keep this
same functionality but have a gesture recognizer perform the recognition of the double tap
instead of touchesBegan:withEvent: . Make sure you remove the following code
from touchesBegan:withEvent: .
if ([t tapCount] > 1) {
[self clearAll];
return;
}