@protocol UIGestureRecognizerDelegate <NSObject>@optional// called when a gesture recognizer attempts to transition out of UIGestureRecognizerStatePossible. returning NO causes it to transition to UIGestureRecognizerStateFailed- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer;// called when the recognition of one of gestureRecognizer or otherGestureRecognizer would be blocked by the other// return YES to allow both to recognize simultaneously. the default implementation returns NO (by default no two gestures can be recognized simultaneously)//// note: returning YES is guaranteed to allow simultaneous recognition. returning NO is not guaranteed to prevent simultaneous recognition, as the other gesture‘s delegate may return YES- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer;// called before touchesBegan:withEvent: is called on the gesture recognizer for a new touch. return NO to prevent the gesture recognizer from seeing this touch- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldReceiveTouch:(UITouch *)touch;@end

// create a relationship with another gesture recognizer that will prevent this gesture‘s actions from being called until otherGestureRecognizer transitions to UIGestureRecognizerStateFailed// if otherGestureRecognizer transitions to UIGestureRecognizerStateRecognized or UIGestureRecognizerStateBegan then this recognizer will instead transition to UIGestureRecognizerStateFailed// example usage: a single tap may require a double tap to fail- (void)requireGestureRecognizerToFail:(UIGestureRecognizer *)otherGestureRecognizer;