@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;