Programming iOS 6, 3rd Edition

Gesture Recognizers

Writing and maintaining a state machine that interprets touches across a combination of three or four touches... methods is hard enough when a view confines itself to expecting only one kind of gesture, such as dragging. It becomes even more involved when a view wants to accept and respond differently to different kinds of gesture. Furthermore, many types of gesture are conventional and standard; it seems insane to require developers to implement independently the elements that constitute what is, in effect, a universal vocabulary.

The solution is gesture recognizers, which standardize common gestures and allow the code for different gestures to be separated and encapsulated into different objects.

Gesture Recognizer Classes

A gesture recognizer (a subclass of UIGestureRecognizer) is an object attached to a UIView, which has for this purpose methods addGestureRecognizer: and removeGestureRecognizer:, and a gestureRecognizers property. A UIGestureRecognizer implements the four touches... handlers, but it is not a responder (a UIResponder), so it does not participate in the responder chain.

If a new touch is going to be delivered to a view, it is also associated with and delivered to that view’s gesture recognizers if it has any, and that view’s superview’s gesture recognizers if it has any, and so on up the view hierarchy. Thus, the place of a gesture recognizer in the view hierarchy matters, even though it isn’t part of the responder chain.

UITouch and UIEvent provide ...