UIResponder

The UIResponder class defines an interface for objects that respond to and handle events. It is the superclass of UIApplication, UIView and its subclasses (which include UIWindow). Instances of these classes are sometimes referred to as responder objects or, simply, responders.


UIButton Hierarchy 

UIButton inherits from UIControl, UIControl inherits from UIView, UIView inherits from UIResponder, UIResponder inherits from the root class NSObject


UIButton <-- UIControl <-- UIView <-- UIResponder <-- NSObject