What is responder chain?
App manages events by using Responder Object. Responder objects are subclass instances of UIResponder which is UIView, UIViewController, UIApplication.
Responder object should manage events by itself or transfer to other responder object.
When a event is occured, app transfers the event to suitable resonder object which is called first responder
Responders receive the raw event data and must either handle the event or forward it to another responder object.