Explain the concept of the responder chain in the event handling system in GUI applications.

The Responder Chain in iOS is a mechanism that allows events (such as touches, gestures, key presses) to be passed along a chain of objects capable of handling them. When an event occurs, it is first sent to the first responder (usually a UIView or UIViewController). If that object does not handle the event, it is passed to the next object in the chain, and so on, until the event is handled or the chain ends.

This allows flexible event handling without tightly coupling to specific objects. For example, if a user taps a button that does not handle the event, it can be passed to a controller or another object that can respond.

Example responder chain: UIView → UIViewController → UIWindow → UIApplication.

Thus, the responder chain provides a convenient way to route events in iOS GUI applications.