Who is in the responder chain in iOS?

Responder chain is a sequence of objects inherited from UIResponder that have the ability to handle an event.

A typical sequence includes:

1. UIResponder that initially receives the event: This could be a UIView (for example, a button) or a UIViewController.
2. Superview: If the responder does not handle the event, it is passed to its superview.
3. View controller (UIViewController): If the superview does not handle the event, it can be passed to its controller.
4. Window (UIWindow): If the controller does not handle the event, it is passed to the UIWindow object.
5. Application (UIApplication): If the window does not handle the event, it is passed to the UIApplication object.
6. Application delegate (AppDelegate): If UIApplication does not handle the event, it can be passed to its delegate.

The event is passed by calling the appropriate event handling method (touchesBegan(_:with:), motionBegan(_:with:), etc.) on the next responder in the chain, if the current responder did not handle the event (i.e., if the method implementation does not call super).