An application is able to create an interface through the use of the UIKit framework. Everytime a new element is added to the GUI, a UIView object is created and all mapped in a hierarchal structure using super/sub view relationships. For example, when a developer adds a text field to the UIView, a UITextField object is created which is a sub view in the hierarchy.

UI Kit Framework

Understanding how the view hierarchy works can be useful when trying to understand how event handling is done in iOS. Basically, all event driven interactions are done through Responder objects. A responder object is any instance of the UIResponder class which includes the subclasses UIView, UIViewController, and UIApplication`.

A good use case might be, a user selects a text field. UIResponder class directs the event to the most appropriate responder object. The Responder object then can handle the user input/actions with pre-defined logic specific for that UI element.

View Hierarchy

Personal note: This is my best stab at responder objects and believe their are many gaps in my understanding.