Using View Controllers in the Responder Chain

View controllers are descendants of the UIResponder class and are therefore capable of handling all sorts of events. When a view does not respond to a given event, it passes that event to its superview, traveling up the view hierarchy all the way to the root view. However, if any view in the chain is managed by a view controller, it passes the event to the view controller object before passing it up to the superview. In this way, the view controller can respond to events that are not handled by its views. If the view controller does not handle the event, that event moves on to the view’s superview as usual.

The Responder Chain Defines How Events Are Propagated to the App

Figure 7-1 demonstrates the flow of events within a view hierarchy. Suppose you have a custom view that is embedded inside a screen-sized generic view object, which in turn is managed by your view controller. Touch events arriving in your custom view’s frame are delivered to that view for processing. If your view does not handle an event, it is passed along to the parent view. Because the generic view does not handle events, it passes those events along to its view controller first. If the view controller does not handle the event, the event is further passed along to the superview of the generic UIView object, which in this case is the window object.

Figure 7-1  Responder chain for view controllers

Although you might not want to handle touch events specifically in your view controller, you could use it to handle motion-based events. You might also use it to coordinate the setting and changing of the first responder. For more information about how events are distributed and handled in iOS apps, see Event Handling Guide for iOS.