After you’ve built a basic view hierarchy, your next step is to control the visual elements and respond to user input. In an iOS app, you use a view controller (
UIViewController) to manage a content view with its hierarchy of subviews.
A view controller isn’t part of the view hierarchy and it’s not an element in your interface. Instead, it manages the view objects in the hierarchy and provides them with behavior. Each content view hierarchy that you build in your storyboard needs a corresponding view controller, responsible for managing the interface elements and performing tasks in response to user interaction. This usually means writing a custom
UIViewController subclass for each content view hierarchy. If your app has multiple content views, you use a different custom view controller class for each content view.
View controllers play many roles. They coordinate the flow of information between the app’s data model and the views that display that data, manage the life cycle of their content views, and handle orientation changes when the device is rotated. But perhaps their most obvious role is to respond to user input.
You also use view controllers to implement transitions from one type of content to another. Because iOS apps have a limited amount of space in which to display content, view controllers provide the infrastructure needed to remove the views of one view controller and replace them with the views of another.
To define interaction in your app, make your view controller files communicate with the views in your storyboard. You do this by defining connections between the storyboard and source code files through actions and outlets.