Explain the role of 'ViewControllers' in iOS.
Instruction: Describe what ViewControllers are and how they are used in iOS app development.
Updated
Official answer
A UIViewController manages a view hierarchy and coordinates its behavior. It responds to user actions, presents model state, participates in layout and appearance changes, and coordinates navigation or other controllers. A controller can own one part of a screen; container controllers such as navigation or tab controllers organize child controllers, so the relationship is not always one controller per screen.
Views are normally loaded lazily. viewDidLoad is for setup after that controller's view hierarchy loads; it is not a once-per-app callback. Appearance methods can run repeatedly as the view appears and disappears. Layout callbacks can also run many times, so they should not trigger an uncontrolled new network request or create duplicate constraints on every pass.
For an item screen, the controller can ask an injected service to load data, render loading/empty/error/success states and route selection. Domain rules, persistence and transport belong in suitable collaborators rather than being accumulated in the controller. Cancel or invalidate work when appropriate and avoid applying a stale result to a different item or released screen.
UIKit's controller/UI boundary is main-actor isolated. Keep blocking work out of those callbacks, preserve containment rules when adding children and call required superclass implementations. Test repeated presentation, resizing, restoration and failure paths as well as the first successful view load.