Managing Your App's Life Cycle

The current state of your app determines what it can and cannot do at any time. For example, a foreground app has the user’s attention, so it has priority over system resources, including the CPU. By contrast, a background app must do as little work as possible, and preferably nothing, because it is offscreen. As your app changes from state to state, you must adjust its behavior accordingly.

When your app’s state changes, UIKit notifies you by calling methods of the appropriate delegate object:

• In iOS 13 and later, use UISceneDelegate objects to respond to life-cycle events in a scene-based app.

• In iOS 12 and earlier, use the UIApplicationDelegate object to respond to life-cycle events.

If your app supports scenes, UIKit delivers separate life-cycle events for each. A scene represents one instance of your app’s UI running on a device. The user can create multiple scenes for each app, and show and hide them separately. Because each scene has its own life cycle, each can be in a different state of execution. For example, one scene might be in the foreground while others are in the background or are suspended.

The following figure shows the state transitions for scenes. When the user or system requests a new scene for your app, UIKit creates it and puts it in the unattached state. User-requested scenes move quickly to the foreground, where they appear onscreen. A system-requested scene typically moves to the background so that it can process an event. For example, the system might launch the scene in the background to process a location event. When the user dismisses your app's UI, UIKit moves the associated scene to the background state and eventually to the suspended state. UIKit can disconnect a background or suspended scene at any time to reclaim its resources, returning that scene to the unattached state.

Use scene transitions to perform the following tasks:

• When UIKit connects a scene to your app, configure your scene’s initial UI and load the data your scene needs.

• When transitioning to the foreground-active state, configure your UI and prepare to interact with the user. See Preparing Your UI to Run in the Foreground.

• Upon leaving the foreground-active state, save data and quiet your app’s behavior. See Preparing Your UI to Run in the Background.

• Upon entering the background state, finish crucial tasks, free up as much memory as possible, and prepare for your app snapshot. See Preparing Your UI to Run in the Background.

• At scene disconnection, clean up any shared resources associated with the scene.

• In addition to scene-related events, you must also respond to the launch of your app using your UIApplicationDelegate object. For information about what to do at app launch, see Responding to the Launch of Your App.

In iOS 12 and earlier, and in apps that don't support scenes, UIKit delivers all life-cycle events to the UIApplicationDelegate object. The app delegate manages all of your app’s windows, including those displayed on separate screens. As a result, app state transitions affect your app's entire UI, including content on external displays.

The following figure shows the state transitions involving the app delegate object. After launch, the system puts the app in the inactive or background state, depending on whether the UI is about to appear onscreen. When launching to the foreground, the system transitions the app to the active state automatically. After that, the state fluctuates between active and background until the app terminates.

Use app transitions to perform the following tasks:

• At launch, initialize your app’s data structures and UI. See Responding to the Launch of Your App.

• At activation, finish configuring your UI and prepare to interact with the user. See Preparing Your UI to Run in the Foreground.

• Upon deactivation, save data and quiet your app’s behavior. See Preparing Your UI to Run in the Background.

• Upon entering the background state, finish crucial tasks, free up as much memory as possible, and prepare for your app snapshot. See Preparing Your UI to Run in the Background.

• At termination, stop all work immediately and release any shared resources. See applicationWillTerminate:.

In addition to handling life-cycle events, apps must also be prepared to handle the events listed in the following table. Use your UIApplicationDelegate object to handle most of these events. In some cases, you may also be able to handle them using notifications, allowing you to respond from other parts of your app.