Question
What Are iOS App Lifecycle Methods?
Answer
The application life cycle constitutes the sequence of events that occurs between the launch and termination of application.
UIKit-based app lifecyle
iOS <= 12
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.
import UIKit
@main
class AppDelegate: UIResponder, UIApplicationDelegate {
func application(_ application: UIApplication, willFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
return true
}
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
return true
}
func applicationWillEnterForeground(_ application: UIApplication) {}
func applicationDidBecomeActive(_ application: UIApplication) {}
func applicationWillResignActive(_ application: UIApplication) {}
func applicationDidEnterBackground(_ application: UIApplication) {}
func applicationWillTerminate(_ application: UIApplication) {}
}API Docs:
iOS >= 13
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.
import UIKit
@main
class AppDelegate: UIApplicationDelegate {
func application(_:didFinishLaunchingWithOptions:) -> Bool {}
func application(_:configurationForConnecting:options:) -> UISceneConfiguration {}
func application(_:didDiscardSceneSessions:) {}
}
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
var window: UIWindow?
func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {}
func sceneDidDisconnect(_ scene: UIScene) {}
func sceneDidBecomeActive(_ scene: UIScene) {}
func sceneWillResignActive(_ scene: UIScene) {}
func sceneWillEnterForeground(_ scene: UIScene) {}
func sceneDidEnterBackground(_ scene: UIScene) {}
}API Docs:
SwiftUI-based app lifecycle
iOS <= 13
class AppDelegate: NSObject, UIApplicationDelegate {
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
print("Your code here")
return true
}
}
@main
struct MainApp: App {
// Attach your AppDelegate to your SwiftUI app using
// UIApplicationDelegateAdaptor property wrapper
// SwiftUI is responsible for creating that delegate
// and looking after its lifetime,
// so you can go ahead and add any other app delegate
// functionality to that class to have it called.
@UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate
var body: some Scene {
WindowGroup {
ContentView()
}
}
}iOS >=14
@main
struct MainApp: App {
// The system moves your app’s Scene instances through
// phases that reflect a scene’s operational state.
// Observe scenePhase by using Environment property wrapper.
@Environment(\.scenePhase) var scenePhase
var body: some Scene {
WindowGroup {
ContentView()
}
// React to changes of scenaPhase by using onChange
.onChange(of: scenePhase) { newScenePhase in
switch newScenePhase {
case .active:
print("App is active")
case .inactive:
print("App is inactive")
case .background:
print("App is in background")
@unknown default:
print("Oh - interesting: I received an unexpected new value.")
}
}
}
}
}API Docs:
