App Lifecycle — Expert Decisions

Expert decision frameworks for app lifecycle choices. Claude knows scenePhase and SceneDelegate — this skill provides judgment calls for architecture decisions and background task trade-offs.

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Decision Trees

Lifecycle API Selection

What's your project setup?
├─ Pure SwiftUI app (iOS 14+)
│  └─ @main App + scenePhase
│     Simplest approach, sufficient for most apps
│
├─ Need UIKit integration
│  └─ SceneDelegate + UIHostingController
│     Required for some third-party SDKs
│
├─ Need pre-launch setup
│  └─ AppDelegate + SceneDelegate
│     SDK initialization, remote notifications
│
└─ Legacy app (pre-iOS 13)
   └─ AppDelegate only
      window property on AppDelegate

The trap: Using SceneDelegate when pure SwiftUI suffices. scenePhase covers most use cases without the boilerplate.

Background Task Strategy

What work needs to happen in background?
├─ Quick save (< 5 seconds)
│  └─ UIApplication.beginBackgroundTask
│     Request extra time in sceneDidEnterBackground
│
├─ Network sync (< 30 seconds)
│  └─ BGAppRefreshTask
│     System schedules, best-effort timing
│
├─ Large download/upload
│  └─ Background URL Session
│     Continues even after app termination
│
├─ Location tracking
│  └─ Location background mode
│     Significant change or continuous
│
└─ Long processing (> 30 seconds)
   └─ BGProcessingTask
      Runs during charging, overnight

State Restoration Approach

What state needs restoration?
├─ Simple navigation state
│  └─ @SceneStorage
│     Per-scene, automatic, Codable types only
│
├─ Complex navigation + data
│  └─ @AppStorage + manual encoding
│     More control, cross-scene sharing
│
├─ UIKit-based navigation
│  └─ State restoration identifiers
│     encodeRestorableState/decodeRestorableState
│
└─ Don't need restoration
   └─ Start fresh each launch
      Some apps are better this way

Launch Optimization Priority

What's blocking your launch time?
├─ SDK initialization
│  └─ Defer non-critical SDKs
│     Analytics can wait, auth cannot
│
├─ Database loading
│  └─ Lazy loading + skeleton UI
│     Show UI immediately, load data async
│
├─ Network requests
│  └─ Cache + background refresh
│     Never block launch for network
│
└─ Asset loading
   └─ Progressive loading
      Load visible content first

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NEVER Do

Launch Time

NEVER block main thread during launch:

// ❌ UI frozen until network completes
func application(_ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    let data = try! Data(contentsOf: remoteURL)  // Synchronous network!
    processData(data)
    return true
}

// ✅ Defer non-critical work
func application(_ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    setupCriticalServices()  // Auth, crash reporting

    Task.detached(priority: .background) {
        await self.setupNonCriticalServices()  // Analytics, prefetch
    }
    return true
}

NEVER initialize all SDKs synchronously:

// ❌ Each SDK adds to launch time
func application(...) -> Bool {
    AnalyticsSDK.initialize()      // 100ms
    CrashReporterSDK.initialize()  // 50ms
    FeatureFlagsSDK.initialize()   // 200ms
    SocialSDK.initialize()         // 150ms
    // Total: 500ms added to launch!
    return true
}

// ✅ Prioritize and defer
func application(...) -> Bool {
    CrashReporterSDK.initialize()  // Critical — catches launch crashes

    DispatchQueue.main.async {
        AnalyticsSDK.initialize()  // Can wait one runloop
    }

    Task.detached(priority: .utility) {
        FeatureFlagsSDK.initialize()
        SocialSDK.initialize()
    }
    return true
}

Background Tasks

NEVER assume background time is guaranteed:

// ❌ May not complete — iOS can terminate anytime
func sceneDidEnterBackground(_ scene: UIScene) {
    performLongSync()  // No protection!
}

// ✅ Request background time and handle expiration
func sceneDidEnterBackground(_ scene: UIScene) {
    var taskId: UIBackgroundTaskIdentifier = .invalid
    taskId = UIApplication.shared.beginBackgroundTask {
        // Expiration handler — save partial progress
        savePartialProgress()
        UIApplication.shared.endBackgroundTask(taskId)
    }

    Task {
        await performSync()
        UIApplication.shared.endBackgroundTask(taskId)
    }
}

NEVER forget to end background tasks:

// ❌ Leaks background task — iOS may terminate app
func saveData() {
    let taskId = UIApplication.shared.beginBackgroundTask { }
    saveToDatabase()
    // Missing: endBackgroundTask!
}

// ✅ Always end in both success and failure
func saveData() {
    var taskId: UIBackgroundTaskIdentifier = .invalid
    taskId = UIApplication.shared.beginBackgroundTask {
        UIApplication.shared.endBackgroundTask(taskId)
    }

    defer { UIApplication.shared.endBackgroundTask(taskId) }

    do {
        try saveToDatabase()
    } catch {
        Logger.app.error("Save failed: \(error)")
    }
}

State Transitions

NEVER trust applicationWillTerminate to be called:

// ❌ May never be called — iOS can kill app without notice
func applicationWillTerminate(_ application: UIApplication) {
    saveCriticalData()  // Not guaranteed to run!
}

// ✅ Save on every background transition
func sceneDidEnterBackground(_ scene: UIScene) {
    saveCriticalData()  // Called reliably
}

// Also save periodically during use
Timer.scheduledTimer(withTimeInterval: 60, repeats: true) { _ in
    saveApplicationState()
}

NEVER do heavy work in sceneWillResignActive:

// ❌ Blocks app switcher animation
func sceneWillResignActive(_ scene: UIScene) {
    generateThumbnails()  // Visible lag in app switcher
    syncToServer()        // Delays user
}

// ✅ Only pause essential operations
func sceneWillResignActive(_ scene: UIScene) {
    pauseVideoPlayback()
    pauseAnimations()
    // Heavy work goes in sceneDidEnterBackground
}

Scene Lifecycle

NEVER confuse scene disconnect with app termination:

// ❌ Wrong assumption
func sceneDidDisconnect(_ scene: UIScene) {
    // App is terminating!  <- WRONG
    cleanupEverything()
}

// ✅ Scene disconnect means scene released, not app death
func sceneDidDisconnect(_ scene: UIScene) {
    // Scene being released — save per-scene state
    // App may continue running with other scenes
    // Or system may reconnect this scene later
    saveSceneState(scene)
}

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Essential Patterns

SwiftUI Lifecycle Handler

@main
struct MyApp: App {
    @Environment(\.scenePhase) private var scenePhase
    @StateObject private var appState = AppState()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(appState)
        }
        .onChange(of: scenePhase) { oldPhase, newPhase in
            handlePhaseChange(from: oldPhase, to: newPhase)
        }
    }

    private func handlePhaseChange(from old: ScenePhase, to new: ScenePhase) {
        switch (old, new) {
        case (_, .active):
            appState.refreshDataIfStale()

        case (.active, .inactive):
            // Transitioning away — pause but don't save yet
            appState.pauseActiveOperations()

        case (_, .background):
            appState.saveState()
            scheduleBackgroundRefresh()

        default:
            break
        }
    }
}

Background Task Manager

final class BackgroundTaskManager {
    static let shared = BackgroundTaskManager()

    func registerTasks() {
        BGTaskScheduler.shared.register(
            forTaskWithIdentifier: "com.app.refresh",
            using: nil
        ) { task in
            self.handleAppRefresh(task as! BGAppRefreshTask)
        }

        BGTaskScheduler.shared.register(
            forTaskWithIdentifier: "com.app.processing",
            using: nil
        ) { task in
            self.handleProcessing(task as! BGProcessingTask)
        }
    }

    func scheduleRefresh() {
        let request = BGAppRefreshTaskRequest(identifier: "com.app.refresh")
        request.earliestBeginDate = Date(timeIntervalSinceNow: 15 * 60)

        try? BGTaskScheduler.shared.submit(request)
    }

    private func handleAppRefresh(_ task: BGAppRefreshTask) {
        scheduleRefresh()  // Schedule next refresh

        let refreshTask = Task {
            await performRefresh()
        }

        task.expirationHandler = {
            refreshTask.cancel()
        }

        Task {
            await refreshTask.value
            task.setTaskCompleted(success: true)
        }
    }
}

Launch Time Optimization

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    private var launchStartTime: CFAbsoluteTime = 0

    func application(_ application: UIApplication,
        willFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        launchStartTime = CFAbsoluteTimeGetCurrent()

        // Phase 1: Absolute minimum (crash reporting)
        CrashReporter.initialize()

        return true
    }

    func application(_ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Phase 2: Required for first frame
        configureAppearance()

        // Phase 3: Deferred to after first frame
        DispatchQueue.main.async {
            self.completePostLaunchSetup()
            let launchTime = CFAbsoluteTimeGetCurrent() - self.launchStartTime
            Logger.app.info("Launch completed in \(launchTime)s")
        }

        return true
    }

    private func completePostLaunchSetup() {
        // Analytics, feature flags, etc.
        Task.detached(priority: .utility) {
            Analytics.initialize()
            FeatureFlags.refresh()
        }
    }
}

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Quick Reference

Lifecycle Events Order

Event	When	Use For
willFinishLaunching	Before UI	Crash reporting only
didFinishLaunching	UI ready	Critical setup
sceneWillEnterForeground	Coming to front	Undo background changes
sceneDidBecomeActive	Fully active	Refresh, restart tasks
sceneWillResignActive	Losing focus	Pause playback
sceneDidEnterBackground	In background	Save state, start bg task
sceneDidDisconnect	Scene released	Save scene state

Background Task Limits

Task Type	Time Limit	When Runs
beginBackgroundTask	~30 seconds	Immediately
BGAppRefreshTask	~30 seconds	System discretion
BGProcessingTask	Minutes	Charging, overnight
Background URL Session	Unlimited	System managed

State Restoration Options

Approach	Scope	Types	Auto-save
@SceneStorage	Per-scene	Codable	Yes
@AppStorage	App-wide	Primitives	Yes
Restoration ID	Per-VC	Custom	Manual

Red Flags

Smell	Problem	Fix
Sync network in launch	Blocked UI	Async + skeleton UI
All SDKs in didFinish	Slow launch	Prioritize + defer
No beginBackgroundTask	Work may not complete	Always request time
Missing endBackgroundTask	Leaked task	Use defer
Heavy work in willResignActive	Laggy app switcher	Move to didEnterBackground
Trust applicationWillTerminate	May not be called	Save on background
Confuse sceneDidDisconnect	Scene != app termination	Save scene state only