App lifecycle

App Lifecycle

Instructions

Mobile apps run in a hostile runtime. The OS freezes, backgrounds, and kills processes to preserve battery and memory. Treating the lifecycle as an afterthought leads to lost data, broken deep links, and angry reviews.

1. The Lifecycle States

Both platforms converge to roughly:

State	iOS term	Android term	Cross-platform term
Visible & interactive	Active	Resumed	Foreground
Visible but not active	Inactive (incoming call, control center)	Paused	Inactive
Not visible	Background	Stopped	Background
Frozen but in memory	Suspended	Stopped	Suspended
Gone	Terminated	Process death	Killed

2. Transitions and Contracts

• Entering background: cancel network calls not needed offline, persist drafts, release camera/audio/sensors, pause timers.
• Entering foreground: refresh data that may be stale, reconnect websockets, re-check auth, reclaim resources.
• Process death: on cold start, detect whether this is a restart from death (saved-state bundle present) or a first launch; restore the previous screen and selection.
• Low memory warning: drop caches, close non-visible images, release heavy objects.

3. Platform Hooks

// Swift / SwiftUI
@main struct App: App {
    @Environment(\.scenePhase) private var scenePhase
    var body: some Scene {
        WindowGroup { RootView() }
            .onChange(of: scenePhase) { _, phase in
                switch phase {
                case .active:     AppEvents.foreground()
                case .inactive:   AppEvents.inactive()
                case .background: AppEvents.background()
                @unknown default: break
                }
            }
    }
}

// Kotlin / Android (Lifecycle-aware observer)
class AppLifecycleObserver : DefaultLifecycleObserver {
    override fun onStart(owner: LifecycleOwner) = AppEvents.foreground()
    override fun onStop(owner: LifecycleOwner)  = AppEvents.background()
}
ProcessLifecycleOwner.get().lifecycle.addObserver(AppLifecycleObserver())

// Flutter
class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override void initState() { super.initState(); WidgetsBinding.instance.addObserver(this); }
  @override void dispose()   { WidgetsBinding.instance.removeObserver(this); super.dispose(); }
  @override void didChangeAppLifecycleState(AppLifecycleState state) {
    switch (state) {
      case AppLifecycleState.resumed:  AppEvents.foreground();
      case AppLifecycleState.inactive: AppEvents.inactive();
      case AppLifecycleState.paused:   AppEvents.background();
      case AppLifecycleState.detached: AppEvents.detached();
      case AppLifecycleState.hidden:   break;
    }
  }
}

// React Native
import { AppState } from 'react-native';
useEffect(() => {
  const sub = AppState.addEventListener('change', (s) => {
    if (s === 'active') AppEvents.foreground();
    else if (s === 'background') AppEvents.background();
  });
  return () => sub.remove();
}, []);

4. State Restoration

State restoration keeps the user's mental context after the OS kills the process.

• iOS: @SceneStorage, StateRestoration with NSUserActivity for deep links.
• Android: SavedStateHandle in ViewModels; onSaveInstanceState for Views; Navigation Component restores the back stack.
• Flutter: RestorationMixin, RestorableProperty, restorationScopeId in MaterialApp.
• React Native: @react-navigation persistence with AsyncStorage.

Keep restored state small (IDs, not payloads). Refetch the data using the ID on resume.

5. Background Work Budgets

The OS is strict. Do not run background network or CPU work without a matching API:

• iOS: BGTaskScheduler (background refresh, processing tasks), URLSession background configuration for downloads, VoIP push for real-time.
• Android: WorkManager for deferrable guaranteed work, ForegroundService for user-visible ongoing work (with the right foreground-service type declared), JobScheduler under the hood.
• Flutter: workmanager plugin, native channel on iOS.
• React Native: expo-background-fetch, react-native-background-fetch, Headless JS on Android.

Assume background execution may never happen. Design for eventual consistency and sync on foreground.

6. Process Death and Cold Start

• Restore navigation first, then restore feature state.
• Re-run auth checks and token refresh on every cold start.
• If a push notification launched the app, route to the target screen only after auth is verified.
• Measure cold start time; store and foreground launches should be under 2 seconds to first meaningful paint.

7. Common Bugs

• Losing form drafts when the user switches apps. Fix: persist on onPause / background.
• Continuing location updates when backgrounded without user awareness. Fix: stop on background unless there is a declared background mode and a UI affordance.
• Showing stale data after a long background. Fix: soft-invalidate caches on foreground and refetch.
• Keeping websockets open while backgrounded. Fix: close on background, reconnect with backoff on foreground, and rely on push for real-time.
• Deep link opens on cold start before the nav graph is ready. Fix: buffer intents until the navigator is mounted.

8. Observability

Log these events (without PII):

• app.foreground, app.background, app.cold_start, app.warm_start.
• restore.success / restore.failure with a bucketed reason.
• Cold-start duration and first-frame-time as metrics, not logs.

9. Anti-Patterns

• Running long work in onPause or applicationWillResignActive. You have milliseconds.
• Storing large objects in saved-state bundles. Store IDs, rehydrate.
• Polling while backgrounded. Use push or periodic WorkManager/BGTask.
• Assuming the app was foreground since last launch. Always check.

Checklist

• Foreground/background transitions have explicit handlers.
• Drafts and in-flight forms persist on background.
• State restoration stores small keys and rehydrates data.
• Background work uses the platform-sanctioned API with correct declarations.
• Cold-start routing waits for auth and navigator readiness.
• Websockets and media resources are released on background.
• Cold-start and warm-start times are measured and budgeted.
• App handles low-memory warnings by dropping caches.