Push Notifications

The two payload types matter: if you want full control over how it looks (custom layout, image, action buttons), use data-only + flutter_local_notifications. If you're OK with system defaults, use notification: payload.

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Code in action

// MUST be top-level + AOT-safe
@pragma('vm:entry-point')
Future<void> onBackgroundMessage(RemoteMessage msg) async {
  await Firebase.initializeApp();
  await DataSync.handle(msg.data);                  // limited window — be fast
}

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  FirebaseMessaging.onBackgroundMessage(onBackgroundMessage);
  runApp(const MyApp());
}

class NotificationService {
  final _fcm = FirebaseMessaging.instance;

  Future<void> init() async {
    // 1. Permission (iOS + Android 13+)
    final settings = await _fcm.requestPermission();
    if (settings.authorizationStatus != AuthorizationStatus.authorized) return;

    // 2. Token (send to your server, refresh on rotation)
    final token = await _fcm.getToken();
    await api.registerDevice(token);
    _fcm.onTokenRefresh.listen(api.registerDevice);

    // 3. Foreground messages — show your own UI
    FirebaseMessaging.onMessage.listen((msg) {
      LocalNotifications.show(
        title: msg.notification?.title ?? '',
        body:  msg.notification?.body  ?? '',
        payload: msg.data,
      );
    });

    // 4. Tapped in background → app was alive in background
    FirebaseMessaging.onMessageOpenedApp.listen(_handleTap);

    // 5. Tapped while terminated → app was relaunched
    final initial = await _fcm.getInitialMessage();
    if (initial != null) _handleTap(initial);
  }

  void _handleTap(RemoteMessage msg) {
    final route = msg.data['route'] as String?;
    if (route != null) router.go(route);            // deep link
  }
}

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Best practices

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Common mistakes to avoid

❌ Expecting the system to show a banner in foreground
   It doesn't (by default). Show your own via flutter_local_notifications.

❌ Calling Navigator from a background callback
   No tree exists. Persist intent (shared prefs / pending nav state) and
   navigate when the app comes to foreground.

❌ Treating delivery as guaranteed
   Both FCM and APNs are best-effort. For mission-critical events,
   pair push with a server-side fallback (sync on open).

❌ Hard-coding token expectations
   Tokens rotate (reinstall, restore from backup, language change).
   Listen to onTokenRefresh, always.

❌ Forgetting onBackgroundMessage requires AOT entry point
   @pragma('vm:entry-point') — without it, release builds fail silently.

❌ Sending PII in the visible notification body
   "John Smith paid you $50" shows on the lock screen.
   Use less specific text + open the app for details.

❌ Not testing terminated-state taps
   getInitialMessage is easy to forget. Test by force-quitting the app
   and tapping a fresh notification.

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Interview follow-ups

1. What's the difference between notification: and data: payloads? notification: is a structured payload (title, body, icon) that the system displays automatically. data: is opaque key/value pairs your app processes. Use data-only for full control (custom UI, silent sync, deep link intents). Most production apps send both — system handles display, data handles routing.

2. How do you deep-link from a notification tap? Include a route (or similar) in the data: payload. In onMessageOpenedApp and getInitialMessage, parse it and call router.go(route). For terminated state, you may need to wait for app init before navigating — show a splash, then deep link.

3. What's the difference between FCM and APNs? FCM is Google's cross-platform service; APNs is Apple's. FCM on iOS delegates to APNs under the hood — you still configure APNs credentials in Firebase. Same on Android except FCM is native. From Flutter's perspective, firebase_messaging abstracts both.

4. How do you handle notification preferences per user? Server-side: store preference flags, only push when the user has opted in. Client-side: use FCM topics (subscribe/unsubscribe based on prefs) plus Android notification channels (let the user mute categories at the OS level). Always provide an in-app settings screen.

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