Firebase Cloud Messaging Skill

This skill defines how to correctly use Firebase Cloud Messaging (FCM) in Flutter applications.

When to Use

Use this skill when:

• Setting up push notifications with FCM in a Flutter project.
• Handling messages in foreground, background, and terminated states.
• Managing notification permissions and FCM tokens.
• Configuring platform-specific notification display behavior.

1. Setup and Configuration

flutter pub add firebase_messaging

iOS:

• Enable Push Notifications and Background Modes in Xcode.
• Upload your APNs authentication key to Firebase before using FCM.
• Do not disable method swizzling — it is required for FCM token handling.
• Ensure the bundle ID for your APNs authentication key matches your app's bundle ID.

Android:

• Devices must run Android 4.4+ with Google Play services installed.
• Check for Google Play services compatibility in both onCreate() and onResume().

Web:

• Create and register a service worker file named firebase-messaging-sw.js in your web/ directory:

importScripts("https://www.gstatic.com/firebasejs/10.7.0/firebase-app-compat.js");
importScripts("https://www.gstatic.com/firebasejs/10.7.0/firebase-messaging-compat.js");

firebase.initializeApp({ /* your config */ });

const messaging = firebase.messaging();

messaging.onBackgroundMessage((message) => {
  console.log("onBackgroundMessage", message);
});

2. Message Handling

Foreground messages:

FirebaseMessaging.onMessage.listen((RemoteMessage message) {
  print('Foreground message data: ${message.data}');
  if (message.notification != null) {
    print('Notification: ${message.notification}');
  }
});

Background messages:

@pragma('vm:entry-point')
Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  // Initialize Firebase before using other Firebase services in background
  await Firebase.initializeApp();
  print("Background message: ${message.messageId}");
}

void main() {
  FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
  runApp(MyApp());
}

Background handler rules:

• Must be a top-level function (not anonymous, not a class method).
• Annotate with @pragma('vm:entry-point') (Flutter 3.3.0+) to prevent removal during tree shaking in release mode.
• Cannot update app state or execute UI-impacting logic — runs in a separate isolate.
• Call Firebase.initializeApp() before using any other Firebase services.

3. Permissions

NotificationSettings settings = await FirebaseMessaging.instance.requestPermission(
  alert: true,
  badge: true,
  sound: true,
  announcement: false,
  carPlay: false,
  criticalAlert: false,
  provisional: false,
);

print('Authorization status: ${settings.authorizationStatus}');

• iOS / macOS / Web / Android 13+: Must request permission before receiving FCM payloads.
• Android < 13: authorizationStatus returns authorized if the user has not disabled notifications in OS settings.
• Android 13+: Track permission requests in your app — there's no way to determine if the user chose to grant/deny.
• Use provisional permissions on iOS (provisional: true) to let users choose notification types after receiving their first notification.

4. Token Management

Get FCM registration token (use to send messages to a specific device):

final fcmToken = await FirebaseMessaging.instance.getToken();

Web — provide VAPID key:

final fcmToken = await FirebaseMessaging.instance.getToken(
  vapidKey: "BKagOny0KF_2pCJQ3m....moL0ewzQ8rZu"
);

Listen for token refresh:

FirebaseMessaging.instance.onTokenRefresh.listen((fcmToken) {
  // Send updated token to your application server
}).onError((err) {
  // Handle error
});

Apple platforms — ensure APNS token is available before FCM calls:

final apnsToken = await FirebaseMessaging.instance.getAPNSToken();
if (apnsToken != null) {
  // Safe to make FCM plugin API requests
}

Token Lifecycle (Auth State): Tokens should be tied to user sessions. Save the token to your database when a user signs in, and delete the token (or remove it from the user's document) when they sign out. An FCM token is device-specific, not inherently tied to user auth data — failing to clear it on sign-out means the next user on that device might receive the previous user's notifications.

5. Platform-Specific Behavior

• iOS: If the user swipes away the app from the app switcher, it must be manually reopened for background messages to work again.
• Android: If the user force-quits from device settings, the app must be manually reopened.
• iOS foreground notifications: Update presentation options to display notifications while the app is in the foreground:

  await FirebaseMessaging.instance.setForegroundNotificationPresentationOptions(
    alert: true,
    badge: true,
    sound: true,
  );

• Android foreground notifications: Notification messages arriving while the app is in the foreground won't display a visible notification by default. You must consume the payload via the onMessage stream and manually display a visual cue (using your own UI logic or a local notifications plugin).
• Android default channel: To set a default channel for background notifications, add this meta-data to your <application> block in AndroidManifest.xml:

  <meta-data
      android:name="com.google.firebase.messaging.default_notification_channel_id"
      android:value="high_importance_channel" />

6. Auto-Initialization Control

Disable auto-init — iOS (Info.plist):

FirebaseMessagingAutoInitEnabled = NO

Disable auto-init — Android (AndroidManifest.xml):

<meta-data android:name="firebase_messaging_auto_init_enabled" android:value="false" />
<meta-data android:name="firebase_analytics_collection_enabled" android:value="false" />

Re-enable at runtime:

await FirebaseMessaging.instance.setAutoInitEnabled(true);

• The auto-init setting persists across app restarts once set.

7. iOS Image Notifications

Important: The iOS simulator does not display images in push notifications. Test on a physical device.

• Add a Notification Service Extension in Xcode.
• Use Messaging.serviceExtension().populateNotificationContent() in the extension for image handling.
• Swift: add the FirebaseMessaging Swift package to your extension target.
• Objective-C: add the Firebase/Messaging pod to your Podfile.

8. Notification Interaction Handling

When a user taps a notification, the app opens (or is brought to the foreground). Handle the interaction in both cases:

App was terminated:

RemoteMessage? initialMessage =
    await FirebaseMessaging.instance.getInitialMessage();
if (initialMessage != null) {
  // Navigate based on message content
}

App was in background:

FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
  // Navigate based on message content
});

Always handle both scenarios to ensure a smooth user experience regardless of app state when the notification was received.

9. Topic Messaging

• Subscribing to a topic allows sending messages to multiple devices that have opted in.
• Topic messages are best suited for publicly available information (e.g., weather updates), optimized for throughput rather than latency.

// Subscribe
await FirebaseMessaging.instance.subscribeToTopic("weather_alerts");

// Unsubscribe
await FirebaseMessaging.instance.unsubscribeFromTopic("weather_alerts");

Note: subscribeToTopic() and unsubscribeFromTopic() are not supported for web clients via the Flutter plugin.

10. Sending a Test Message

The official Firebase documentation often obscures the exact steps for sending a test push notification. To fire a push (test or real) using the Firebase Console:

1. Obtain your device's FCM registration token (see Section 4).
2. Go to the Firebase Console and select your project.
3. In the left navigation panel, find the Engage (or Run) section and click Messaging (or Cloud Messaging).
4. Click New campaign and select Notifications.
5. Enter a Notification title and Notification text.
6. Click Send test message (often a button on the right side of the screen).
7. In the dialog, enter your FCM registration token and click the + icon to add it.
8. Make sure the token is checked, then click Test.

To send real automated push notifications to production users, you must use a server implementation (via the FCM HTTP v1 API or the Firebase Admin SDK) rather than the console.

11. Server-Side Credentials & Security

The legacy FCM server key endpoint was deprecated in June 2024 — HTTP v1 is the only supported option for sending pushes.

To authenticate server-to-server calls for HTTP v1, you need a Service Account:

1. Go to Firebase Console → Project settings → Service accounts.
2. Click Generate new private key (downloads a .json file).
3. CRITICAL: This file contains highly sensitive secrets and must never be committed to git.
4. Store the file securely (e.g., in a Secret Manager) or pass its stringified contents as an environment variable (like FIREBASE_SERVICE_ACCOUNT) to your backend.

12. Sending Messages (HTTP v1)

To send an FCM HTTP v1 message, your backend must:

1. Complete an OAuth2 JWT exchange (sign with RS256, scope https://www.googleapis.com/auth/firebase.messaging, endpoint https://oauth2.googleapis.com/token).
2. Construct and POST a JSON payload to https://fcm.googleapis.com/v1/projects/{project_id}/messages:send.

Here is a minimal, complete working example using Node.js and the google-auth-library:

const { GoogleAuth } = require('google-auth-library');

// Read the securely-stored service account JSON from environment
const credentials = JSON.parse(process.env.FIREBASE_SERVICE_ACCOUNT);

async function getAccessToken() {
  const auth = new GoogleAuth({
    credentials,
    scopes: ['https://www.googleapis.com/auth/firebase.messaging']
  });
  const client = await auth.getClient();
  const token = await client.getAccessToken();
  return token.token;
}

async function sendPushNotification(fcmToken, title, body) {
  const accessToken = await getAccessToken();
  const projectId = credentials.project_id;
  const url = `https://fcm.googleapis.com/v1/projects/${projectId}/messages:send`;
  
  const payload = {
    message: {
      token: fcmToken,
      notification: {
        title: title,
        body: body,
      },
      // Target specific platform features (e.g., channel on Android, sound on iOS)
      android: {
        notification: {
          channel_id: 'high_importance_channel',
        }
      },
      apns: {
        payload: {
          aps: {
            sound: 'default',
          }
        }
      }
    }
  };

  const response = await fetch(url, {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${accessToken}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify(payload)
  });

  return response.json();
}

References

• Firebase Cloud Messaging Flutter documentation
• Send a test message to a backgrounded app
• Receive messages in Flutter
• Topic messaging on Flutter
• Migrate from legacy FCM APIs to HTTP v1
• Server environment authorization (for OAuth2 / Service Accounts)
• FCM HTTP v1 API Reference
• Android Receive Docs (for default channel ID)
• FirebaseMessaging setForegroundNotificationPresentationOptions (API Reference)