How to handle notification when app in background in Firebase

Handling application notifications in the background with Firebase primarily involves Firebase Cloud Messaging (FCM) and potentially integrating Firebase Functions. The following are the steps to set up and handle background notifications in Firebase:

1. Set up Firebase project and FCM

First, ensure your application is registered and configured with a Firebase project. Add your application in the Firebase console and configure Firebase Cloud Messaging.

Step Examples:

• In the Firebase console, create a new project.
• Add your application (Android/iOS/Web) to this Firebase project.
• Download the configuration file (e.g., google-services.json or GoogleService-Info.plist) and add it to your project.

2. Integrate FCM SDK into your application

Integrate the FCM SDK into your application according to your platform (Android/iOS/Web).

Example code (Android):

java
// Add Firebase dependencies to build.gradle file
implementation 'com.google.firebase:firebase-messaging:20.2.0'

// Initialize FCM in your Application or Activity
FirebaseMessaging.getInstance().subscribeToTopic("news");

3. Handle received notifications

Process received notifications within the relevant service. On Android, this is commonly achieved by extending FirebaseMessagingService and overriding the onMessageReceived method.

Android Example:

java
public class MyFirebaseMessagingService extends FirebaseMessagingService {

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        if (remoteMessage.getData().size() > 0) {
            handleNow(remoteMessage.getData());
        }
    }

    private void handleNow(Map<String, String> data) {
        // Process received data
    }
}

4. Use Firebase Functions for background logic

If you require executing background operations (e.g., database updates, user notifications) when a notification is received, utilize Firebase Functions to achieve this.

Example (Node.js):

javascript
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

exports.sendNotification = functions.firestore.document('messages/{messageId}')
    .onCreate((snapshot, context) => {
        const messageData = snapshot.data();
        const payload = {
            notification: {
                title: 'New Message',
                body: messageData.text
            }
        };

        return admin.messaging().sendToTopic('news', payload);
    });

This function listens for new document creation events in the Firestore messages collection and sends notifications to all users subscribed to the news topic.

5. Test and debug

During development, test the notification functionality to ensure it correctly receives and processes notifications on different devices and in various scenarios.

• Use the Firebase console to send test notifications.
• Use Postman or similar tools to simulate backend notifications.
• Debug the application services and Firebase Functions to ensure data is processed correctly.

By following these steps, you can effectively set up and handle application background notifications in Firebase, ensuring a great user experience and app interactivity.