Where to call the Update chat message "delivered" function when app is on Background in Flutter?

Question

Im trying to implement the whatsapp like functionality of message being delivered when the receiver's app is in background state and the wifi is on.

I want to call the function to update the message document status from 'sent' to 'delivered'. But cant find where to call it from. I tried calling it inside FirebaseMessaging.onMessage.listen but it is still not working on background.

static setDeliveredStatus(
          {required String senderId,
          required String receiverId,
          required String receiverName}) async {
        print('setting = ${receiverName + receiverId}');
        print('${currentUser!.displayName!} + ${senderId}');
        QuerySnapshot query = await _firestore
            .collection(CollectionKeys.messages)
            .doc(receiverName + receiverId)
            .collection(currentUser!.displayName! + senderId)
            .where('status', isEqualTo: describeEnum(MessageStatus.sent))
            .get();
        query.docs.forEach((doc) async {
          await doc.reference
              .update({'status': describeEnum(MessageStatus.delivered)});
        });
      }

This above function is currently being called inside initState of my home.dart screen in a onMessage Listener:

FirebaseMessaging.onMessage.listen((message) {
      if (message.notification != null) {
        LocalNotificationService.createAndDisplayNotificationChannel(message);
        FirebaseServices.setDeliveredStatus(
            senderId: message.data['senderId'],
            receiverId: message.data['receiverId'],
            receiverName: message.data['receiverName']);
      }
    });

Answer 1

The FCM SDK has an onBackgroundMessage function where you can configure a callback function when receiving messages while the app is in the background.

FirebaseMessaging.onBackgroundMessage(_onBackgroundHandler);

See this document for reference. In the call back method you could put the logic to update the document.