Flutter: How to check if app is running on background from firebase messaging plugins onBackgroundMessage

Question

I'm using firebase_messaging plugin to register a callback handler with onBackgroundMessage for my data-only payload of firebase messaging.

If the app is in foreground or in background, the normal way of operation is using sockets to get the data from network and show notification from the app.

But when the app is in killed state, I would like to show the notification by fetching the data from network. But these operations conflicts when the app is in background as onBackgroundMessage is getting called in background also.

If I'm not wrong, the handler is running on a separate isolate and it has no access to the main contents. So how can I differentiate the killed and background state of the app from this isolated function?

Answer 1

You can use IsolateNameServer to register a ReceiverPort from the foreground when it is running and remove it when the foreground is not running. Then on the background isolate check if it exists and if so redirect the FCM message through the port to the foreground for handling on foreground.

Something along the lines of this:

const FOREGROUND_ISOLATE_PORT_NAME = 'foreground_port';

class NotificationManager {
  ReceivePort? _foregroundReceivePort;
  StreamSubscription<RemoteMessage>? _fcmMessageSubscription;

  init() async {
    FirebaseMessaging.onBackgroundMessage(_fcmMessageHandlerBackground);
    _fcmMessageSubscription = FirebaseMessaging.onMessage.listen(_fcmMessageHandlerForeground);
    _foregroundReceivePort = ReceivePort();
    IsolateNameServer.registerPortWithName(
      _foregroundReceivePort!.sendPort,
      FOREGROUND_ISOLATE_PORT_NAME,
    );
    _foregroundReceivePort!.listen((message) {
      if (message is RemoteMessage) {
        log('got fcm message for handling in foreground');
        _fcmMessageHandlerForeground(message);
      }
    });
  }

  shutdown() async {
    _fcmMessageSubscription?.cancel();
    IsolateNameServer.removePortNameMapping(FOREGROUND_ISOLATE_PORT_NAME);
    _foregroundReceivePort!.close();
    _foregroundReceivePort = null;
  }

}

With these two top level functions:

Future<void> _fcmMessageHandlerForeground(RemoteMessage message) async {
  // ... handle message in foreground ...
}

Future<void> _fcmMessageHandlerBackground(RemoteMessage message) async {
  final foreground = IsolateNameServer.lookupPortByName(FOREGROUND_ISOLATE_PORT_NAME);
  if (foreground != null) {
    log("redirecting FCM message to foreground");
    foreground.send(message);
  } else {
    // ... handle message in background ...
  }
}