How to push notifications in the background (even when the app is closed), when a boolean flag in firebase database is set to true in flutter?

Question

I want to push a notification in both foreground and background in a flutter app when a bool flag is set to true. I gone through the example given in flutter_background_service package but I can't unserstand where to write the code to check the flag and call the method to push a notification.

What I did understand is that when the app switches to background, the onStart method is called. I have a unique username for each user, which I retrieve in the main function using shared preferences. So after fetching the username, the service should be running (in case of both foreground and background).

The following is the code I tried, but didn't get the expected result

Future<void> initializeService() async {
 final service = FlutterBackgroundService();
 final initializationSettingsAndroid =
      AndroidInitializationSettings('@mipmap/ic_launcher');
  final initializationSettings = InitializationSettings(
    android:initializationSettingsAndroid);
  await _flutterLocalNotificationsPlugin.initialize(initializationSettings);
  
  await service.configure(
    androidConfiguration: AndroidConfiguration(
      // this will be executed when app is in foreground or background in separated isolate
      onStart: onStart,

      // auto start service
      autoStart: true,
      isForegroundMode: true,
    ),
    iosConfiguration: IosConfiguration(
      // auto start service
      autoStart: true,

      // this will be executed when app is in foreground in separated isolate
      onForeground: onStart,

      // you have to enable background fetch capability on xcode project
      onBackground: onIosBackground,
    ),
  );

  service.startService();
}
@pragma('vm:entry-point')
Future<bool> onIosBackground(ServiceInstance service) async {
  WidgetsFlutterBinding.ensureInitialized();
  DartPluginRegistrant.ensureInitialized();

  print("IOS background"); // Since I am just working with android, I am just ignoring the IOS //function

  return true;
}
@pragma('vm:entry-point')
void onStart(ServiceInstance service) async {
  // Only available for flutter 3.0.0 and later
  await Firebase.initializeApp();
  DartPluginRegistrant.ensureInitialized();

  Timer.periodic(const Duration(seconds: 5), (timer) async {
    if (service is AndroidServiceInstance) {
      

    // uname is the variable I retrieve using shared preferences in the main function...
     databaseReference.child('Users/${uname}/Panic').onValue.listen((event) {
      final boolFlag = event.snapshot.value;
      if (boolFlag == true) {
        sendNotification();
    }
  });
      
      }
    }


Future<void> sendNotification() async {
  const channel = AndroidNotificationChannel(
    'Id1',
    'Alert notification channel',
    importance: Importance.high,
  );
  await _flutterLocalNotificationsPlugin
      .resolvePlatformSpecificImplementation<
          AndroidFlutterLocalNotificationsPlugin>()
      ?.createNotificationChannel(channel);

  final notificationDetails = NotificationDetails(
    android: AndroidNotificationDetails(
      channel.id,
      channel.name,
      importance: Importance.high,
    ),
  );

  await _flutterLocalNotificationsPlugin.show(
    0,
    'Alert !!!',
    'Alert!!! Alert !!!',
    notificationDetails,
    payload: "Payload",
  );
}


Future <void> main() async{
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  await _firebaseMessaging.requestPermission();
  final token = await _firebaseMessaging.getToken();
  print('FCM Token: $token');
  sharedPreferences = await SharedPreferences.getInstance();
  dBr = await FirebaseDatabase.instance.ref();
  vals = await keepMeSignedIn();
  userName = vals[0] ?? "User";
  uname = vals[1];
  val = vals[2] ?? false;
  print("Details from main page: $userName  $uname $val");
  await initializeService(); // Initializing the bg service
  runApp(
  MaterialApp(
  home: SplashScreen(uname,val),
  debugShowCheckedModeBanner: false,
  ));
  
}