flutter how to always listen for push notifications code example

Example: flutter show push notification on forgrounf

FirebaseMessaging _firebaseMessaging = FirebaseMessaging();
FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin =
      new FlutterLocalNotificationsPlugin();
  @override
  void initState() {
var initializationSettingsAndroid =
        new AndroidInitializationSettings('@mipmap/ic_launcher');
    var initializationSettingsIOS = new IOSInitializationSettings();
    var initializationSettings = new InitializationSettings(
        initializationSettingsAndroid, initializationSettingsIOS);
    flutterLocalNotificationsPlugin.initialize(initializationSettings,
        onSelectNotification: onSelectNotification);
    _firebaseMessaging.configure(
      onMessage: (Map<String, dynamic> message) async {
        showNotification(
            message['notification']['title'], message['notification']['body']);
        print("onMessage: $message");
      },
      onLaunch: (Map<String, dynamic> message) async {
        print("onLaunch: $message");
        Navigator.pushNamed(context, '/notify');
      },
      onResume: (Map<String, dynamic> message) async {
        print("onResume: $message");
      },
    );
}
Future onSelectNotification(String payload) async {
    showDialog(
      context: context,
      builder: (_) {
        return new AlertDialog(
          title: Text("PayLoad"),
          content: Text("Payload : $payload"),
        );
      },
    );
  }
void showNotification(String title, String body) async {
    await _demoNotification(title, body);
  }

  Future<void> _demoNotification(String title, String body) async {
    var androidPlatformChannelSpecifics = AndroidNotificationDetails(
        'channel_ID', 'channel name', 'channel description',
        importance: Importance.Max,
        playSound: true,
        sound: 'sound',
        showProgress: true,
        priority: Priority.High,
        ticker: 'test ticker');

    var iOSChannelSpecifics = IOSNotificationDetails();
    var platformChannelSpecifics = NotificationDetails(
        androidPlatformChannelSpecifics, iOSChannelSpecifics);
    await flutterLocalNotificationsPlugin
        .show(0, title, body, platformChannelSpecifics, payload: 'test');
  }