FCM push notifications FLUTTER when app is terminated

first answer, I hope will be good.

As you can see here notifications are handled with onBackgroundMessage even if the app is terminated. Maybe your problem is that you are not sending a Notification but a "Data only" message. In that case you have to set the field "priority": "high"

Also, with the latest release of Flutter and firebase_messaging the methods onResume, onMessage and onLaunch are deprecated. You should substitute them with onMessage, onMessageOpened, onBackground, but you can find definitely more info with the official documentation

Just an addition to @samUser1 answer. When app is terminated this code will handle events.

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

And outside main function parse it. The example below uses AwesomeNotifications package. https://github.com/rafaelsetragni/awesome_notifications/blob/master/example/lib/main.dart

Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  await Firebase.initializeApp();

  final res = await AwesomeNotifications().createNotification(
    content: NotificationContent(
      id: Random().nextInt(1000000),
      channelKey: 'channel_name',
      title: 'Hello!',
      body: 'World!',
      notificationLayout: NotificationLayout.Default,
    ),
  );
}