How to do it...

In this recipe, you will see how to send notification messages when your app is in the background:

1. Add the latest version of the firebase_messaging plugin to the pubspec.yaml file of your app.

2. At the top of the login_screen.dart file in your app, import the firebase_messaging.dart file:

import 'package:firebase_messaging/firebase_messaging.dart';

3. At the top of the _LoginScreenState class, add the following declaration:

final FirebaseMessaging messaging = FirebaseMessaging.instance;

4. In the initState method, edit the code in the whenComplete callback of the Firebase initializeApp method as follows (you will get an error in _firebaseBackgroundMessageReceived that we will fix in the next step):

Firebase.initializeApp().whenComplete(() {
 auth = FirebaseAuthentication();
  FirebaseMessaging.onBackgroundMessage(
    _firebaseBackgroundMessageReceived);
  setState(() {});
});

5. Outside of the _LoginScreenState class, add a method, called _firebaseBackgroundMessageReceived...