Firebase Login with Flutter using onAuthStateChanged

Question

Outside of Flutter, when I implement firebase authentication I always use the onAuthStateChanged listener provided by firebase to determine if the user is logged in or not and respond accordingly.

I am trying to do something similar using flutter, but I can find a way to access onAuthStateChanged of Firebase. I am using the firebase_auth, and google_signin Flutter plugins. I am working of example code that is included with the firebase_auth Flutter plugin. Below is the sample code. I can login successfully with google sign in, but the example is too simple, because I want to have an observer/listener to detect the user's signed in/out state.

Is there a way to detect via observer/listener using the firebase_auth/google_signin flutter plugins to determine the status of a user?

Ultimately I want the app to determine if the user is logged in (yes/no). If not then show a login screen, if yes then show my main app page.

import 'dart:async';
import 'dart:io';

import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

final FirebaseAuth _auth = FirebaseAuth.instance;
final GoogleSignIn _googleSignIn = new GoogleSignIn();

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Firebase Auth Demo',
      home: new MyHomePage(title: 'Firebase Auth Demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Future<String> _message = new Future<String>.value('');

  Future<String> _testSignInAnonymously() async {
    final FirebaseUser user = await _auth.signInAnonymously();
    assert(user != null);
    assert(user == _auth.currentUser);
    assert(user.isAnonymous);
    assert(!user.isEmailVerified);
    assert(await user.getToken() != null);
    if (Platform.isIOS) {
      // Anonymous auth doesn't show up as a provider on iOS
      assert(user.providerData.isEmpty);
    } else if (Platform.isAndroid) {
      // Anonymous auth does show up as a provider on Android
      assert(user.providerData.length == 1);
      assert(user.providerData[0].providerId == 'firebase');
      assert(user.providerData[0].uid != null);
      assert(user.providerData[0].displayName == null);
      assert(user.providerData[0].photoUrl == null);
      assert(user.providerData[0].email == null);
    }
    return 'signInAnonymously succeeded: $user';
  }

  Future<String> _testSignInWithGoogle() async {
    final GoogleSignInAccount googleUser = await _googleSignIn.signIn();
    final GoogleSignInAuthentication googleAuth =
        await googleUser.authentication;
    final FirebaseUser user = await _auth.signInWithGoogle(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );
    assert(user.email != null);
    assert(user.displayName != null);
    assert(!user.isAnonymous);
    assert(await user.getToken() != null);
    return 'signInWithGoogle succeeded: $user';
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          new MaterialButton(
              child: const Text('Test signInAnonymously'),
              onPressed: () {
                setState(() {
                  _message = _testSignInAnonymously();
                });
              }),
          new MaterialButton(
              child: const Text('Test signInWithGoogle'),
              onPressed: () {
                setState(() {
                  _message = _testSignInWithGoogle();
                });
              }),
          new FutureBuilder<String>(
              future: _message,
              builder: (_, AsyncSnapshot<String> snapshot) {
                return new Text(snapshot.data ?? '',
                    style: const TextStyle(
                        color: const Color.fromARGB(255, 0, 155, 0)));
              }),
        ],
      ),
    );
  }
}

Here are links to the flutter packages in question: https://github.com/flutter/plugins/tree/master/packages/firebase_auth https://github.com/flutter/plugins/tree/master/packages/google_sign_in

Answer 1

I know this question is pretty old, but here is the answer if anybody is still looking for it.

Firebase returns a Stream of FirebaseUser with it's onAuthStateChanged function. There are many ways to listen to the user's authentication state change. This is how I do it:

Solution 1

I return a StreamBuilder to my App's home page, and the StreamBuilder returns specific pages based on the auth status of the user.

@override
Widget build(BuildContext context) {
  return MaterialApp(
      title: 'Your App Name',
      home: _getLandingPage()
  );
}

Widget _getLandingPage() {
  return StreamBuilder<FirebaseUser>(
    stream: FirebaseAuth.instance.onAuthStateChanged,
    builder: (BuildContext context, snapshot) {
      if (snapshot.hasData) {
        if (snapshot.data.providerData.length == 1) { // logged in using email and password
          return snapshot.data.isEmailVerified
              ? MainPage()
              : VerifyEmailPage(user: snapshot.data);
        } else { // logged in using other providers
          return MainPage();
        }
      } else {
        return LoginPage();
      }
    },
  );
}

Solution 2

You can create a listener in your app's initState() function as well. Make sure the firebase app has been initialized before registering the listener.

@override
void initState() {
  super.initState();

  FirebaseAuth.instance.authStateChanges().listen((firebaseUser) {
    // do whatever you want based on the firebaseUser state
  });
}