Flutter state management with Provider

Question

I have a simple Provider class:

import 'package:flutter/foundation.dart';

class AppState with ChangeNotifier {
  bool _isLoggedIn = false;
  bool get isLoggedIn => _isLoggedIn;

  set isLoggedIn(bool newValue) {
    _isLoggedIn = newValue;
    notifyListeners();
  }
}

And in the login class I just set isLoggedIn to true if login is successful:

 void _signInWithEmailAndPassword(appState) async {
    try {
      final FirebaseUser user = await _auth.signInWithEmailAndPassword(
        ...
      );

      if (user != null) {
        appState.isLoggedIn = true;
        appState.userData = user.providerData;
        ...
      }
    } catch (e) {
      setState(() {
        _errorMessage = e.message;
      });
    }
  }

Pressing the back button on Android lets users go back to this page even after successfully logging in. So I wanted to know if Provider.of can be accessed before Widget build and redirect a user if isLoggedIn is true.

Now I have something like:

@override
  Widget build(BuildContext context) {
    final appState = Provider.of<AppState>(context);
...

This is only one use case for the login view, but I'm sure this functionality can be used in other cases.

Answer 1

If you are going to use the FirebaseUser or Logged in user throughout your app, i would suggest that you add the Provider on the highest level of your app. Example

void main() {

   runApp(MyApp());
   }

class MyApp extends StatelessWidget {
  MyApp();

  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        StreamProvider<FirebaseUser>.value(
          stream: FirebaseAuth.instance.onAuthStateChanged, // Provider here
        ),
      ],
      child: MaterialApp(
        title: 'My App',
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
          primaryColor: Colors.green,
          primarySwatch: Colors.green,
          accentColor: Colors.yellow,
        ),
        home: MainPage(),
      ),
    );
  }
}

class MainPage extends StatefulWidget {
  MainPage({Key key, this.storage}) : super(key: key);
  final FirebaseStorage storage;
  @override
  _MainPageState createState() => _MainPageState();
}

class _MainPageState extends State<MainPage>
    with SingleTickerProviderStateMixin {
  @override
  Widget build(BuildContext context) {
    final user = Provider.of<FirebaseUser>(context); // gets the firebase user
    bool loggedIn = user != null;

    return loggedIn ? HomePage() : LoginPage(); // Will check if the user is logged in. And will change anywhere in the app if the user logs in
  }
}

References

Fireship 185 Provider

Great Youtube video explaining the code