Flutter state management with Provider - flutter

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