Flutter Firebase Auth - change home widget if the user is logged in - firebase

Question

I'm building an app that supports only google login using google_sign_in package. When the app is run, it first checks if FirebaseAuth is alive. If user in FirebaseAuth.instance.authStateChanges().listen((User? user) is not null, HomePage() should be shown. If the user is null it should go to AuthPage() which has a google sign-in button.
The code for main.dart is shown below.
void main() async {
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
Widget _defaultHome = HomePage();
// Check if user already logged in
FirebaseAuth auth = FirebaseAuth.instance;
auth.authStateChanges().listen((User? user) {
if (user == null) {
_defaultHome = AuthPage();
}
print(user);
});
runApp(MultiProvider(
providers: [
ChangeNotifierProvider(create: (_) => SignInProvider()),
ChangeNotifierProvider(create: (_) => DataProvider()),
],
child: new MaterialApp(
title: 'Mentea',
home: _defaultHome,
routes: {
'auth': (context) => AuthPage(),
'home': (context) => HomePage(),
},
),
));
}
But when I run it, console gives
I/flutter (14933): null
which means no logged in user, but the emulator shows HomePage() instead of AuthPage(). Anybody know how to fix this? I tried changing 'home:' attribute to 'initialRoute:', but it doesn't work either (directs to HomePage() while printing null).
String _defaultHome = 'home';
...
auth.authStateChanges().listen((User? user) {
if (user == null) {
_defaultHome = 'auth;
}
});
...
child: new MaterialApp(
title: 'Mentea',
initialRoute: _defaultHome,
routes: {
'auth': (context) => AuthPage(),
'home': (context) => HomePage(),
},
),

Answer 1

We can assign a StreamBuilder that listens to FirebaseAuth state changes, to the home property:
StreamBuilder(
stream: FirebaseAuth.instance.onAuthStateChanged,
builder: (user) {
return user == null ? AuthPage() : HomePage(),
}
),
Follows a full example:
void main() async {
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
runApp(MultiProvider(
providers: [
ChangeNotifierProvider(create: (_) => SignInProvider()),
ChangeNotifierProvider(create: (_) => DataProvider()),
],
child: MaterialApp(
title: 'Mentea',
home: StreamBuilder(
stream: FirebaseAuth.instance.onAuthStateChanged,
builder: (user) {
return user == null ? AuthPage() : HomePage(),
},
),
routes: {
'auth': (context) => AuthPage(),
'home': (context) => HomePage(),
},
),
));
}