Provider isn't notified when users sign up - flutter

Question

Context
I am trying to build an app, using Flutter, which requires the users to sign up and/or sign in.
Using a provider, I was able to determine at each launch of the app if the user was already signed in or not, thus showing respectively the MainScreen or the SignInScreen, as you can see from the code below.
void main() {
runApp(Stuff());
}
class Stuff extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
StreamProvider<FirebaseUser>.value(value: AuthService().onAuthChanged),
],
child: Consumer<FirebaseUser>(builder: (context, user, _) {
return MaterialApp(
title: 'Stuff',
home: user != null ? MainScreen() : SignInScreen(),
routes: {
'/main_screen' : (context) => MainScreen(),
'/signup_screen': (context) => SignUpScreen(),
'/signin_screen': (context) => SignInScreen(),
},
);
}),
);
}
}
where AuthService for now is just the following:
class AuthService{
final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;
#override
Stream<FirebaseUser> get onAuthChanged => _firebaseAuth.onAuthStateChanged;
}
If the user is not signed in, then theSignInScreen is shown, and after the sign in process the StreamProvider is notified correctly and the home property of the MaterialApp updates automatically showing the MainScreen.
Problem
When users get to the SignInScreen they have the choice to either log in or get to another the SignUpScreen to sign up (app screen), like this:
GestureDetector(
child: Text('Sign Up.'),
onTap: () {
Navigator.pushReplacementNamed(context, '/signup_screen');
},
),
If they choose to sign in, when they're done, the home is correctly updated and the MainScreen is shown as I said above. But if they choose to sign up, when they're done, nothing happens. All listeners are notified, but the StreamProvider doesn't receive this notification. The only reason I can think of for this behavior, is that the SignUpScreen is outside the scope of the Provider, but the widget tree seems to suggets that it is.
Widget Tree
It it possibile that the problem is pushReplacementNamed?
P.S: I should mention that the GestureDetector is part of the following method that I pass to the appBar property of Scaffold.
PreferredSize _buildAppBar() {
return PreferredSize(
preferredSize: Size.fromHeight(MediaQuery.of(context).size.height * 0.18),
child: Container(
constraints: BoxConstraints(
maxHeight: MediaQuery.of(context).size.height * 0.18),
padding: EdgeInsets.only(left: 45.0, right: 45.0),
child: Column(
mainAxisAlignment: MainAxisAlignment.end,
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Text('Sign In'),
SizedBox(
height: 2.5,
),
Row(
children: [
Text('Have an account already?'),
SizedBox(
width: 4.0,
),
GestureDetector(
child: Text('Sign In.'),
onTap: () {
Navigator.pushReplacementNamed(context, '/signin_screen');
},
),
],
),
],
),
),
);
}

Answer 1

I think the issue here is because you don't notify Stuff widget when user signs in. You can wrap it a Streambuilder in order to listen to AuthState changes like this.
class Stuff extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
Provider<AuthService>(create: (_) => AuthService()),
],
child: Consumer<AuthService>(builder: (context, auth, _) {
return StreamBuilder<FirebaseUser>(ctx,AsyncSnapshot<FirebaseUser> snapshot)=> MaterialApp(
title: 'Stuff',
home: snapshot.data!= null && snapshot.data.user != null ? MainScreen() : SignInScreen(),
routes: {
'/main_screen' : (context) => MainScreen(),
'/signup_screen': (context) => SignUpScreen(),
'/signin_screen': (context) => SignInScreen(),
},
);
}),
);
}
}