Proxy provider with firebase authentication

main.dart

void main() {
  Provider.debugCheckInvalidValueType = null;
  runApp(MyAppExample());
}

class MyAppExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MultiProvider(providers: [
        StreamProvider<FirebaseUser>(
          create: (_) => FirebaseAuth.instance.onAuthStateChanged,
          initialData: null,
        ),
        ProxyProvider<FirebaseUser, String>(
          update: (_, user, __) {
            return user?.uid;
          },
        )
      ], child: AppView()),
    );
  }
}

class AppView extends StatelessWidget {
  const AppView({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final uid = Provider.of<String>(context);
    print(uid);
    final isLoggedIn = uid != null;

    return Scaffold(
      body: Center(
        child: Column(
          children: <Widget>[
            SizedBox(height: 300),
            Text(
              'Has user signed in?',
              style: Theme.of(context).textTheme.headline4,
            ),
            SizedBox(height: 50),
            AnimatedDefaultTextStyle(
              child: Text(
                isLoggedIn ? 'Yes' : 'No',
              ),
              style: Theme.of(context).textTheme.headline2.copyWith(
                    color: isLoggedIn ? Colors.greenAccent : Colors.redAccent,
                  ),
              duration: Duration(milliseconds: 1500),
              curve: Curves.ease,
            ),
            SizedBox(
              height: 100,
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceAround,
              children: <Widget>[
                FlatButton(
                    color: Colors.yellowAccent,
                    onPressed: () => FirebaseAuth.instance.signInAnonymously(),
                    child: Text('Sign In')),
                FlatButton(
                    color: Colors.tealAccent,
                    onPressed: () => FirebaseAuth.instance.signOut(),
                    child: Text('Sign Out')),
              ],
            ),
          ],
        ),
      ),
    );
  }
}