String _uid = '';
      void initState() {
        super.initState();
        auth.currentUser().then((userId) {
          setState(() {
            _uid = userId;
          });
        });
      }

class RootPage extends StatefulWidget {

  RootPage({this.auth});
  final BaseAuth auth;

  @override
  State<StatefulWidget> createState() => new _RootPageState();

}

enum AuthStatus {
  notSignedIn,
  signedIn
}

class _RootPageState extends State<RootPage>{

  AuthStatus authStatus = AuthStatus.notSignedIn;

  @override
  void initState() {
    super.initState();
    widget.auth.currentUser().then((userId) {
      setState(() {
        authStatus = userId == null ? AuthStatus.notSignedIn : AuthStatus.signedIn;
      });
    });
  }

  void _signedIn(){
    setState(() {
      authStatus = AuthStatus.signedIn;
    });
  }

  void _signedOut() {
    setState(() {
      authStatus = AuthStatus.notSignedIn;
    });
  }

  @override
  Widget build(BuildContext context) {
    switch(authStatus) {
      case AuthStatus.notSignedIn:
        return new LoginPage(
          auth: widget.auth,
          onSignedIn: _signedIn,
        );
      case AuthStatus.signedIn:
        /*
        return new HomePageState(
          auth: widget.auth,
          onSignedOut: _signedOut,
        );
        */
        return new HomePage(
          auth: widget.auth,
          onSignedOut: _signedOut,
        );
    }
  }
}

abstract class BaseAuth{
  Future<String> signInWithEmailAndPassword(String email, String password);
  Future<String> createUserWithEmailAndPassword(String email, String password);
  Future<String> currentUser();
  Future<void> signOut();
}

class Auth implements BaseAuth{

  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;

  Future<String> signInWithEmailAndPassword(String email, String password) async {
    FirebaseUser user = await _firebaseAuth.signInWithEmailAndPassword(email: email, password: password);
    return user.uid;
  }

  Future<String> createUserWithEmailAndPassword(String email, String password) async {
    FirebaseUser user = await _firebaseAuth.createUserWithEmailAndPassword(email: email, password: password);
    return user.uid;
  }

  Future<String> currentUser() async {
    FirebaseUser user = await _firebaseAuth.currentUser();
    return user.uid;
  }

  Future<void> signOut() async {
    return _firebaseAuth.signOut();
  }

}

class HomePage extends StatefulWidget {

  HomePage({this.auth, this.onSignedOut});

  final BaseAuth auth;
  final VoidCallback onSignedOut;

  void _signOut() async {
    try {
      await auth.signOut();
      onSignedOut();
    } catch (e) {
      print(e);
    }
  }
  @override
  State<StatefulWidget> createState() => new _HomePageState();
}

class _HomePageState extends State<HomePage> {
/*
  _HomePageState({this.auth, this.onSignedOut});
  final BaseAuth auth;
  final VoidCallback onSignedOut;
  */
  //final BaseAuth auth;

  String _uid = '';
  void initState() {
    super.initState();
    auth.currentUser().then((userId) {
      setState(() {
        _uid = userId;
      });
    });
  }