当我希望使用currentUser（）的Firebase用户ID时为Null错误

Question

Error

NoSuchMethodError: The method 'currentUser' was called on null.

Receiver: null

Tried calling: currentUser()

I'm interesting in know where i need to using this code:

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

This code is from the home_page.dart (see below)

I had also tried to using:

widget.auth.current....

root_page

In bottom of the file i had tried different approach. Instead of HomePage then _HomePageState(comments out)

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,
        );
    }
  }
}

auth.dart

This is the class I'm using to get current user from auth.

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();
  }

}

home_page.dart

I had tried different things in this file but get different errors.

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 = '';

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

Answer 1

_auth.currentUser() does not return the user ID, it returns a FirebaseUser object .

Change it like this :

auth.currentUser().then((user) {
  if (user == null || user.isAnonymous) {
    // what will you do?
    return;
  }
  setState(() {
    _uid = user.uid;
  });
});

Yet, I would recommend to monitor the onAuthStateChanged stream instead . This way you will be informed when the user logs in or logs out immediately.

Check this article , it covers it in depth.

Answer 2

You can use like this.

someMethod() async {
   FirebaseUser user = await FirebaseAuth.instance.currentUser();
   print(user.uid);
}