The argument type 'Stream<MyUser?>' can't be assigned to the parameter type &#0-CodePudding

I'm trying to create a user system with assistment of this playlist. But, it's kind of outdated so I'm trying to keeping up with today. I'm a totally beginner. I read few things from the stackoverflow but I couldn't keep up. Here is what error it gave:

The argument type 'Stream<MyUser?>' can't be assigned to the parameter type 'Stream<MyUser?>?'.

And here is the main.dart: (To be specific error is right in the "value: AuthService().user," line. The "AuthService().user" part is underline in red.)

import 'package:flutter/material.dart';
import 'package:unistuff_main/screens/wrapper.dart';
import 'package:provider/provider.dart';
import 'package:unistuff_main/services/auth.dart';
import 'package:unistuff_main/models/myuser.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return StreamProvider<MyUser?>.value(
      initialData: null,
      value: AuthService().user,
      child: MaterialApp(
        home: Wrapper(),
      ),
    );
  }
}

And here is my myuser.dart file:

class MyUser {
  final String? uid;

  MyUser({this.uid});
}

And here is auth.dart: (If you noticed I didn't add any login method yet. Is that can cause the problem?)

import 'package:firebase_auth/firebase_auth.dart';
import 'package:unistuff_main/models/MyUser.dart';

class AuthService {
  final FirebaseAuth _auth = FirebaseAuth.instance; //private data member

  //create MyUser object based on FirebaseUser
  MyUser? _userfromFirebase(User user) {
    return user != null ? MyUser(uid: user.uid) : null;
  }

  //auth change user stream
  Stream<MyUser?> get user {
    return _auth
        .authStateChanges()
        .map((User? user) => _userfromFirebase(user!));
  }

  //sign in with email & password
  Future signInWithMail() async {
    try {} catch (e) {}
  }

  //register with email & password

  //sign out
}

Thanks for any help..

CodePudding user response：

It seems that the type of your AuthService().user isn't null-able, so try to change the type of the user from your auth.dart to Stream<MyUser?>?.

Stream<MyUser?>? get onAuthStateChanges => FirebaseAuth.instance
  .authStateChanges()
  .map((currentUser) => MyUser.fromSnapshot(currentUser!));

I'm not sure, but you probably are using the authStateChanges() from FirebaseAuth.instance. So if it doesn't solve your problem, print the AuthService().user :)

CodePudding user response：

In the line when error occurs, try to use operator ?? and pass the raw object if null.

For example:

MyObject(
  stringField: myString ?? '', //you are ensuring that if your String is null, there will be no error
);