flutter - '用户模型？函数（用户）”不能分配给参数类型“用户模型函数（用户？）”，因为“用户模型？” 可以为空，并且 'UserModel' 不是

Question

#我一直在关注 twitter 克隆的 youtube 项目，将 _userFromFirebaseUser 放入 map() 时出现错误；#

##我在 13:29 收到错误，这是链接。https://www.youtube.com/watch?v=YI7avcWI3aQ&t=919s ##

##我在终端收到的错误是Launching lib\main.dart on sdk gphone x86 arm in debug mode... lib\main.dart:1 lib/services/auth/auth.dart:13:40: Error ：参数类型'UserModel？函数（用户）”不能分配给参数类型“用户模型函数（用户？）”，因为“用户模型？” 可以为空，而 'UserModel' 不是。

• 'UserModel' 来自'package:twitterclone/models/users.dart' ('lib/models/users.dart')。
• '用户'来自'package:firebase_auth/firebase_auth.dart'（'../../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/firebase_auth-1.0.1/lib/firebase_auth.dart' ）。返回 auth.authStateChanges().map(_userFromFirebaseUser);

代码和目录

-users.dart

class UserModel {
 final String id;
 UserModel({required this.id});
}

-auth.dart

import 'package:firebase_auth/firebase_auth.dart';
import 'package:twitterclone/models/users.dart';

class AuthService {
 FirebaseAuth auth = FirebaseAuth.instance;

  UserModel? _userFromFirebaseUser(User user) {
  // ignore: unnecessary_null_comparison
     return user != null ? UserModel(id: user.uid) : null;
  }

  Stream<UserModel> get user {
    return auth.authStateChanges().map(_userFromFirebaseUser);
  }

  Future<void> signUp(email, password) async {
    try {
      User user = (await auth.createUserWithEmailAndPassword(
          email: email, password: password)) as User;
      _userFromFirebaseUser(user);
    } on FirebaseAuthException catch (e) {
      if (e.code == 'user-not-found') {
        print('No user found for that email.');
      } else if (e.code == 'wrong-password') {
        print('Wrong password provided for that user.');
      }
    } catch (e) {
      print(e);
    }
  }

  Future<void> signIn(email, password) async {
    try {
      User user = (await auth.signInWithEmailAndPassword(
          email: email, password: password)) as User;

      _userFromFirebaseUser(user);
    } on FirebaseAuthException catch (e) {
      print(e);
    } catch (e) {
      print(e);
    }
  }
}

标签： flutterdartuiddart-null-safety

Answer 1

在

UserModel? _userFromFirebaseUser(User user) {
  // ignore: unnecessary_null_comparison
     return user != null ? UserModel(id: user.uid) : null;
  }

由于UserModel可以为空，因此也将参数更改为可以为空

UserModel? _userFromFirebaseUser(User? user) ...