Angular 8 订阅服务中的变量的问题

Question

我正在学习 Angular 并尝试监听一个保存 http 请求值的变量。这是服务

export class UsersService {
 message = new Subject<string>();
 constructor(private http: HttpClient) { }
 addUser(userData: {username, email, password, school}) {
   this.http.post<any>('http://127.0.0.1/apiapp/insert.php', userData).subscribe( (r) => {
     this.message = r.message;
   });
}

当我记录消息时，我收到success 现在我想调用这个函数并从组件中监听那个变量

result = '';
private resultSubscription: Subscription;

  ngOnInit() {
    this.resultSubscription = this.userS.message.subscribe( (r) => {
      this.result = r;
    });
 }

  addPost(userData: {username: string, email: string, password: string, school: string}) {
    this.userS.addUser(userData);
    console.log(this.result);    
 }

我得到一个空白数据（没有记录只是控制台中的一个空白行）。为什么会这样以及如何解决？ 提前致谢

Answer 1

这里有 3 个问题。

• 您没有通过主题正确发送值
• 您的console.log(this.result) 的位置存在一些异步问题

• 您需要在您的主题上调用 .asObservable 以获得可观察的主题

  export class UsersService {
      message = new Subject<string>();
      constructor(private http: HttpClient) { }
  
      addUser(userData: {username, email, password, school}) {
          this.http.post<any>('http://127.0.0.1/apiapp/insert.php', userData).subscribe( (r) => {
          // .next is how you send data via a subject
          this.message.next(r.message);
      });
  }
  
  
  
  
  result = '';
  private resultSubscription: Subscription;
  
  ngOnInit() {
      // Get subject as observable
      this.resultSubscription = this.userS.message.asObservable().subscribe( (r) => {
          this.result = r;
          // by logging here we can ensure that the result will be populated
          console.log(this.result);  
      });
  }
  
  addPost(userData: {username: string, email: string, password: string, school: string}) {
      this.userS.addUser(userData);
  }
  
  // It is important to unsubscribe for your observable when the component is destroyed
  ngOnDestroy(): void { this.resultSubscription.unsubscribe(); }