Angular 2+ 仅在变化时订阅 observable

Question

所以这是场景。我有一个带有BehaviorSubject 的用户服务和一个返回此BehaviorSubject 的可观察值的方法。我的第二个文件是订阅可观察对象的 header 组件。问题是..是否可以仅订阅更改，或者我是否需要在 this.userSubject.next(this.user) 之前添加一些逻辑？

这里是代码供引用:

// user.service.ts
user: User;
private userSubject = new BehaviorSubject<User>(new User({}));

keepUpdated = () => {
  this.tokenService.tokenStream()
    .subscribe(token => {
      this.user.update(token);
      this.userSubject.next(this.user);
    });
}

这里

// header.component.ts
ngOnInit() {
  this.userService.keepUpdated();
  this.userService.userStream()
    .subscribe(user => {
      // Here is the problem. This console.log gets called everytime userSubject.next(this.user) send something. I would like it only only to be called if the user is different from the previous one.
      console.log(user);
    });
}

Answer 1

您可以使用 distinctUntilChanged 运算符 ( documentation ):

ngOnInit() {
  this.userService.keepUpdated();
  this.userService.userStream()
    .distinctUntilChanged()
    .subscribe(user => {
      console.log(user);
    });
}

这应该通过与前一个值进行比较来过滤每个发出的值，如果不同，则将调用订阅，否则不会。