为什么在共享服务中使用 Subject 或 BehaviorSubject，没有 observable 就不能实现？

为了在组件之间共享数据，我们使用共享服务与 Subject 或 BrhaviorSubject observable，如下所示，

import { Injectable } from '@angular/core';
import { Observable, Subject } from 'rxjs';

@Injectable()
export class DataService {
    private subject = new Subject<any>();

    // Method to set navmenu //
    public sendData(data: boolean) {
        this.subject.next(data);
    }

    // Method to clear navmenu //
    public clearData() {
        this.subject.next();
    }

    // Method to get navmenu //
    public getData(): Observable<any> {
        return this.subject.asObservable();
    }
}

但是我们可以在不使用 Subject 或 BehaviorSubject observable 的情况下实现相同的目标。在下面的示例中，组件 A 和 B 都共享 EmailService 的实例，因此如果组件 A 更改了值 (emailService.apiKey)，则组件 B 中的值也会相同，反之亦然，如下所示，

class EmailService{
  public apiKey: string = 'ABCD1234';
}

@Component({
  selector: 'component-a',
  template: `
  Component A - {{emailService.apiKey}}
  <button (click)="changeApiKey()">Change Key</button>
  `
})

export class A{
  public emailService: EmailService;
  constructor(emailService: EmailService){
    this.emailService = emailService;
  }

  changeApiKey(){
    this.emailService.apiKey = "XYZ1234";
  }
}

@Component({
  selector: 'component-b',
  template: `
  Component B - {{emailService.apiKey}}
  <button (click)="changeApiKey()">Change Key</button>
  `
})

export class B{
  public emailService: EmailService;
  constructor(emailService: EmailService){
    this.emailService = emailService;
  }

  changeApiKey(){
    this.emailService.apiKey = "ABCD1234";
  }
}

@Component({
  selector: 'app-root',
  template: `

    <component-a></component-a>
    <br />
    <component-b></component-b>
  `,
  providers: [
    EmailService
  ]
})

export class AppComponent {

}

Can someone please explain why most of the examples and tutorials use observable in shared service. What's the advantage in using that ??