Angular + rxjs : No provider for Subscription

Question

当服务持有一个值并且组件订阅它时，我正在尝试以 Angular 实现服务组件通信。我正在使用 rxjs 订阅，但我得到了

Uncaught (in promise): Error: No provider for Subscription!

这是我在服务中所做的:

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

@Injectable()
export class PracticeContextService {
  practice: Subject<any> = new Subject<any>();
  public practiceSelected$ = this.practice.asObservable();

 setPractice(providerId: string) {
   this.practice.next({ providerId: providerId });
 }

 getPractice(): Observable<any> {
   return this.practice.asObservable();
 }
}

在组件中:

import { Component, OnDestroy, OnInit } from '@angular/core';
import { Subscription } from 'rxjs/Subscription';
import { PracticeContextService } from '../practice-context.service';

@Component({
  selector : 'practice-context-selector',
  templateUrl : './practice-context-selector.component.html',
  styleUrls : ['./practice-context-selector.component.css']
})

export class PracticeContextSelectorComponent implements OnInit, OnDestroy {

  practice: string;

  constructor(private context: PracticeContextService,
          private subscription: Subscription) {}

  ngOnInit() {
    this.subscription = this.context.practiceSelected$.subscribe(
      practice => {
        this.practice = practice;
      });
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}

然后将组件和服务捆绑到模块中，然后将其注入(inject)另一个模块。

import { NgModule }           from '@angular/core';
import { CommonModule }       from '@angular/common';

import { PracticeContextSelectorComponent } from './practice-context-selector/practice-context-selector.component';
import { PracticeContextService } from './practice-context.service';

@NgModule({
  imports : [
    CommonModule
  ],
  declarations : [
    PracticeContextSelectorComponent
  ],
  providers : [
    PracticeContextService,
  ],
  exports: [
    PracticeContextSelectorComponent
  ]
})

export class PracticeContextModule {}

显然，我在这里做错了什么

Answer 1

只需删除 private subscription: Subscription来自构造函数并将其放入类的属性中。