RxJS 订阅未触发

目前正在学习 RxJS。我有一个整数selectedCourseIndex在我想要订阅一个单独的组件的服务中。

课程部分.service.ts

private selectedCourseIndex: number = -1; //this number can change from another method
private selectedCourseIndexUpdated = new Subject<number>();

getSelectedCourseUpdateListener(){
    return this.selectedCourseIndexUpdated.asObservable();
}

课程部分.component.ts

  private selectedCourseIndex: number = -1; //Placeholder initial selectedCourseIndex until new one given on init from courses-section service
  private selectedCourseSub: Subscription;

  constructor( public coursesSectionService: CoursesSectionService ){}

  ngOnInit(){
   this.selectedCourseIndex = 
   this.coursesSectionService.getSelectedCourseIndex();
   this.selectedCourseSub = this.coursesSectionService
    .getSelectedCourseUpdateListener()
    .subscribe((selectedCourseIndex: number) => {
      this.selectedCourseIndex = selectedCourseIndex;
      console.log('this fires when subscription updates'); //this never fires once
    });
  }

但是，订阅永远不会触发，无论是在最初订阅时还是在整数更改时。怎么了？

“Subject”可观察对象不会在订阅时重放之前的值。仅当您[以某种方式]在服务中执行以下操作时，您的订阅才会触发 -

this.selectedCourseIndexUpdated.next(<your new number>);

要在每个订阅上触发订阅[以获取最后发出的值]，您应该使用“BehaviorSubject”而不是像这样的主题 -

private selectedCourseIndexUpdated = new BehaviorSubject<number>(0);

BehaviorSubject 重放每个订阅上最后发出的值。请注意，BehaviorSubject 构造函数采用默认值。