rxjs - 同步 RxJs 取消订阅不起作用

Question

演示：https ://stackblitz.com/edit/rxjs-unsubscribe-issue?file=index.ts

下面的代码不起作用

错误：无法读取未定义的属性“取消订阅”

    const a = (): Observable<any> =>
    new Observable(sub => {
      sub.next(1);
      return () => {
        console.log('unsubscribe');
      };
    });
    const observer = a().subscribe(
      value => {
        console.log('Subscription');
        observer.unsubscribe();
      },
      e => console.log(e),
      () => console.log('complete')
    );

但是下面的代码正在工作

    const b = (): Observable<any> =>
    new Observable(sub => {
      setTimeout(()=>sub.next(1),0);
      return () => {
        console.log('unsubscribe');
      };
    });
    const observer2 = b().subscribe(
      value => {
        console.log('Subscription b');
        observer2.unsubscribe();
      },
      e => console.log(e),
      () => console.log('complete')
    );

帮我理解背后的原因

Answer 1

正如您在问题的标题中提到的，第一个示例是同步的，因此您在仍在.subscribe()方法内部时获得了第一个值。自然，observer，它应该有一个Subscription对象还没有被初始化。

如果您想在收到单个值后取消订阅，我建议您使用.take(1)