javascript - 如何在 RxJS 中取消 switchmap 链？

Question

以下switchMap的使用...

Rx.Observable.timer(0, 1000)

  // First switchMap
  .switchMap(i => {
    if (i % 2 === 0) {
      console.log(i, 'is even, continue');
      return Rx.Observable.of(i);
    }
    console.log(i, 'is odd, cancel');
    return Rx.Observable.empty();
  })

  // Second switchMap
  .switchMap(i => Rx.Observable.timer(0, 500).mapTo(i))

  .subscribe(i => console.log(i));

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.5.6/Rx.min.js"></script>

...产生以下输出:

0 is even, continue
0
0
1 is odd, cancel
0 // <-------------------- why?
0 // <-------------------- why?
2 is even, continue
2
2
3 is odd, cancel
2 // <-------------------- why?
2 // <-------------------- why?
4 is even, continue
.
.
.

我期望以下输出:

0 is even, continue
0
0
1 is odd, cancel
2 is even, continue
2
2
3 is odd, cancel
4 is even, continue
.
.
.

我预计第一个 switchMap 每次返回时都会取消下游的所有内容，我猜这是不正确的。

有办法实现所需的行为吗？

Answer 1

原因是返回一个空的 observable 不会导致它切换到新的 observable，即第二个 switchMap 永远不会被调用。

一个真正 hacky 的解决方案是返回一个稍后可以忽略的神奇值

const CANCEL = {};
Rx.Observable.timer(0, 1000)

  // First switchMap
  .switchMap(i => {
    if (i % 2 === 0) {
      console.log(i, 'is even, continue');
      return Rx.Observable.of(i);
    }
    console.log(i, 'is odd, send CANCEL observable');
    return Rx.Observable.of(CANCEL);
  })


  // Second switchMap
  .switchMap(i => Rx.Observable.timer(0, 500).mapTo(i))

  // Filter out cancelled events
  .filter(i => i != CANCEL)
  
  .subscribe(i => console.log(i));

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.5.6/Rx.min.js"></script>