Emitting only after set of events?

问题

I'm trying to do something like the following - B is being emitted independently of A, but I want to emit when there is a B that happens after an A (in that order).

----A--------A-->
B--B--B--B----B->

------B-------B->

Thanks!

回答1:

In case you have a hot observable and you want to emit the values from B, then you might want to use a switchMap in combination with a take(1): (basically just the 3 last lines are relevant, the upper part is just mocking some data-stream)

// Mocking A and B
const streamA$ = Rx.Observable
  .interval(2500)
  .do(() => console.log("Emitting on A => TAKE NEXT B!!!"))
  .share();
const streamB$ = Rx.Observable
  .interval(1200)
  .do(data => console.log("Emitting on B: " + data))
  .publish();
streamB$.connect();
// \End of mocking

streamA$
  .switchMap(() => streamB$.take(1))
  .subscribe(data => console.info("Took value: " + data));

<script src="https://unpkg.com/rxjs/bundles/Rx.min.js"></script>

---

If you have cold observables you could use window in combination with map:

// Mocking A and B
const streamA$ = Rx.Observable
  .interval(4000)
  .do(() => console.log("Emitting on A => TAKE NEXT B!!!"));
const streamB$ = Rx.Observable
  .interval(900)
  .do(data => console.log("Emitting on B: " + data));
// \End of mocking

streamB$.window(streamA$)
  .skip(1)                 // skip the first window, since this will be emitted before A emitted the first time
  .mergeMap(win => win.take(1)) // each window should have at most 1 emission
  
  .subscribe(data => console.info("Took value: " + data));

<script src="https://unpkg.com/rxjs/bundles/Rx.min.js"></script>

来源：https://stackoverflow.com/questions/43188180/emitting-only-after-set-of-events