rxjs仅在第一次执行tap [英] rxjs execute tap only at the first time

问题描述

我只想在获得第一个发射值时才执行tap()

I want to execute tap() only when i get the first emitted value

类似的东西:

Observable
  .pipe(
     tap(() => { /* execute only when I get the first emitted value */ })
  )
  .subscribe(() => {
     // .....
  })

推荐答案

您可以在地图操作符(如 concatMap )中使用索引.与其他方法不同，这对于所选索引是完全灵活的.假设您要点击第二个发射 index === 1 或任何谓词，例如 index％2 === 0

You can use the index in map operators like concatMap. Unlike other approaches this is totally flexible about the chosen index. Let's say if you want tap on 2nd emission index === 1 or any predicate like index % 2 === 0

// these are because of using rxjs from CDN in code snippet, ignore them
const {of, interval} = rxjs;
const {take, tap, concatMap} = rxjs.operators;


// main code
const stream = interval(250).pipe(take(4))

stream.pipe(
  concatMap((value, index) => index === 0
    ? of(value).pipe(
        tap(() => console.log('tap'))
      )
    : of(value)
  )
)
.subscribe(x => console.log(x));

<script src="https://unpkg.com/@reactivex/rxjs@6.x/dist/global/rxjs.umd.js"></script>

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