javascript - rxjs 只在第一次执行tap

Question

我只想在获得第一个发射值时才执行 tap()

类似于:

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

Answer 1

您可以在 concatMap 等 map 运算符中使用索引。与其他方法不同，这对于所选索引是完全灵活的。假设您想要点击第二次发射 index === 1 或任何谓词，例如 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));

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