javascript - RxJS reduce doesn't continue

Question

Why doesn't the flatMap cause downstream reductions to fire?

I got code like:

handleFiles.flatMap(files =>
  Rx.Observable.from(files).
  flatMap((file, i) => fileReader(file, i)).
  reduce((form, file, i) => {
    form.append('file[' + i + ']', result);
    console.log('reduce step', file);
    return form;
  }, new FormData()).
  tap(console.log.bind(console, 'after reduce'))
).
subscribe(console.log.bind(console, 'response'));

And the problem is that the 'after reduce' tap is never hit. Why?

The log is like:

reduce step [data]
reduce step [data]

Screenshot:

Answer 1

问题不在flatMap；它的工作方式reduce。

reduce读取整个流并将其缩减为单个值，仅在源流关闭时发出。如果您的from(files)流没有结束，那么reduce将永远不会输出它的值。

尝试scan改用；它发出每个中间步骤，似乎是您正在寻找的。

Answer 2

如果 files 是一个数组，那么如果从 fileReader 返回的 observable 是，reduce 应该终止。因此，对于这段代码，问题在于 fileReader 返回了一个未完成的 observable。

Answer 3

这是一个使用 reduce 和非终止 observable 的示例；

使用窗口时间：

import { fromEvent, interval, timer } from 'rxjs';
import { reduce, filter, windowTime, map, mergeMap } from 'rxjs/operators';

const interval$ = interval(1000);
const observable = interval$.pipe(
  windowTime(2000), // each window is 2s
  mergeMap(window$ => window$.pipe(
    reduce((a,x) => { // reduce to array
      return [...a,x];
    }, []),
    filter(x => !!x.length) // in the background timer is still running so suppress empty events
  )), // flatten the Observable-of-Observables
);
const subscription = observable.subscribe(x => console.log(x));
setTimeout(() => subscription.unsubscribe(), 10000);

使用缓冲时间：

import { fromEvent } from 'rxjs';
import { bufferTime, filter, map } from 'rxjs/operators';

let count = 1;
const clicks = fromEvent(document, 'click');
const observable = clicks.pipe(
  bufferTime(1000), // batch into array every 1s
  filter(x => !!x.length), // ignore events without clicks
  map(x => x.reduce((a,y) => ({...a, [count++]: y}), {})),
);
observable.subscribe(x => console.log(x));

使用审计时间：

import { fromEvent } from 'rxjs';
import { tap, auditTime, map } from 'rxjs/operators';

let buffer = [];
const clicks = fromEvent(document, 'click');
const observable = clicks.pipe(
  tap((event) => buffer.push(event)),
  auditTime(1000), // buffer every 1s after 1st click is detected
  map((_lastEvent) => { // ignore last event
    const events = buffer; // save off buffer
    buffer = []; // clear buffer
    return events.reduce((a,e,i) => ({...a, [i]: e}),{});
  }),
);
observable.subscribe((events) => console.log(events));

使用 takeUntil 并重复：

注意：take/repeat 将重置 observable（即，interval-counter 保持在 0 并且事件可能会丢失）

import { fromEvent, timer, interval } from 'rxjs';
import { takeUntil, reduce, repeat, filter } from 'rxjs/operators';

const interval$ = interval(1000);
const timer$ = timer(2000);

const observable = interval$.pipe(
  takeUntil(timer$), // unsubscribe from stream every 2s so reduce terminates
  reduce((acc, event) => [...acc, event], []), // reduce to array of events
  filter(x => !!x.length), // suppress emission of empty stream
  repeat(), // resubscribe to stream
);
// console will only show array of [0] since takeUntil stops right when interval emits
const subscription = observable.subscribe(x => console.log(x));
setTimeout(() => subscription.unsubscribe(), 10000);