如何让 rxjs 暂停/恢复？

Question

现在有一个数组，数组值为图片link，例如：

const imageList = [
  'https://cdn.pixabay.com/photo/2020/02/16/20/29/new-york-4854718_960_720.jpg',
  'https://cdn.pixabay.com/photo/2020/02/14/16/04/mallorca-4848741_960_720.jpg',
  'https://cdn.pixabay.com/photo/2020/02/14/04/20/old-city-4847469_960_720.jpg',
  // more...
];

我想用rxjs顺序下载（我是Electron app，可以下载）

第一张图片下载完成后，再下载第二张图片。当正在下载第三张图片时，用户点击暂停按钮，等待第三张图片下载完成。然后就不用下载了。当用户点击继续按钮时，从第四张图片开始下载。

我参考了这篇文章：Buffering (lossless) 部分 https://medium.com/@kddsky/pauseable-observables-in-rxjs-58ce2b8c7dfd。本文代码为：

merge(
  source$.pipe( bufferToggle(off$, ()=>on$)  ),
  source$.pipe( windowToggle(on$, ()=>off$) )
).pipe(
  // then flatten buffer arrays and window Observables
  flatMap(x => x)
)

演示 URL 是：https://thinkrx.io/gist/cef1572743cbf3f46105ec2ba56228cd

但是这段代码有两个问题，不符合我的需求。不知道怎么修改。

我使用redux-observable，所以我想用两个动作触发它们：(this.props.start() / this.props.pause())
恢复后数据依然是一个一个传输，不是一次性释放

当前代码如下所示：

export const epicDownloadResources = (
  action$: ActionsObservable<AnyAction>,
  store$: StateObservable<RootState>,
) => {
  return action$.pipe(
    ofType(appActions.other.start()),
    of([
      'https://cdn.pixabay.com/photo/2020/02/16/20/29/new-york-4854718_960_720.jpg',
      'https://cdn.pixabay.com/photo/2020/02/14/16/04/mallorca-4848741_960_720.jpg',
      'https://cdn.pixabay.com/photo/2020/02/14/04/20/old-city-4847469_960_720.jpg',
    ]),
    mergeMap(() => {
      // code
    }),
    mergeMap((url: string) => {
      // downloading
    })
}

In the real product, it will be downloading the company's internal picture resources, not other non-copyright pictures.

Answer 1

这是我的尝试：

const urlArr = Array.from({ length: 10 }, (_, idx) => 'url/' + idx);
let idx = 0;

const urlEmitter = new Subject();
const url$ = urlEmitter.asObservable();
const stopEmitter = new Subject();
const stopValues$ = stopEmitter.asObservable();

const start$ = fromEvent(start, 'click');
start$.pipe(take(1)).subscribe(() => (stopEmitter.next(false), urlEmitter.next(urlArr[idx++]))); // Start emitting valeus

const stopSubscription = fromEvent(stop, 'click').pipe(mapTo(true)).subscribe(stopEmitter);

const shouldContinue$ = stopValues$.pipe(map(shouldStop => !shouldStop));

const subsequentStartClicks$ = start$.pipe(
  skip(1), // Skip the first time the `start` button is clicked
  observeOn(asyncScheduler), // Make sure it emits after the buffer has been initialized
  tap(() => stopEmitter.next(false)), // shouldContinue$ will emit `true`
);

const downloadedUrls$ = url$.pipe(
  mergeMap(url => of(url).pipe(delay(idx * 500))), // Simulate a file downloading
  combineLatest(shouldContinue$), // Make sure it acts according to `shouldContinue$`
  filter(([_, shouldContinue]) => shouldContinue),
  map(([v]) => v),
  tap((v) => console.warn(v)), // Debugging purposes...

  // Because of `combineLatest`
  // If you click `start` and wait some time, then you click `stop`
  // then you click again `start`, you might get the last value added to the array
  // this is because `shouldContinue$` emitted a new value
  // So you want to make sure you won't get the same value multiple times
  distinctUntilChanged(), 

  tap(() => urlEmitter.next(urlArr[idx++])),

  bufferToggle(
    start$,
    () => stopValues$.pipe(filter(v => !!v)),
  )
);

merge(
  subsequentStartClicks$.pipe(mapTo(false)), // Might not be interested in click events 
  downloadedUrls$
)
  .pipe(filter(v => !!v))
  .subscribe(console.log);

我受到 bufferToggle's 图表的启发。

我的想法是遵循相同的方法，除了仅当 start$ 流发出时才应发出值并且应在 stop$ 发出时停止。

----X--X----------------------------------> urls$

-Y----------------------------------------> start$

-----------Z------------------------------> end$


-----------[X, X]-------------------------------> urls$

每次按下 stop 按钮时，都会将一个 true 值推送到 stopValues$ 流中。 shouldContinue$ 确定 url$ 流是否应继续推送值，具体取决于 stopValues$。

StackBlitz.

Answer 2

我采取了完全不同的方法。

如果我没理解错的话，你想在用户恢复后按顺序继续。这实际上意味着进行窗口化或缓冲是没有意义的。

我认为简单使用 concatMap 嵌套就足够了。

const pause$ = fromEvent(pauseButton, "click").pipe(
 mapTo(false),
);

const resume$ = fromEvent(resumeButton, "click").pipe(
 mapTo(true),
);

const pauseResume$ = merge(pause$,resume$).pipe(
 startWith(true),
 shareReplay(1),
)

const source = of(...imageList).pipe(
 concatMap((url, i) =>
   pauseResume$.pipe(
     tap(v => console.log(`should resume ${v}`)),
     filter(v => v), // Only resume if true
     take(1),
     concatMap(() =>
       from(fetch(url)).pipe(
         delay(1000), // Simulate slow request
         mapTo(i) // just for logging which request we just completed
       )
     )
   )
 )
);
source.subscribe(x => console.log(x));

这将暂停开始新请求，直到 resume$ 发出新值。根据您的情况，我相信这就是您想要的。

我不确定您是否希望第三个请求在用户暂停时在您的场景中完成。我假设你这样做了，但如果没有，你可以在请求后使用另一个 concatMap 来 pauseResume$ 和过滤器。

stackblitz

Answer 3

delayWhen is a very powerful operator. My solution uses mergeMap and delayWhen.

features: retry, throttle, pause, resume

Create and subscribe to Observable

const concurrentLimit = 5
const retryLimit = 10
const source$ = from(new Array(100).fill(0).map((_, i) => i))
// remove <boolean> if not typescript
const pause$ = new BehaviorSubject<boolean>(false);
const pass$ = pause$.pipe(filter((v) => !v));

const throttledTask$ = source$.pipe(
  mergeMap((item) => {
    return of(item).pipe(
      delayWhen(() => pass$),
      mergeMap(async (item) => {
         // you can also throw some errors
         return await new Promise((resolve)=>
             setTimeout(resolve(item), Math.random()*1000))
      }),
      retryWhen((errors$) => errors$.pipe(delay(1000), take(retryLimit)))
    );
  }, concurrentLimit)

const subscription = throttledTask$.subscribe(x => console.log(x))

add Pause/Resume event handlers

const pause = () => { pause$.next(true) }
const resume = () => { pause$.next(false) }

Explanation:

delayWhen will pause the stream and wait until the pass$ signals emit.

BehaviorSubject is used to compose pass$ signals, which will emit the last value when subscribed.

mergeMap can handle asynchronous tasks and has a concurrent thread count limit parameter. When delayWhen pauses a stream, that stream will remain inside mergeMap and occupies a concurrent 'thread'.

retryWhen will resubscribe until errors$.pipe(delay(1000), take(retryLimit)) emits complete or error.

如何让 rxjs 暂停/恢复？

How to make rxjs pause / resume?

rxjs

reactjs

redux

redux-observable

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