RxJS throttle behavior; get first value immediately

Question

Example Plunkr: https://plnkr.co/edit/NZwb3ol8CbZFtSc6Q9zm?p=preview

I am aware that there are these 3 throttle methods for RxJS (5.0 beta.4):

auditTime(), throttleTime() and debounceTime()

The behavior I am looking for is the one lodash does by default on throttle:

• 1. Give me the first value immediately!
• 2. on consecutive values, hold values for the given delay, then emit last occurred value
• 3. when throttle delay expired, go back to state (1)

In theory this should look like:

inputObservable
  .do(() => cancelPreviousRequest())
  .throttleTime(500)
  .subscribe((value) => doNextRequest(value))

But

• throttleTime never gives me the last value, if emitted in the throttle timeout
• debounceTime doesn't trigger immediately
• auditTime doesn't trigger immediately

Could I combine any of the RxJS methods to achieve the described behavior?

Answer 1

For anyone looking for this after 2018: This has been added over a year ago, but for some reason, the documentation has not been updated yet.

RxJS commit

You can simply pass a config object to throttleTime. The default is { leading: true, trailing: false }. To achieve the behavior discussed here you simply have to set trailing to true: { leading: true, trailing: true }

EDIT:

For completeness, here is a working snippet:

import { asyncScheduler } from 'rxjs'
import { throttleTime } from 'rxjs/operators'

...

observable.pipe(
  throttleTime(100, asyncScheduler, { leading: true, trailing: true })
)

Answer 2

For older RxJs, I wrote a concatLatest operator that does most of what you want. With it, you could get your throttling behavior with this code:

const delay = Rx.Observable.empty().delay(500);
inputObservable
    .map(value => Rx.Observable.of(value).concat(delay))
    .concatLatest()
    .subscribe(...);

Here's the operator. I took a stab at updating it to work with RxJS5:

Rx.Observable.prototype.concatLatest = function () {
    /// <summary>
    /// Concatenates an observable sequence of observable sequences, skipping sequences that arrive while the current sequence is being observed.
    /// If N new observables arrive while the current observable is being observed, the first N-1 new observables will be thrown
    /// away and only the Nth will be observed.
    /// </summary>
    /// <returns type="Rx.Observable"></returns>
    var source = this;

    return Rx.Observable.create(function (observer) {
        var latest,
            isStopped,
            isBusy,
            outerSubscription,
            innerSubscription,
            subscriptions = new Rx.Subscription(function () {
              if (outerSubscription) {
                outerSubscription.unsubscribe();
              }
              if (innerSubscription) {
                innerSubscription.unsubscribe();
              }
            }),
            onError = observer.error.bind(observer),
            onNext = observer.next.bind(observer),
            innerOnComplete = function () {
                var inner = latest;
                if (inner) {
                    latest = undefined;
                    if (innerSubscription) {
                      innerSubscription.unsubscribe();
                    }
                    innerSubscription = inner.subscribe(onNext, onError, innerOnComplete);
                }
                else {
                    isBusy = false;
                    if (isStopped) {
                        observer.complete();
                    }
                }
            };

        outerSubscription = source.subscribe(function (newInner) {
            if (isBusy) {
                latest = newInner;
            }
            else {
                isBusy = true;
                if (innerSubscription) {
                  innerSubscription.unsubscribe();
                }
                innerSubscription = newInner.subscribe(onNext, onError, innerOnComplete);
            }
        }, onError, function () {
            isStopped = true;
            if (!isBusy) {
                observer.complete();
            }
        });

        return subscriptions;
    });
};

And here's an updated plunkr: https://plnkr.co/edit/DSVmSPRijJwj9msefjRi?p=preview

Note I updated your lodash version to the latest version. In lodash 4.7, I rewrote the throttle/debounce operators to fix some edge case bugs. You were using 4.6.1 which still had some of those bugs, though I don't think they were affecting your test.

Answer 3

I took the auditTime operator and changed 2 lines to achieve the desired behavior.

New plunker: https://plnkr.co/edit/4NkXsOeJOSrLUP9WEtp0?p=preview

Original:

• https://github.com/ReactiveX/rxjs/blob/master/src/operator/auditTime.ts

Changes:

from (auditTime):

protected _next(value: T): void {
  this.value = value;
  this.hasValue = true;
  if (!this.throttled) {
    this.add(this.throttled = this.scheduler.schedule(dispatchNext, this.duration, this));
  }
}

clearThrottle(): void {
  const { value, hasValue, throttled } = this;
  if (throttled) {
    this.remove(throttled);
    this.throttled = null;
    throttled.unsubscribe();
  }
  if (hasValue) {
    this.value = null;
    this.hasValue = false;
    this.destination.next(value);
  }
}

to (auditTimeImmediate):

protected _next(value: T): void {
    this.value = value;
    this.hasValue = true;
    if (!this.throttled) {
        // change 1:
        this.clearThrottle();
    }
}

clearThrottle(): void {
    const { value, hasValue, throttled } = this;
    if (throttled) {
        this.remove(throttled);
        this.throttled = null;
        throttled.unsubscribe();
    }
    if (hasValue) {
        this.value = null;
        this.hasValue = false;
        this.destination.next(value);
        // change 2:
        this.add(this.throttled = this.scheduler.schedule(dispatchNext, this.duration, this));
    }
}

So I start the timeout after the value was nexted.

Usage:

inputObservable
  .do(() => cancelPreviousRequest())
  .auditTimeImmediate(500)
  .subscribe((value) => doNextRequest(value))