@reactivex/rxjs/src/operator/sample.ts

Reactive Extensions for modern JavaScript

import {Operator} from '../Operator';
import {Observable} from '../Observable';
import {Subscriber} from '../Subscriber';

import {OuterSubscriber} from '../OuterSubscriber';
import {InnerSubscriber} from '../InnerSubscriber';
import {subscribeToResult} from '../util/subscribeToResult';

/**
 * Emits the most recently emitted value from the source Observable whenever
 * another Observable, the `notifier`, emits.
 *
 * <span class="informal">It's like {@link sampleTime}, but samples whenever
 * the `notifier` Observable emits something.</span>
 *
 * <img src="./img/sample.png" width="100%">
 *
 * Whenever the `notifier` Observable emits a value or completes, `sample`
 * looks at the source Observable and emits whichever value it has most recently
 * emitted since the previous sampling, unless the source has not emitted
 * anything since the previous sampling. The `notifier` is subscribed to as soon
 * as the output Observable is subscribed.
 *
 * @example <caption>On every click, sample the most recent "seconds" timer</caption>
 * var seconds = Rx.Observable.interval(1000);
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var result = seconds.sample(clicks);
 * result.subscribe(x => console.log(x));
 *
 * @see {@link audit}
 * @see {@link debounce}
 * @see {@link sampleTime}
 * @see {@link throttle}
 *
 * @param {Observable<any>} notifier The Observable to use for sampling the
 * source Observable.
 * @return {Observable<T>} An Observable that emits the results of sampling the
 * values emitted by the source Observable whenever the notifier Observable
 * emits value or completes.
 * @method sample
 * @owner Observable
 */
export function sample<T>(notifier: Observable<any>): Observable<T> {
  return this.lift(new SampleOperator(notifier));
}

export interface SampleSignature<T> {
  (notifier: Observable<any>): Observable<T>;
}

class SampleOperator<T> implements Operator<T, T> {
  constructor(private notifier: Observable<any>) {
  }

  call(subscriber: Subscriber<T>, source: any): any {
    return source._subscribe(new SampleSubscriber(subscriber, this.notifier));
  }
}

/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
class SampleSubscriber<T, R> extends OuterSubscriber<T, R> {
  private value: T;
  private hasValue: boolean = false;

  constructor(destination: Subscriber<any>, notifier: Observable<any>) {
    super(destination);
    this.add(subscribeToResult(this, notifier));
  }

  protected _next(value: T) {
    this.value = value;
    this.hasValue = true;
  }

  notifyNext(outerValue: T, innerValue: R,
             outerIndex: number, innerIndex: number,
             innerSub: InnerSubscriber<T, R>): void {
    this.emitValue();
  }

  notifyComplete(): void {
    this.emitValue();
  }

  emitValue() {
    if (this.hasValue) {
      this.hasValue = false;
      this.destination.next(this.value);
    }
  }
}