import { Observable } from '../Observable';
import { MonoTypeOperatorFunction } from '../interfaces';
/**
 * 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 declare function sample<T>(notifier: Observable<any>): MonoTypeOperatorFunction<T>;