import { Observable } from '../Observable';
/**
 * Converts a higher-order Observable into a first-order Observable by dropping
 * inner Observables while the previous inner Observable has not yet completed.
 *
 * <span class="informal">Flattens an Observable-of-Observables by dropping the
 * next inner Observables while the current inner is still executing.</span>
 *
 * <img src="./img/exhaust.png" width="100%">
 *
 * `exhaust` subscribes to an Observable that emits Observables, also known as a
 * higher-order Observable. Each time it observes one of these emitted inner
 * Observables, the output Observable begins emitting the items emitted by that
 * inner Observable. So far, it behaves like {@link mergeAll}. However,
 * `exhaust` ignores every new inner Observable if the previous Observable has
 * not yet completed. Once that one completes, it will accept and flatten the
 * next inner Observable and repeat this process.
 *
 * @example <caption>Run a finite timer for each click, only if there is no currently active timer</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var higherOrder = clicks.map((ev) => Rx.Observable.interval(1000).take(5));
 * var result = higherOrder.exhaust();
 * result.subscribe(x => console.log(x));
 *
 * @see {@link combineAll}
 * @see {@link concatAll}
 * @see {@link switch}
 * @see {@link mergeAll}
 * @see {@link exhaustMap}
 * @see {@link zipAll}
 *
 * @return {Observable} An Observable that takes a source of Observables and propagates the first observable
 * exclusively until it completes before subscribing to the next.
 * @method exhaust
 * @owner Observable
 */
export declare function exhaust<T>(this: Observable<T>): Observable<T>;