import {Subscribable} from '../Observable';
import {MergeAllOperator} from './mergeAll';

/**
 * Converts a higher-order Observable into a first-order Observable by
 * concatenating the inner Observables in order.
 *
 * <span class="informal">Flattens an Observable-of-Observables by putting one
 * inner Observable after the other.</span>
 *
 * <img src="./img/concatAll.png" width="100%">
 *
 * Joins every Observable emitted by the source (a higher-order Observable), in
 * a serial fashion. It subscribes to each inner Observable only after the
 * previous inner Observable has completed, and merges all of their values into
 * the returned observable.
 *
 * __Warning:__ If the source Observable emits Observables quickly and
 * endlessly, and the inner Observables it emits generally complete slower than
 * the source emits, you can run into memory issues as the incoming Observables
 * collect in an unbounded buffer.
 *
 * Note: `concatAll` is equivalent to `mergeAll` with concurrency parameter set
 * to `1`.
 *
 * @example <caption>For each click event, tick every second from 0 to 3, with no concurrency</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var higherOrder = clicks.map(ev => Rx.Observable.interval(1000).take(4));
 * var firstOrder = higherOrder.concatAll();
 * firstOrder.subscribe(x => console.log(x));
 *
 * @see {@link combineAll}
 * @see {@link concat}
 * @see {@link concatMap}
 * @see {@link concatMapTo}
 * @see {@link exhaust}
 * @see {@link mergeAll}
 * @see {@link switch}
 * @see {@link zipAll}
 *
 * @return {Observable} An Observable emitting values from all the inner
 * Observables concatenated.
 * @method concatAll
 * @owner Observable
 */
export function concatAll<T>(): T {
  return this.lift(new MergeAllOperator<T>(1));
}

export interface ConcatAllSignature<T> {
  (): T;
  <R>(): Subscribable<R>;
}