es6/operator/multicast.js

import { multicast as higherOrder } from '../operators/multicast';
/* tslint:enable:max-line-length */
/**
 * Allows source Observable to be subscribed only once with a Subject of choice,
 * while still sharing its values between multiple subscribers.
 *
 * <span class="informal">Subscribe to Observable once, but send its values to multiple subscribers.</span>
 *
 * <img src="./img/multicast.png" width="100%">
 *
 * `multicast` is an operator that works in two modes.
 *
 * In the first mode you provide a single argument to it, which can be either an initialized Subject or a Subject
 * factory. As a result you will get a special kind of an Observable - a {@link ConnectableObservable}. It can be
 * subscribed multiple times, just as regular Observable, but it won't subscribe to the source Observable at that
 * moment. It will do it only if you call its `connect` method. This means you can essentially control by hand, when
 * source Observable will be actually subscribed. What is more, ConnectableObservable will share this one subscription
 * between all of its subscribers. This means that, for example, `ajax` Observable will only send a request once,
 * even though usually it would send a request per every subscriber. Since it sends a request at the moment of
 * subscription, here request would be sent when the `connect` method of a ConnectableObservable is called.
 *
 * The most common pattern of using ConnectableObservable is calling `connect` when the first consumer subscribes,
 * keeping the subscription alive while several consumers come and go and finally unsubscribing from the source
 * Observable, when the last consumer unsubscribes. To not implement that logic over and over again,
 * ConnectableObservable has a special operator, `refCount`. When called, it returns an Observable, which will count
 * the number of consumers subscribed to it and keep ConnectableObservable connected as long as there is at least
 * one consumer. So if you don't actually need to decide yourself when to connect and disconnect a
 * ConnectableObservable, use `refCount`.
 *
 * The second mode is invoked by calling `multicast` with an additional, second argument - selector function.
 * This function accepts an Observable - which basically mirrors the source Observable - and returns Observable
 * as well, which should be the input stream modified by any operators you want. Note that in this
 * mode you cannot provide initialized Subject as a first argument - it has to be a Subject factory. If
 * you provide selector function, `multicast` returns just a regular Observable, instead of ConnectableObservable.
 * Thus, as usual, each subscription to this stream triggers subscription to the source Observable. However,
 * if inside the selector function you subscribe to the input Observable multiple times, actual source stream
 * will be subscribed only once. So if you have a chain of operators that use some Observable many times,
 * but you want to subscribe to that Observable only once, this is the mode you would use.
 *
 * Subject provided as a first parameter of `multicast` is used as a proxy for the single subscription to the
 * source Observable. It means that all values from the source stream go through that Subject. Thus, if a Subject
 * has some special properties, Observable returned by `multicast` will have them as well. If you want to use
 * `multicast` with a Subject that is one of the ones included in RxJS by default - {@link Subject},
 * {@link AsyncSubject}, {@link BehaviorSubject}, or {@link ReplaySubject} - simply use {@link publish},
 * {@link publishLast}, {@link publishBehavior} or {@link publishReplay} respectively. These are actually
 * just wrappers around `multicast`, with a specific Subject hardcoded inside.
 *
 * Also, if you use {@link publish} or {@link publishReplay} with a ConnectableObservables `refCount` operator,
 * you can simply use {@link share} and {@link shareReplay} respectively, which chain these two.
 *
 * @example <caption>Use ConnectableObservable</caption>
 * const seconds = Rx.Observable.interval(1000);
 * const connectableSeconds = seconds.multicast(new Subject());
 *
 * connectableSeconds.subscribe(value => console.log('first: ' + value));
 * connectableSeconds.subscribe(value => console.log('second: ' + value));
 *
 * // At this point still nothing happens, even though we subscribed twice.
 *
 * connectableSeconds.connect();
 *
 * // From now on `seconds` are being logged to the console,
 * // twice per every second. `seconds` Observable was however only subscribed once,
 * // so under the hood Observable.interval had only one clock started.
 *
 * @example <caption>Use selector</caption>
 * const seconds = Rx.Observable.interval(1000);
 *
 * seconds
 *     .multicast(
 *         () => new Subject(),
 *         seconds => seconds.zip(seconds) // Usually zip would subscribe to `seconds` twice.
 *                                         // Because we are inside selector, `seconds` is subscribed once,
 *     )                                   // thus starting only one clock used internally by Observable.interval.
 *     .subscribe();
 *
 * @see {@link publish}
 * @see {@link publishLast}
 * @see {@link publishBehavior}
 * @see {@link publishReplay}
 * @see {@link share}
 * @see {@link shareReplay}
 *
 * @param {Function|Subject} subjectOrSubjectFactory - Factory function to create an intermediate Subject through
 * which the source sequence's elements will be multicast to the selector function input Observable or
 * ConnectableObservable returned by the operator.
 * @param {Function} [selector] - Optional selector function that can use the input stream
 * as many times as needed, without causing multiple subscriptions to the source stream.
 * Subscribers to the input source will receive all notifications of the source from the
 * time of the subscription forward.
 * @return {Observable<T>|ConnectableObservable<T>} An Observable that emits the results of invoking the selector
 * on the source stream or a special {@link ConnectableObservable}, if selector was not provided.
 *
 * @method multicast
 * @owner Observable
 */
export function multicast(subjectOrSubjectFactory, selector) {
    return higherOrder(subjectOrSubjectFactory, selector)(this);
}
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