docs(multicast): enhance the documentation

src/operator/multicast.ts

@@ -10,21 +10,96 @@ export function multicast<T>(SubjectFactory: (this: Observable<T>) => Subject<T>
 /* tslint:enable:max-line-length */
 
 /**
- * Returns an Observable that emits the results of invoking a specified selector on items
- * emitted by a ConnectableObservable that shares a single subscription to the underlying stream.
+ * 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%">
  *
- * @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
- * or Subject to push source elements into.
- * @param {Function} [selector] - Optional selector function that can use the multicasted source stream
+ * `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 given source will receive all notifications of the source from the
+ * Subscribers to the input source will receive all notifications of the source from the
  * time of the subscription forward.
- * @return {Observable} An Observable that emits the results of invoking the selector
- * on the items emitted by a `ConnectableObservable` that shares a single subscription to
- * the underlying stream.
+ * @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
  */