share.ts

import { Observable } from '../Observable';
import { from } from '../observable/from';
import { take } from '../operators/take';
import { Subject } from '../Subject';
import { SafeSubscriber } from '../Subscriber';
import { Subscription } from '../Subscription';
import { MonoTypeOperatorFunction, SubjectLike } from '../types';
import { operate } from '../util/lift';

export interface ShareConfig<T> {
  /**
   * The factory used to create the subject that will connect the source observable to
   * multicast consumers.
   */
  connector?: () => SubjectLike<T>;
  /**
   * If true, the resulting observable will reset internal state on error from source and return to a "cold" state. This
   * allows the resulting observable to be "retried" in the event of an error.
   * If false, when an error comes from the source it will push the error into the connecting subject, and the subject
   * will remain the connecting subject, meaning the resulting observable will not go "cold" again, and subsequent retries
   * or resubscriptions will resubscribe to that same subject. In all cases, RxJS subjects will emit the same error again, however
   * {@link ReplaySubject} will also push its buffered values before pushing the error.
   * It is also possible to pass a notifier factory returning an observable instead which grants more fine-grained
   * control over how and when the reset should happen. This allows behaviors like conditional or delayed resets.
   */
  resetOnError?: boolean | ((error: any) => Observable<any>);
  /**
   * If true, the resulting observable will reset internal state on completion from source and return to a "cold" state. This
   * allows the resulting observable to be "repeated" after it is done.
   * If false, when the source completes, it will push the completion through the connecting subject, and the subject
   * will remain the connecting subject, meaning the resulting observable will not go "cold" again, and subsequent repeats
   * or resubscriptions will resubscribe to that same subject.
   * It is also possible to pass a notifier factory returning an observable instead which grants more fine-grained
   * control over how and when the reset should happen. This allows behaviors like conditional or delayed resets.
   */
  resetOnComplete?: boolean | (() => Observable<any>);
  /**
   * If true, when the number of subscribers to the resulting observable reaches zero due to those subscribers unsubscribing, the
   * internal state will be reset and the resulting observable will return to a "cold" state. This means that the next
   * time the resulting observable is subscribed to, a new subject will be created and the source will be subscribed to
   * again.
   * If false, when the number of subscribers to the resulting observable reaches zero due to unsubscription, the subject
   * will remain connected to the source, and new subscriptions to the result will be connected through that same subject.
   * It is also possible to pass a notifier factory returning an observable instead which grants more fine-grained
   * control over how and when the reset should happen. This allows behaviors like conditional or delayed resets.
   */
  resetOnRefCountZero?: boolean | (() => Observable<any>);
}

export function share<T>(): MonoTypeOperatorFunction<T>;

export function share<T>(options: ShareConfig<T>): MonoTypeOperatorFunction<T>;

/**
 * Returns a new Observable that multicasts (shares) the original Observable. As long as there is at least one
 * Subscriber this Observable will be subscribed and emitting data. When all subscribers have unsubscribed it will
 * unsubscribe from the source Observable. Because the Observable is multicasting it makes the stream `hot`.
 * This is an alias for `multicast(() => new Subject()), refCount()`.
 *
 * ![](share.png)
 *
 * ## Example
 * Generate new multicast Observable from the source Observable value
 * ```ts
 * import { interval } from 'rxjs';
 * import { share, map } from 'rxjs/operators';
 *
 * const source = interval(1000)
 *   .pipe(
 *         map((x: number) => {
 *             console.log('Processing: ', x);
 *             return x*x;
 *         }),
 *         share()
 * );
 *
 * source.subscribe(x => console.log('subscription 1: ', x));
 * source.subscribe(x => console.log('subscription 1: ', x));
 *
 * // Logs:
 * // Processing:  0
 * // subscription 1:  0
 * // subscription 1:  0
 * // Processing:  1
 * // subscription 1:  1
 * // subscription 1:  1
 * // Processing:  2
 * // subscription 1:  4
 * // subscription 1:  4
 * // Processing:  3
 * // subscription 1:  9
 * // subscription 1:  9
 * // ... and so on
 * ```
 *
 * ## Example with notifier factory: Delayed reset
 * ```ts
 * import { interval } from 'rxjs';
 * import { share, take, timer } from 'rxjs/operators';
 *
 * const source = interval(1000).pipe(take(3), share({ resetOnRefCountZero: () => timer(1000) }));
 *
 * const subscriptionOne = source.subscribe(x => console.log('subscription 1: ', x));
 * setTimeout(() => subscriptionOne.unsubscribe(), 1300);
 *
 * setTimeout(() => source.subscribe(x => console.log('subscription 2: ', x)), 1700);
 *
 * setTimeout(() => source.subscribe(x => console.log('subscription 3: ', x)), 5000);
 *
 * // Logs:
 * // subscription 1:  0
 * // (subscription 1 unsubscribes here)
 * // (subscription 2 subscribes here ~400ms later, source was not reset)
 * // subscription 2:  1
 * // subscription 2:  2
 * // (subscription 2 unsubscribes here)
 * // (subscription 3 subscribes here ~2000ms later, source did reset before)
 * // subscription 3:  0
 * // subscription 3:  1
 * // subscription 3:  2
 * ```
 *
 * @see {@link api/index/function/interval}
 * @see {@link map}
 *
 * @return A function that returns an Observable that mirrors the source.
 */
export function share<T>(options: ShareConfig<T> = {}): MonoTypeOperatorFunction<T> {
  const { connector = () => new Subject<T>(), resetOnError, resetOnComplete, resetOnRefCountZero } = options;

  let connection: SafeSubscriber<T> | null = null;
  let resetConnection: Subscription | null = null;
  let subject: SubjectLike<T> | null = null;
  let refCount = 0;
  let hasCompleted = false;
  let hasErrored = false;

  const cancelReset = () => {
    resetConnection?.unsubscribe();
    resetConnection = null;
  };
  // Used to reset the internal state to a "cold"
  // state, as though it had never been subscribed to.
  const reset = () => {
    cancelReset();
    connection = subject = null;
    hasCompleted = hasErrored = false;
  };
  const resetAndUnsubscribe = () => {
    // We need to capture the connection before
    // we reset (if we need to reset).
    const conn = connection;
    reset();
    conn?.unsubscribe();
  };

  return operate((source, subscriber) => {
    refCount++;
    if (!hasErrored && !hasCompleted) {
      cancelReset();
    }

    // Create the subject if we don't have one yet.
    subject = subject ?? connector();

    // The following line adds the subscription to the subscriber passed.
    // Basically, `subscriber === subject.subscribe(subscriber)` is `true`.
    subject.subscribe(subscriber);

    if (!connection) {
      // We need to create a subscriber here - rather than pass an observer and
      // assign the returned subscription to connection - because it's possible
      // for reentrant subscriptions to the shared observable to occur and in
      // those situations we want connection to be already-assigned so that we
      // don't create another connection to the source.
      connection = new SafeSubscriber({
        next: (value: T) => subject!.next(value),
        error: (err: any) => {
          hasErrored = true;
          // We need to capture the subject before
          // we reset (if we need to reset).
          const dest = subject!;
          cancelReset();
          resetConnection = handleReset(reset, resetOnError, err);
          dest.error(err);
        },
        complete: () => {
          hasCompleted = true;
          // We need to capture the subject before
          // we reset (if we need to reset).
          const dest = subject!;
          cancelReset();
          resetConnection = handleReset(reset, resetOnComplete);
          dest.complete();
        },
      });
      from(source).subscribe(connection);
    }

    // This is also added to `subscriber`, technically.
    return () => {
      refCount--;

      // If we're resetting on refCount === 0, and it's 0, we only want to do
      // that on "unsubscribe", really. Resetting on error or completion is a different
      // configuration.
      if (refCount === 0 && !hasErrored && !hasCompleted) {
        cancelReset(); // paranoia, there should never be a resetConnection, if we reached this point
        resetConnection = handleReset(resetAndUnsubscribe, resetOnRefCountZero);
      }
    };
  });
}

function handleReset<T extends unknown[] = never[]>(
  fn: () => void,
  on: boolean | ((...args: T) => Observable<any>) = true,
  ...args: T
): Subscription | null {
  if (on === true) {
    fn();

    return null;
  }

  if (on === false) {
    return null;
  }

  return on(...args)
    .pipe(take(1))
    .subscribe(() => fn());
}