on-promise.ts

import { EventReceiver } from '../base';
import { alwaysReceiveValue, neverReceiveBecause } from '../base/impl';
import { OnEvent, onEventBy } from '../on-event';

/**
 * Represents a promise as event sender.
 *
 * When the `promise` resolves successfully the resolved value is sent to registered event receivers. The events
 * supply is {@link Supply.off cut off} immediately after that without any reason specified.
 *
 * When the `promise` is rejected the events supply is {@link Supply.off cut off} with promise rejection reason.
 *
 * @category Core
 * @typeParam T - A type of value the promise is resolved to.
 * @param promise - A promise-like instance or raw value to represent as event sender.
 *
 * @returns An {@link OnEvent} sender of the given `promise` settlement event.
 */
export function onPromise<T>(promise: PromiseLike<T> | T): OnEvent<[T]> {
  const whenResolved = Promise.resolve(promise);
  let receive = (receiver: EventReceiver.Generic<[T]>): void => {
    whenResolved.then(
      () => receive(receiver),
      () => receive(receiver),
    );
  };

  whenResolved
    .then(value => {
      receive = alwaysReceiveValue(value);
    })
    .catch(e => {
      receive = neverReceiveBecause(e);
    });

  return onEventBy(receiver => receive(receiver));
}