shaka-player/lib/util/public_promise.js

DASH/EME video player library

/*! @license
 * Shaka Player
 * Copyright 2016 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

goog.provide('shaka.util.PublicPromise');


/**
 * @summary
 * A utility to create Promises with convenient public resolve and reject
 * methods.
 *
 * @extends {Promise<T>}
 * @template T
 */
shaka.util.PublicPromise = class {
  /**
   * @return {!Promise<T>}
   */
  constructor() {
    let resolvePromise;
    let rejectPromise;

    // Promise.call causes an error.  It seems that inheriting from a native
    // Promise is not permitted by JavaScript interpreters.

    // The work-around is to construct a Promise object, modify it to look like
    // the compiler's picture of PublicPromise, then return it.  The caller of
    // new PublicPromise will receive |promise| instead of |this|, and the
    // compiler will be aware of the additional properties |resolve| and
    // |reject|.

    const promise = new Promise(((resolve, reject) => {
      resolvePromise = resolve;
      rejectPromise = reject;
    }));

    // Now cast the Promise object to our subclass PublicPromise so that the
    // compiler will permit us to attach resolve() and reject() to it.
    const publicPromise = /** @type {shaka.util.PublicPromise} */(promise);
    publicPromise.resolve = resolvePromise;
    publicPromise.reject = rejectPromise;

    return publicPromise;
  }


  /** @param {T=} value */
  resolve(value) {}


  /** @param {*=} reason */
  reject(reason) {}
};