gecko-dev/testing/marionette/sync.js

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

const {
  error,
  TimeoutError,
} = ChromeUtils.import("chrome://marionette/content/error.js", {});

/* exported PollPromise, TimedPromise */
this.EXPORTED_SYMBOLS = ["PollPromise", "TimedPromise"];

const {TYPE_ONE_SHOT, TYPE_REPEATING_SLACK} = Ci.nsITimer;

/**
 * @callback Condition
 *
 * @param {function(*)} resolve
 *     To be called when the condition has been met.  Will return the
 *     resolved value.
 * @param {function} reject
 *     To be called when the condition has not been met.  Will cause
 *     the condition to be revaluated or time out.
 *
 * @return {*}
 *     The value from calling ``resolve``.
 */

/**
 * Runs a Promise-like function off the main thread until it is resolved
 * through ``resolve`` or ``rejected`` callbacks.  The function is
 * guaranteed to be run at least once, irregardless of the timeout.
 *
 * The ``func`` is evaluated every ``interval`` for as long as its
 * runtime duration does not exceed ``interval``.  Evaluations occur
 * sequentially, meaning that evaluations of ``func`` are queued if
 * the runtime evaluation duration of ``func`` is greater than ``interval``.
 *
 * ``func`` is given two arguments, ``resolve`` and ``reject``,
 * of which one must be called for the evaluation to complete.
 * Calling ``resolve`` with an argument indicates that the expected
 * wait condition was met and will return the passed value to the
 * caller.  Conversely, calling ``reject`` will evaluate ``func``
 * again until the ``timeout`` duration has elapsed or ``func`` throws.
 * The passed value to ``reject`` will also be returned to the caller
 * once the wait has expired.
 *
 * Usage::
 *
 *     let els = new PollPromise((resolve, reject) => {
 *       let res = document.querySelectorAll("p");
 *       if (res.length > 0) {
 *         resolve(Array.from(res));
 *       } else {
 *         reject([]);
 *       }
 *     });
 *
 * @param {Condition} func
 *     Function to run off the main thread.
 * @param {number=} [timeout=2000] timeout
 *     Desired timeout.  If 0 or less than the runtime evaluation
 *     time of ``func``, ``func`` is guaranteed to run at least once.
 *     The default is 2000 milliseconds.
 * @param {number=} [interval=10] interval
 *     Duration between each poll of ``func`` in milliseconds.
 *     Defaults to 10 milliseconds.
 *
 * @return {Promise.<*>}
 *     Yields the value passed to ``func``'s
 *     ``resolve`` or ``reject`` callbacks.
 *
 * @throws {*}
 *     If ``func`` throws, its error is propagated.
 */
function PollPromise(func, {timeout = 2000, interval = 10} = {}) {
  const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);

  return new Promise((resolve, reject) => {
    const start = new Date().getTime();
    const end = start + timeout;

    let evalFn = () => {
      new Promise(func).then(resolve, rejected => {
        if (error.isError(rejected)) {
          throw rejected;
        }

        // return if timeout is 0, allowing |func| to be evaluated at
        // least once
        if (start == end || new Date().getTime() >= end) {
          resolve(rejected);
        }
      }).catch(reject);
    };

    // the repeating slack timer waits |interval|
    // before invoking |evalFn|
    evalFn();

    timer.init(evalFn, interval, TYPE_REPEATING_SLACK);

  }).then(res => {
    timer.cancel();
    return res;
  }, err => {
    timer.cancel();
    throw err;
  });
}

/**
 * Represents the timed, eventual completion (or failure) of an
 * asynchronous operation, and its resulting value.
 *
 * In contrast to a regular Promise, it times out after ``timeout``.
 *
 * @param {Condition} func
 *     Function to run, which will have its ``reject``
 *     callback invoked after the ``timeout`` duration is reached.
 *     It is given two callbacks: ``resolve(value)`` and
 *     ``reject(error)``.
 * @param {timeout=} [timeout=1500] timeout
 *     ``condition``'s ``reject`` callback will be called
 *     after this timeout.
 * @param {Error=} [throws=TimeoutError] throws
 *     When the ``timeout`` is hit, this error class will be
 *     thrown.  If it is null, no error is thrown and the promise is
 *     instead resolved on timeout.
 *
 * @return {Promise.<*>}
 *     Timed promise.
 */
function TimedPromise(fn, {timeout = 1500, throws = TimeoutError} = {}) {
  const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);

  return new Promise((resolve, reject) => {
    // Reject only if |throws| is given.  Otherwise it is assumed that
    // the user is OK with the promise timing out.
    let bail = () => {
      if (throws !== null) {
        let err = new throws();
        reject(err);
      } else {
        resolve();
      }
    };

    timer.initWithCallback({notify: bail}, timeout, TYPE_ONE_SHOT);

    try {
      fn(resolve, reject);
    } catch (e) {
      reject(e);
    }

  }).then(res => {
    timer.cancel();
    return res;
  }, err => {
    timer.cancel();
    throw err;
  });
}