gecko-dev/toolkit/modules/Task.jsm

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=2 et sw=2 tw=80 filetype=javascript: */
/* 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";

this.EXPORTED_SYMBOLS = [
  "Task"
];

/**
 * This module implements a subset of "Task.js" <http://taskjs.org/>.
 *
 * Paraphrasing from the Task.js site, tasks make sequential, asynchronous
 * operations simple, using the power of JavaScript's "yield" operator.
 *
 * Tasks are built upon generator functions and promises, documented here:
 *
 * <https://developer.mozilla.org/en/JavaScript/Guide/Iterators_and_Generators>
 * <http://wiki.commonjs.org/wiki/Promises/A>
 *
 * The "Task.spawn" function takes a generator function and starts running it as
 * a task.  Every time the task yields a promise, it waits until the promise is
 * fulfilled.  "Task.spawn" returns a promise that is resolved when the task
 * completes successfully, or is rejected if an exception occurs.
 *
 * -----------------------------------------------------------------------------
 *
 * Cu.import("resource://gre/modules/Task.jsm");
 *
 * Task.spawn(function* () {
 *
 *   // This is our task. Let's create a promise object, wait on it and capture
 *   // its resolution value.
 *   let myPromise = getPromiseResolvedOnTimeoutWithValue(1000, "Value");
 *   let result = yield myPromise;
 *
 *   // This part is executed only after the promise above is fulfilled (after
 *   // one second, in this imaginary example).  We can easily loop while
 *   // calling asynchronous functions, and wait multiple times.
 *   for (let i = 0; i < 3; i++) {
 *     result += yield getPromiseResolvedOnTimeoutWithValue(50, "!");
 *   }
 *
 *   return "Resolution result for the task: " + result;
 * }).then(function (result) {
 *
 *   // result == "Resolution result for the task: Value!!!"
 *
 *   // The result is undefined if no value was returned.
 *
 * }, function (exception) {
 *
 *   // Failure!  We can inspect or report the exception.
 *
 * });
 *
 * -----------------------------------------------------------------------------
 *
 * This module implements only the "Task.js" interfaces described above, with no
 * additional features to control the task externally, or do custom scheduling.
 * It also provides the following extensions that simplify task usage in the
 * most common cases:
 *
 * - The "Task.spawn" function also accepts an iterator returned by a generator
 *   function, in addition to a generator function.  This way, you can call into
 *   the generator function with the parameters you want, and with "this" bound
 *   to the correct value.  Also, "this" is never bound to the task object when
 *   "Task.spawn" calls the generator function.
 *
 * - In addition to a promise object, a task can yield the iterator returned by
 *   a generator function.  The iterator is turned into a task automatically.
 *   This reduces the syntax overhead of calling "Task.spawn" explicitly when
 *   you want to recurse into other task functions.
 *
 * - The "Task.spawn" function also accepts a primitive value, or a function
 *   returning a primitive value, and treats the value as the result of the
 *   task.  This makes it possible to call an externally provided function and
 *   spawn a task from it, regardless of whether it is an asynchronous generator
 *   or a synchronous function.  This comes in handy when iterating over
 *   function lists where some items have been converted to tasks and some not.
 */

////////////////////////////////////////////////////////////////////////////////
//// Globals

const Cc = Components.classes;
const Ci = Components.interfaces;
const Cu = Components.utils;
const Cr = Components.results;

Cu.import("resource://gre/modules/Promise.jsm");

// The following error types are considered programmer errors, which should be
// reported (possibly redundantly) so as to let programmers fix their code.
const ERRORS_TO_REPORT = ["EvalError", "RangeError", "ReferenceError", "TypeError"];

/**
 * Detect whether a value is a generator.
 *
 * @param aValue
 *        The value to identify.
 * @return A boolean indicating whether the value is a generator.
 */
function isGenerator(aValue) {
  return Object.prototype.toString.call(aValue) == "[object Generator]";
}

////////////////////////////////////////////////////////////////////////////////
//// Task

/**
 * This object provides the public module functions.
 */
this.Task = {
  /**
   * Creates and starts a new task.
   *
   * @param aTask
   *        - If you specify a generator function, it is called with no
   *          arguments to retrieve the associated iterator.  The generator
   *          function is a task, that is can yield promise objects to wait
   *          upon.
   *        - If you specify the iterator returned by a generator function you
   *          called, the generator function is also executed as a task.  This
   *          allows you to call the function with arguments.
   *        - If you specify a function that is not a generator, it is called
   *          with no arguments, and its return value is used to resolve the
   *          returned promise.
   *        - If you specify anything else, you get a promise that is already
   *          resolved with the specified value.
   *
   * @return A promise object where you can register completion callbacks to be
   *         called when the task terminates.
   */
  spawn: function Task_spawn(aTask) {
    return createAsyncFunction(aTask).call(undefined);
  },

  /**
   * Create and return an 'async function' that starts a new task.
   *
   * This is similar to 'spawn' except that it doesn't immediately start
   * the task, it binds the task to the async function's 'this' object and
   * arguments, and it requires the task to be a function.
   *
   * It simplifies the common pattern of implementing a method via a task,
   * like this simple object with a 'greet' method that has a 'name' parameter
   * and spawns a task to send a greeting and return its reply:
   *
   * let greeter = {
   *   message: "Hello, NAME!",
   *   greet: function(name) {
   *     return Task.spawn((function* () {
   *       return yield sendGreeting(this.message.replace(/NAME/, name));
   *     }).bind(this);
   *   })
   * };
   *
   * With Task.async, the method can be declared succinctly:
   *
   * let greeter = {
   *   message: "Hello, NAME!",
   *   greet: Task.async(function* (name) {
   *     return yield sendGreeting(this.message.replace(/NAME/, name));
   *   })
   * };
   *
   * While maintaining identical semantics:
   *
   * greeter.greet("Mitchell").then((reply) => { ... }); // behaves the same
   *
   * @param aTask
   *        The task function to start.
   *
   * @return A function that starts the task function and returns its promise.
   */
  async: function Task_async(aTask) {
    if (typeof(aTask) != "function") {
      throw new TypeError("aTask argument must be a function");
    }

    return createAsyncFunction(aTask);
  },

  /**
   * Constructs a special exception that, when thrown inside a legacy generator
   * function (non-star generator), allows the associated task to be resolved
   * with a specific value.
   *
   * Example: throw new Task.Result("Value");
   */
  Result: function Task_Result(aValue) {
    this.value = aValue;
  }
};

function createAsyncFunction(aTask) {
  let asyncFunction = function () {
    let result = aTask;
    if (aTask && typeof(aTask) == "function") {
      if (aTask.isAsyncFunction) {
        throw new TypeError(
          "Cannot use an async function in place of a promise. " +
          "You should either invoke the async function first " +
          "or use 'Task.spawn' instead of 'Task.async' to start " +
          "the Task and return its promise.");
      }

      try {
        // Let's call into the function ourselves.
        result = aTask.apply(this, arguments);
      } catch (ex if ex instanceof Task.Result) {
        return Promise.resolve(ex.value);
      } catch (ex) {
        return Promise.reject(ex);
      }
    }

    if (isGenerator(result)) {
      // This is an iterator resulting from calling a generator function.
      return new TaskImpl(result).deferred.promise;
    }

    // Just propagate the given value to the caller as a resolved promise.
    return Promise.resolve(result);
  };

  asyncFunction.isAsyncFunction = true;

  return asyncFunction;
}

////////////////////////////////////////////////////////////////////////////////
//// TaskImpl

/**
 * Executes the specified iterator as a task, and gives access to the promise
 * that is fulfilled when the task terminates.
 */
function TaskImpl(iterator) {
  this.deferred = Promise.defer();
  this._iterator = iterator;
  this._isStarGenerator = !("send" in iterator);
  this._run(true);
}

TaskImpl.prototype = {
  /**
   * Includes the promise object where task completion callbacks are registered,
   * and methods to resolve or reject the promise at task completion.
   */
  deferred: null,

  /**
   * The iterator returned by the generator function associated with this task.
   */
  _iterator: null,

  /**
   * Whether this Task is using a star generator.
   */
  _isStarGenerator: false,

  /**
   * Main execution routine, that calls into the generator function.
   *
   * @param aSendResolved
   *        If true, indicates that we should continue into the generator
   *        function regularly (if we were waiting on a promise, it was
   *        resolved). If true, indicates that we should cause an exception to
   *        be thrown into the generator function (if we were waiting on a
   *        promise, it was rejected).
   * @param aSendValue
   *        Resolution result or rejection exception, if any.
   */
  _run: function TaskImpl_run(aSendResolved, aSendValue) {
    if (this._isStarGenerator) {
      try {
        let result = aSendResolved ? this._iterator.next(aSendValue)
                                   : this._iterator.throw(aSendValue);

        if (result.done) {
          // The generator function returned.
          this.deferred.resolve(result.value);
        } else {
          // The generator function yielded.
          this._handleResultValue(result.value);
        }
      } catch (ex) {
        // The generator function failed with an uncaught exception.
        this._handleException(ex);
      }
    } else {
      try {
        let yielded = aSendResolved ? this._iterator.send(aSendValue)
                                    : this._iterator.throw(aSendValue);
        this._handleResultValue(yielded);
      } catch (ex if ex instanceof Task.Result) {
        // The generator function threw the special exception that allows it to
        // return a specific value on resolution.
        this.deferred.resolve(ex.value);
      } catch (ex if ex instanceof StopIteration) {
        // The generator function terminated with no specific result.
        this.deferred.resolve();
      } catch (ex) {
        // The generator function failed with an uncaught exception.
        this._handleException(ex);
      }
    }
  },

  /**
   * Handle a value yielded by a generator.
   *
   * @param aValue
   *        The yielded value to handle.
   */
  _handleResultValue: function TaskImpl_handleResultValue(aValue) {
    // If our task yielded an iterator resulting from calling another
    // generator function, automatically spawn a task from it, effectively
    // turning it into a promise that is fulfilled on task completion.
    if (isGenerator(aValue)) {
      aValue = Task.spawn(aValue);
    }

    if (aValue && typeof(aValue.then) == "function") {
      // We have a promise object now. When fulfilled, call again into this
      // function to continue the task, with either a resolution or rejection
      // condition.
      aValue.then(this._run.bind(this, true),
                  this._run.bind(this, false));
    } else {
      // If our task yielded a value that is not a promise, just continue and
      // pass it directly as the result of the yield statement.
      this._run(true, aValue);
    }
  },

  /**
   * Handle an uncaught exception thrown from a generator.
   *
   * @param aException
   *        The uncaught exception to handle.
   */
  _handleException: function TaskImpl_handleException(aException) {
    if (aException && typeof aException == "object" && "name" in aException &&
        ERRORS_TO_REPORT.indexOf(aException.name) != -1) {

      // We suspect that the exception is a programmer error, so we now
      // display it using dump().  Note that we do not use Cu.reportError as
      // we assume that this is a programming error, so we do not want end
      // users to see it. Also, if the programmer handles errors correctly,
      // they will either treat the error or log them somewhere.

      let stack = ("stack" in aException) ? aException.stack : "not available";
      dump("*************************\n");
      dump("A coding exception was thrown and uncaught in a Task.\n\n");
      dump("Full message: " + aException + "\n");
      dump("Full stack: " + stack + "\n");
      dump("*************************\n");
    }

    this.deferred.reject(aException);
  }
};