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 /* -*- 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);

   }

 };