michael@0: /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
michael@0: /* vim: set ts=2 et sw=2 tw=80 filetype=javascript: */
michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: "use strict";
michael@0: 
michael@0: this.EXPORTED_SYMBOLS = [
michael@0:   "Promise"
michael@0: ];
michael@0: 
michael@0: /**
michael@0:  * This module implements the "promise" construct, according to the
michael@0:  * "Promises/A+" proposal as known in April 2013, documented here:
michael@0:  *
michael@0:  * <http://promises-aplus.github.com/promises-spec/>
michael@0:  *
michael@0:  * A promise is an object representing a value that may not be available yet.
michael@0:  * Internally, a promise can be in one of three states:
michael@0:  *
michael@0:  * - Pending, when the final value is not available yet.  This is the only state
michael@0:  *   that may transition to one of the other two states.
michael@0:  *
michael@0:  * - Resolved, when and if the final value becomes available.  A resolution
michael@0:  *   value becomes permanently associated with the promise.  This may be any
michael@0:  *   value, including "undefined".
michael@0:  *
michael@0:  * - Rejected, if an error prevented the final value from being determined.  A
michael@0:  *   rejection reason becomes permanently associated with the promise.  This may
michael@0:  *   be any value, including "undefined", though it is generally an Error
michael@0:  *   object, like in exception handling.
michael@0:  *
michael@0:  * A reference to an existing promise may be received by different means, for
michael@0:  * example as the return value of a call into an asynchronous API.  In this
michael@0:  * case, the state of the promise can be observed but not directly controlled.
michael@0:  *
michael@0:  * To observe the state of a promise, its "then" method must be used.  This
michael@0:  * method registers callback functions that are called as soon as the promise is
michael@0:  * either resolved or rejected.  The method returns a new promise, that in turn
michael@0:  * is resolved or rejected depending on the state of the original promise and on
michael@0:  * the behavior of the callbacks.  For example, unhandled exceptions in the
michael@0:  * callbacks cause the new promise to be rejected, even if the original promise
michael@0:  * is resolved.  See the documentation of the "then" method for details.
michael@0:  *
michael@0:  * Promises may also be created using the "Promise.defer" function, the main
michael@0:  * entry point of this module.  The function, along with the new promise,
michael@0:  * returns separate methods to change its state to be resolved or rejected.
michael@0:  * See the documentation of the "Deferred" prototype for details.
michael@0:  *
michael@0:  * -----------------------------------------------------------------------------
michael@0:  *
michael@0:  * Cu.import("resource://gre/modules/Promise.jsm");
michael@0:  *
michael@0:  * // This function creates and returns a new promise.
michael@0:  * function promiseValueAfterTimeout(aValue, aTimeout)
michael@0:  * {
michael@0:  *   let deferred = Promise.defer();
michael@0:  *
michael@0:  *   try {
michael@0:  *     // An asynchronous operation will trigger the resolution of the promise.
michael@0:  *     // In this example, we don't have a callback that triggers a rejection.
michael@0:  *     do_timeout(aTimeout, function () {
michael@0:  *       deferred.resolve(aValue);
michael@0:  *     });
michael@0:  *   } catch (ex) {
michael@0:  *     // Generally, functions returning promises propagate exceptions through
michael@0:  *     // the returned promise, though they may also choose to fail early.
michael@0:  *     deferred.reject(ex);
michael@0:  *   }
michael@0:  *
michael@0:  *   // We don't return the deferred to the caller, but only the contained
michael@0:  *   // promise, so that the caller cannot accidentally change its state.
michael@0:  *   return deferred.promise;
michael@0:  * }
michael@0:  *
michael@0:  * // This code uses the promise returned be the function above.
michael@0:  * let promise = promiseValueAfterTimeout("Value", 1000);
michael@0:  *
michael@0:  * let newPromise = promise.then(function onResolve(aValue) {
michael@0:  *   do_print("Resolved with this value: " + aValue);
michael@0:  * }, function onReject(aReason) {
michael@0:  *   do_print("Rejected with this reason: " + aReason);
michael@0:  * });
michael@0:  *
michael@0:  * // Unexpected errors should always be reported at the end of a promise chain.
michael@0:  * newPromise.then(null, Components.utils.reportError);
michael@0:  *
michael@0:  * -----------------------------------------------------------------------------
michael@0:  */
michael@0: 
michael@0: // These constants must be defined on the "this" object for them to be visible
michael@0: // by subscripts in B2G, since "this" does not match the global scope.
michael@0: this.Cc = Components.classes;
michael@0: this.Ci = Components.interfaces;
michael@0: this.Cu = Components.utils;
michael@0: this.Cr = Components.results;
michael@0: 
michael@0: this.Cc["@mozilla.org/moz/jssubscript-loader;1"]
michael@0:     .getService(this.Ci.mozIJSSubScriptLoader)
michael@0:     .loadSubScript("resource://gre/modules/Promise-backend.js", this);