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