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109 lines
3.5 KiB
JavaScript
109 lines
3.5 KiB
JavaScript
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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"use strict";
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/**
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* Helpers for async functions. Async functions are generator functions that are
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* run by Tasks. An async function returns a Promise for the resolution of the
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* function. When the function returns, the promise is resolved with the
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* returned value. If it throws the promise rejects with the thrown error.
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*
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* See Task documentation at https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Task.jsm.
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*/
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var {Cu} = require("chrome");
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var {Task} = require("resource://gre/modules/Task.jsm");
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var Promise = require("promise");
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/**
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* Create an async function that only executes once per instance of an object.
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* Once called on a given object, the same promise will be returned for any
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* future calls for that object.
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*
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* @param Function func
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* The generator function that to wrap as an async function.
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* @return Function
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* The async function.
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*/
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exports.asyncOnce = function asyncOnce(func) {
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const promises = new WeakMap();
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return function(...args) {
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let promise = promises.get(this);
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if (!promise) {
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promise = Task.spawn(func.apply(this, args));
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promises.set(this, promise);
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}
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return promise;
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};
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};
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/**
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* Adds an event listener to the given element, and then removes its event
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* listener once the event is called, returning the event object as a promise.
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* @param nsIDOMElement element
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* The DOM element to listen on
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* @param String event
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* The name of the event type to listen for
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* @param Boolean useCapture
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* Should we initiate the capture phase?
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* @return Promise
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* The promise resolved with the event object when the event first
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* happens
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*/
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exports.listenOnce = function listenOnce(element, event, useCapture) {
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return new Promise(function(resolve, reject) {
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var onEvent = function(ev) {
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element.removeEventListener(event, onEvent, useCapture);
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resolve(ev);
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}
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element.addEventListener(event, onEvent, useCapture);
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});
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};
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/**
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* Call a function that expects a callback as the last argument and returns a
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* promise for the result. This simplifies using callback APIs from tasks and
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* async functions.
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*
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* @param Any obj
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* The |this| value to call the function on.
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* @param Function func
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* The callback-expecting function to call.
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* @param Array args
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* Additional arguments to pass to the method.
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* @return Promise
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* The promise for the result. If the callback is called with only one
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* argument, it is used as the resolution value. If there's multiple
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* arguments, an array containing the arguments is the resolution value.
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* If the method throws, the promise is rejected with the thrown value.
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*/
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function promisify(obj, func, args) {
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return new Promise(resolve => {
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args.push((...results) => {
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resolve(results.length > 1 ? results : results[0]);
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});
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func.apply(obj, args);
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});
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}
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/**
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* Call a method that expects a callback as the last argument and returns a
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* promise for the result.
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*
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* @see promisify
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*/
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exports.promiseInvoke = function promiseInvoke(obj, func, ...args) {
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return promisify(obj, func, args);
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};
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/**
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* Call a function that expects a callback as the last argument.
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*
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* @see promisify
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*/
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exports.promiseCall = function promiseCall(func, ...args) {
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return promisify(undefined, func, args);
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};
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