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/* 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";
/**
* Helpers for async functions. Async functions are generator functions that are
* run by Tasks. An async function returns a Promise for the resolution of the
* function. When the function returns, the promise is resolved with the
* returned value. If it throws the promise rejects with the thrown error.
*
* See Task documentation at https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Task.jsm.
*/
var {Task} = require("devtools/shared/task");
var Promise = require("promise");
/**
* Create an async function that only executes once per instance of an object.
* Once called on a given object, the same promise will be returned for any
* future calls for that object.
*
* @param Function func
* The generator function that to wrap as an async function.
* @return Function
* The async function.
*/
exports.asyncOnce = function asyncOnce(func) {
const promises = new WeakMap();
return function (...args) {
let promise = promises.get(this);
if (!promise) {
promise = Task.spawn(func.apply(this, args));
promises.set(this, promise);
}
return promise;
};
};
/**
* Adds an event listener to the given element, and then removes its event
* listener once the event is called, returning the event object as a promise.
* @param nsIDOMElement element
* The DOM element to listen on
* @param String event
* The name of the event type to listen for
* @param Boolean useCapture
* Should we initiate the capture phase?
* @return Promise
* The promise resolved with the event object when the event first
* happens
*/
exports.listenOnce = function listenOnce(element, event, useCapture) {
return new Promise(function (resolve, reject) {
let onEvent = function (ev) {
element.removeEventListener(event, onEvent, useCapture);
resolve(ev);
};
element.addEventListener(event, onEvent, useCapture);
});
};
/**
* Call a function that expects a callback as the last argument and returns a
* promise for the result. This simplifies using callback APIs from tasks and
* async functions.
*
* @param Any obj
* The |this| value to call the function on.
* @param Function func
* The callback-expecting function to call.
* @param Array args
* Additional arguments to pass to the method.
* @return Promise
* The promise for the result. If the callback is called with only one
* argument, it is used as the resolution value. If there's multiple
* arguments, an array containing the arguments is the resolution value.
* If the method throws, the promise is rejected with the thrown value.
*/
function promisify(obj, func, args) {
return new Promise(resolve => {
args.push((...results) => {
resolve(results.length > 1 ? results : results[0]);
});
func.apply(obj, args);
});
}
/**
* Call a method that expects a callback as the last argument and returns a
* promise for the result.
*
* @see promisify
*/
exports.promiseInvoke = function promiseInvoke(obj, func, ...args) {
return promisify(obj, func, args);
};
/**
* Call a function that expects a callback as the last argument.
*
* @see promisify
*/
exports.promiseCall = function promiseCall(func, ...args) {
return promisify(undefined, func, args);
};
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