import FIREFOX_52_6_0esr_RELEASE from mozilla-esr52 hg repo

devtools/shared/async-utils.js

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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);
+};