Bug 974065 - Add async function helpers to devtools. r=fitzgen, r=robcee

toolkit/devtools/async-utils.js

@@ -0,0 +1,100 @@
+/* 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.
+ */
+
+let {Cu} = require("chrome");
+let {Task} = require("resource://gre/modules/Task.jsm");
+let {Promise} = require("resource://gre/modules/Promise.jsm");
+
+/**
+ * Create an async function from a generator function.
+ *
+ * @param Function func
+ *        The generator function that to wrap as an async function.
+ * @return Function
+ *         The async function.
+ */
+exports.async = function async(func) {
+  return function(...args) {
+    return Task.spawn(func.apply(this, args));
+  };
+};
+
+/**
+ * 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;
+  };
+};
+
+
+/**
+ * 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);
+};