devtools/shared/async-utils.js

   1 /* This Source Code Form is subject to the terms of the Mozilla Public
   2  * License, v. 2.0. If a copy of the MPL was not distributed with this
   3  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   4
   5 "use strict";
   6
   7 /**
   8  * Helpers for async functions. An async function returns a Promise for the
   9  * resolution of the function. When the function returns, the promise is
  10  * resolved with the returned value. If it throws the promise rejects with
  11  * the thrown error.
  12  */
  13
  14 /**
  15  * Adds an event listener to the given element, and then removes its event
  16  * listener once the event is called, returning the event object as a promise.
  17  * @param  Element element
  18  *         The DOM element to listen on
  19  * @param  String event
  20  *         The name of the event type to listen for
  21  * @param  Boolean useCapture
  22  *         Should we initiate the capture phase?
  23  * @return Promise
  24  *         The promise resolved with the event object when the event first
  25  *         happens
  26  */
  27 exports.listenOnce = function listenOnce(element, event, useCapture) {
  28   return new Promise(function (resolve) {
  29     const onEvent = function (ev) {
  30       element.removeEventListener(event, onEvent, useCapture);
  31       resolve(ev);
  32     };
  33     element.addEventListener(event, onEvent, useCapture);
  34   });
  35 };
  36
  37 // Return value when `safeAsyncMethod` catches an error.
  38 const SWALLOWED_RET = Symbol("swallowed");
  39
  40 /**
  41  * Wraps the provided async method in a try/catch block.
  42  * If an error is caught while running the method, check the provided condition
  43  * to decide whether the error should bubble or not.
  44  *
  45  * @param  {Function} asyncFn
  46  *         The async method to wrap.
  47  * @param  {Function} shouldSwallow
  48  *         Function that will run when an error is caught. If it returns true,
  49  *         the error will be swallowed. Otherwise, it will bubble up.
  50  * @param {Mixed} retValue
  51  *         Optional value to return when an error is caught and is swallowed.
  52  * @return {Function} The wrapped method.
  53  */
  54 exports.safeAsyncMethod = function (
  55   asyncFn,
  56   shouldSwallow,
  57   retValue = SWALLOWED_RET
  58 ) {
  59   return async function (...args) {
  60     try {
  61       const ret = await asyncFn(...args);
  62       return ret;
  63     } catch (e) {
  64       if (shouldSwallow()) {
  65         console.warn("Async method failed in safeAsyncMethod", e);
  66         return retValue;
  67       }
  68       throw e;
  69     }
  70   };
  71 };