async-node-events

async-node-events

An EventEmitter replacement that allows both asynchronous and synchronous emissions and handlers. This is entirely based off of and almost entirely written by @dfellis in his excellent async-cancelable-events module. Even this README is primarily written by @dfellis. Recent updates are from Digital Bazaar.

This version is primarily targeted at using Promises and async/await with emit and listeners. Namely, the asynchronous emit will return a Promise which can be awaited upon. Listeners are run in series and their result is awaited upon. Throwing an error from a listener that executed asynchronously will terminate the listener chain (subsequent listeners will not receive the emitted event and the emit Promise will be rejected. Events may also be canceled by a listener resolving to or returning false. This will stop further event emission to other listeners and return false rather than undefined, but not raise an error.

The synchronous emitSync variant acts like the asynchronous emit but only allows synchronous listeners.

The listener manipulation methods (on, off, etc) are synchronous and use emitSync to emit newListener, removeListener, and maxListenersPassed. Listeners for these events must be synchronous.

• Credit for the original project goes to David Ellis.
• Improvements and updates by Digital Bazaar.

Install

npm install async-node-events

Usage

var EventEmitter = require('async-node-events').EventEmitter;
var util = require('util');

(async () => {
  const myEmitter = new EventEmitter();
  myEmitter.on('test', async (...) => {
    ...
  });
  await myEmitter.emit('test', ...);
})();

function MyEmittingObject() {
  EventEmitter.call(this);
  ...
}

util.inherits(MyEmittingObject, EventEmitter);

The API is intended to be a mostly-drop-in replacement for Node.js' EventEmitter object, except with support for asynchronous listeners.

The primary differences between the EventEmitter and async-node-events are:

1. Passing the maximum number of listeners allowed will fire off a maxListenersPassed event with the event name and listener count as arguments. The warning the official EventEmitter prints is simply a listener for async-node-events, and can be disabled by running this.removeAllListeners('maxListenersPassed') just after the EventEmitter.call(this) listed above.
2. The various synchronous method calls are chainable, so foo.on('bar', func1).on('baz', func2) is valid.
3. emit and emitSync return undefined if the even was emitted to all listeners and false when a listener caused cancellation. This differs from returning an indication if there were listeners.

License (MIT)

Copyright (C) 2012-2013 by David Ellis

Copyright (C) 2014-2019 Digital Bazaar

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Changelog

2.0.0 - 2019-11-08

Changed

• BREAKING: EventEmitter is exported from module.
  • For CommonJS use const {EventEmitter} = require('async-node-events');
  • For ES Modules use import {EventEmitter} from 'async-node-events';
• BREAKING: Make emit() async function and resolve to true/false if listeners were called rather than returning this.
• BREAKING: Listeners called through sync functions (on, off, etc) will all be called in a sync manner, and must not return a Promise.
• BREAKING: emitSync is now like emit but only handles synchronous listeners.
• BREAKING: emit and emitSync return false when cancelled and true if not cancelled. This differs from Node.js API that indicates if there were listeners. Use listenerCount to check for listeners.
• Update dependencies.
• Switch from gulp to simple npm scripts and nyc.
• Switch to ES modules using 'esm' for Node.js support.
• Update test suite.
• Modernize code.
  • async/await
  • arrow functions
  • various new ES features
  • use Map

Added

• Add eslint support and update style.
• off() as alias for removeListener.
• Instance listenerCount().

Removed

• BREAKING: Listener manipulation API is now always synchronous. Remove old explicit sync/async API.
  • onSync() / onceSync() / addListenerSync()
  • onAsync() / onceAsync() / addListenerAsync()