https://github.com/qubyte/vertebrate-event-emitter
An event emitter implementation robust against memory leaks.
https://github.com/qubyte/vertebrate-event-emitter
es2015-modules eventemitter javascript memory-leak umd
Last synced: 2 months ago
JSON representation
An event emitter implementation robust against memory leaks.
- Host: GitHub
- URL: https://github.com/qubyte/vertebrate-event-emitter
- Owner: qubyte
- License: mit
- Created: 2016-02-26T12:57:01.000Z (about 9 years ago)
- Default Branch: main
- Last Pushed: 2022-01-25T11:28:39.000Z (over 3 years ago)
- Last Synced: 2025-02-28T23:21:16.782Z (3 months ago)
- Topics: es2015-modules, eventemitter, javascript, memory-leak, umd
- Language: JavaScript
- Size: 321 KB
- Stars: 10
- Watchers: 2
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- Changelog: HISTORY.md
- Contributing: CONTRIBUTING.md
- License: LICENSE
- Code of conduct: CODE_OF_CONDUCT.md
Awesome Lists containing this project
README
# Vertebrate event emitter
This repository contains an implementation of an event emitter. Working with
emitters can be frustrating. These frustrations led me to make an ES2015 based
implementation contained in this repository.
Key features:
- Based on `WeakMap` so you don't have to unregister callbacks when you drop
references to an emitter.
- Unregister a callback using a reference (like `setTimeout`).
- Give a registered callback a call count to unregister itself.
[See below](#problems-with-existing-emitters) for why these are important.
This library comes with both UMD and ES2015 versions. If you're using
[rollup](http://rollupjs.org/), it'll use the ES2015 version. Node will use
the UMD version automatically. Since this library provides UMD and ES2015
versions, it's supported by every major module system (or no module system at
all). It has no production dependencies, making it easy to include too!
## API
### Class `EventEmitter`
The `EventEmitter` is used to construct new emitter objects. It takes
no arguments.
```javascript
const emitter = new EventEmitter();
```
### `reference = emitter.on(name, callback, count = Infinity)`
Add a listener function `callback` to the emitter for the given `name`. `name`
must be a string. `count` is the number of times the event can be called before
the listener is automatically unregistered. `count` defaults to `Infinity` when
not given. When given it must be a positive integer greater than `1`, or
`Infinity`.
A `reference` object is returned, which may later be used to unregister the
listener.
```javascript
// Callback called every time 'event-name' triggered.
emitter.on('event-name', callback);
// Callback each time 'event-name' triggered, unregistering after 10 calls.
emitter.on('event-name', callback, 10);
```
The count parameter can be used to simulate a Node.js style `once` method.
### `emitter.off(reference)`
Unregister an event listener using the `reference` object returned by
`emitter.on`.
```javascript
// The on method returns a reference object.
const ref = emitter.on('event-name', callback);
// Use the off method to unregister the callback.
emitter.off(ref);
```
### `emitter.allOff(name = undefined)`
When called with a name string, all events for that name are removed from the
emitter. When called without a name string, all events for all names are
removed.
### `emitter.trigger(name, ...args)`
Trigger all handlers for the given name with the remaining arguments `args`. The
callbacks are called with the emitter as `this`.
```javascript
function testCallback(a, b, c) {
console.log(a, b, c);
}
emitter.on('some-event', testCallback);
emitter.trigger('some-event', 1, 2, 3) // logs: 1, 2, 3
```
### `emitter.emit(name, ...args)`
Alias for `emitter.trigger`.
## Problems with existing emitters
### Emitters make memory leaks too easy to create
For example, if you add an event listener to a Backbone event emitter using
`on`, it will stay there until something removes it. Backbone tries to get
around this with `listenTo`, which allows the emitter itself to unregister
events in batch. This is no fault of existing implementations. JavaScript itself
made it an impossible problem to solve until recently.
Luckily, one of the earlier features of ES2015 to make it into browsers was
`WeakMap`, which allows the garbage collector to clean up members when no other
references to them remain. The Vertebrate event emitter uses these to avoid
memory leaks.
### Most implementations are keyed on event name and a callback
In Node, you might have code like:
```javascript
import EventEmitter from 'events';
const emitter = new EventEmitter();
function testCallback() {
console.log('Hello, world!');
}
emitter.on('test', testCallback); // Add a listener to the 'test' event.
emitter.removeListener('test', testCallback); // Remove the listener.
```
That looks fine, but what happens when you add the same callback for an event
twice? Does the callback get called twice per emission, or just once? If twice,
what happens when you remove the listener? Does it remove both or just one?
This ambiguity bothers me.
When a Vertebrate emitter has a listener registered for an event, it returns a
reference object, a lot like `setTimeout` does. Unregistering the event is done
using this reference object:
```javascript
import EventEmitter from 'vertebrate-event-emitter';
const emitter = new EventEmitter();
function testCallback() {
console.log('Hello, world!');
}
const ref = emitter.on('test', testCallback); // Add a listener.
emitter.off(ref); // Remove the listener. No need to use the event name.
```
You get a fresh reference object each time a listener is registered, so the
ambiguity never arises.