A JS EventEmitter foundation for evented code
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flarnie and zpao Add Code of Conduct (#32)
* Move 'Contribute' instructions into separate doc

We are about to add a `CONTRIBUTING.md` doc that will have the following piece, taken from the `README.md`

```
## Contribute

The main purpose of this repository is to share Facebook's implementation of an emitter. Please see React's [contributing article](https://github.com/facebook/react/blob/master/CONTRIBUTING.md), which generally applies to `fbemitter`, if you are interested in submitting a pull request.
```

* Add `CONTRIBUTING.md`

Even though it just points to the React 'Contributing' article, this is where people will conventionally expect to find details about Contributing.

* Link to COC in `CONTRIBUTING.md`

We are about to add a Code of Conduct, and it makes sense to surface that in the `CONTRIBUTING.md`.

* Add `CODE_OF_CONDUCT.md`

In the past Facebook didn't promote including a Code of Conduct when creating new projects, and many projects skipped this important document. Let's fix it. :)

Even though this project is not super active, it still makes sense to have a Code of Conduct.

**why make this change?:**
Facebook Open Source provides a Code of Conduct statement for all
projects to follow, to promote a welcoming and safe open source community.

Exposing the COC via a separate markdown file is a standard being
promoted by Github via the Community Profile in order to meet their Open
Source Guide's recommended community standards.

As you can see, adding this file will improve [the emitter community profile](https://github.com/facebook/emitter/community)
checklist and increase the visibility of our COC.

**test plan:**
Viewing it on my branch -
(Flarnie will insert screenshots)

**issue:**
internal task t23481323
Latest commit e916e97 Nov 27, 2017

README.md

EventEmitter

Facebook's EventEmitter is a simple emitter implementation that prioritizes speed and simplicity. It is conceptually similar to other emitters like Node's EventEmitter, but the precise APIs differ. More complex abstractions like the event systems used on facebook.com and m.facebook.com can be built on top of EventEmitter as well DOM event systems.

API Concepts

EventEmitter's API shares many concepts with other emitter APIs. When events are emitted through an emitter instance, all listeners for the given event type are invoked.

var emitter = new EventEmitter();
emitter.addListener('event', function(x, y) { console.log(x, y); });
emitter.emit('event', 5, 10);  // Listener prints "5 10".

EventEmitters return a subscription for each added listener. Subscriptions provide a convenient way to remove listeners that ensures they are removed from the correct emitter instance.

var subscription = emitter.addListener('event', listener);
subscription.remove();

Usage

First install the fbemitter package via npm, then you can require or import it.

var {EventEmitter} = require('fbemitter');
var emitter = new EventEmitter();

Building from source

Once you have the repository cloned, building a copy of fbemitter is easy, just run gulp build. This assumes you've installed gulp globally with npm install -g gulp.

gulp build

API

constructor()

Create a new emitter using the class' constructor. It accepts no arguments.

var {EventEmitter} = require('fbemitter');
var emitter = new EventEmitter();

addListener(eventType, callback)

Register a specific callback to be called on a particular event. A token is returned that can be used to remove the listener.

var token = emitter.addListener('change', (...args) => {
  console.log(...args);
});

emitter.emit('change', 10); // 10 is logged
token.remove();
emitter.emit('change', 10); // nothing is logged

once(eventType, callback)

Similar to addListener() but the callback is removed after it is invoked once. A token is returned that can be used to remove the listener.

var token = emitter.once('change', (...args) => {
  console.log(...args);
});

emitter.emit('change', 10); // 10 is logged
emitter.emit('change', 10); // nothing is logged

removeAllListeners(eventType)

Removes all of the registered listeners. eventType is optional, if provided only listeners for that event type are removed.

var token = emitter.addListener('change', (...args) => {
  console.log(...args);
});

emitter.removeAllListeners();
emitter.emit('change', 10); // nothing is logged

listeners(eventType)

Return an array of listeners that are currently registered for the given event type.

emit(eventType, ...args)

Emits an event of the given type with the given data. All callbacks that are listening to the particular event type will be notified.

var token = emitter.addListener('change', (...args) => {
  console.log(...args);
});

emitter.emit('change', 10); // 10 is logged

__emitToSubscription(subscription, eventType, ...args)

It is reasonable to extend EventEmitter in order to inject some custom logic that you want to do on every callback that is called during an emit, such as logging, or setting up error boundaries. __emitToSubscription() is exposed to make this possible.

class MyEventEmitter extends EventEmitter {
  __emitToSubscription(subscription, eventType) {
    var args = Array.prototype.slice.call(arguments, 2);
    var start = Date.now();
    subscription.listener.apply(subscription.context, args);
    var time = Date.now() - start;
    MyLoggingUtility.log('callback-time', {eventType, time});
  }
}

And then you can create instances of MyEventEmitter and use it like a standard EventEmitter. If you just want to log on each emit and not on each callback called during an emit you can override emit() instead of this method.