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Revealing constructor pattern event-emitter
// This event emitter emits events, but reserves the right to publish events to
// for its creator. It uses a WeakMap for true encapsulation.
const eesToEventMaps = new WeakMap();
export default class EventEmitter {
constructor(publisher) {
const eventMap = Object.create(null);
eesToEventMaps.set(this, eventMap);
on(eventName, handler) {
const eventMap = eesToEventMaps.get(this);
let handlers = eventMap[eventName];
if (!handlers) {
handlers = eventMap[eventName] = [];
off(eventName, handler) {
const eventMap = eesToEventMaps.get(this);
const handlers = eventMap[eventName];
if (!handlers) {
const index = handlers.indexOf(handler);
if (index === -1) {
handlers.splice(index, -1);
function makePublish(ee) {
const eventMap = eesToEventMaps.get(ee);
return function (eventName, ...args) {
const handlers = eventMap[eventName];
if (handlers) {
handlers.forEach(h => h(...args));
const myEE = new EventEmitter(publish => {
// Wait for interesting things to happen, then call
// `publish("eventName", ...args)`.
// The other code only gets access to `on` and `off`: it cannot trigger spurious
// events, but only listen for them. This makes it safe to pass `myEE` to
// multiple consumers without worrying about them accidentally stepping on each
// other's toes.
// (Of course, they could intentionally step on each other's toes, e.g. by
// overwriting `on` or `off`. For true security you'll need to deeply-freeze
// `myEE`, and do a few more things, e.g. to protect against plan interference
// attacks. We're not focused on that right now, but instead on the API
// ergonomics.)
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trusktr commented Aug 21, 2016

This pattern is awesome!

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trusktr commented Aug 29, 2017

@domenic, please let me know if I'm close: this pattern of the constructor accepting a publish makes this EventEmitter class not ideal for extending, and more ideal for other code to contain references to an EventEmitter.

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