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/**
* @fileoverview A variant of EventEmitter which does not give listeners information about each other
* @author Teddy Katz
*/
"use strict";
//------------------------------------------------------------------------------
// Typedefs
//------------------------------------------------------------------------------
/**
* An event emitter
* @typedef {Object} SafeEmitter
* @property {function(eventName: string, listenerFunc: Function): void} on Adds a listener for a given event name
* @property {function(eventName: string, arg1?: any, arg2?: any, arg3?: any)} emit Emits an event with a given name.
* This calls all the listeners that were listening for that name, with `arg1`, `arg2`, and `arg3` as arguments.
* @property {function(): string[]} eventNames Gets the list of event names that have registered listeners.
*/
/**
* Creates an object which can listen for and emit events.
* This is similar to the EventEmitter API in Node's standard library, but it has a few differences.
* The goal is to allow multiple modules to attach arbitrary listeners to the same emitter, without
* letting the modules know about each other at all.
* 1. It has no special keys like `error` and `newListener`, which would allow modules to detect when
* another module throws an error or registers a listener.
* 2. It calls listener functions without any `this` value. (`EventEmitter` calls listeners with a
* `this` value of the emitter instance, which would give listeners access to other listeners.)
* 3. Events can be emitted with at most 3 arguments. (For example: when using `emitter.emit('foo', a, b, c)`,
* the arguments `a`, `b`, and `c` will be passed to the listener functions.)
* @returns {SafeEmitter} An emitter
*/
module.exports = () => {
const listeners = Object.create(null);
return Object.freeze({
on(eventName, listener) {
if (eventName in listeners) {
listeners[eventName].push(listener);
} else {
listeners[eventName] = [listener];
}
},
emit(eventName, a, b, c) {
if (eventName in listeners) {
listeners[eventName].forEach(listener => listener(a, b, c));
}
},
eventNames() {
return Object.keys(listeners);
}
});
};
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