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