1 | /**
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2 | * @fileoverview A variant of EventEmitter which does not give listeners information about each other
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3 | * @author Teddy Katz
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4 | */
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5 |
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6 | ;
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7 |
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8 | //------------------------------------------------------------------------------
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9 | // Typedefs
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10 | //------------------------------------------------------------------------------
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11 |
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12 | /**
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13 | * An event emitter
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14 | * @typedef {Object} SafeEmitter
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15 | * @property {function(eventName: string, listenerFunc: Function): void} on Adds a listener for a given event name
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16 | * @property {function(eventName: string, arg1?: any, arg2?: any, arg3?: any)} emit Emits an event with a given name.
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17 | * This calls all the listeners that were listening for that name, with `arg1`, `arg2`, and `arg3` as arguments.
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18 | * @property {function(): string[]} eventNames Gets the list of event names that have registered listeners.
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19 | */
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20 |
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21 | /**
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22 | * Creates an object which can listen for and emit events.
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23 | * This is similar to the EventEmitter API in Node's standard library, but it has a few differences.
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24 | * The goal is to allow multiple modules to attach arbitrary listeners to the same emitter, without
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25 | * letting the modules know about each other at all.
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26 | * 1. It has no special keys like `error` and `newListener`, which would allow modules to detect when
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27 | * another module throws an error or registers a listener.
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28 | * 2. It calls listener functions without any `this` value. (`EventEmitter` calls listeners with a
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29 | * `this` value of the emitter instance, which would give listeners access to other listeners.)
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30 | * 3. Events can be emitted with at most 3 arguments. (For example: when using `emitter.emit('foo', a, b, c)`,
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31 | * the arguments `a`, `b`, and `c` will be passed to the listener functions.)
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32 | * @returns {SafeEmitter} An emitter
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33 | */
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34 | module.exports = () => {
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35 | const listeners = Object.create(null);
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36 |
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37 | return Object.freeze({
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38 | on(eventName, listener) {
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39 | if (eventName in listeners) {
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40 | listeners[eventName].push(listener);
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41 | } else {
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42 | listeners[eventName] = [listener];
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43 | }
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44 | },
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45 | emit(eventName, a, b, c) {
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46 | if (eventName in listeners) {
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47 | listeners[eventName].forEach(listener => listener(a, b, c));
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48 | }
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49 | },
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50 | eventNames() {
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51 | return Object.keys(listeners);
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52 | }
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53 | });
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54 | };
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