89 lines
2.7 KiB
Markdown
89 lines
2.7 KiB
Markdown
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---
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tags:
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- Programming_Languages
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- backend
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- node-js
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- node-modules
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---
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* Much of the NodeJS core is built around an [asynchronous event-driven architecture](Single-threaded%20asynchronous%20architecture.md) in which certain kinds of objects (called "emitters") emit named events that cause `Function` objects ("listeners") to be called.
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* For example: a `fs.ReadStream` emits an event when the file is opened
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## Event Emitters
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* All objects that emit events are instances of the `EventEmitter` class. These objects expose an `eventEmitter.on()` function that allows one or more functions to be attached to named events emitted by the object.
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* These functions are listeners of the emitter.
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## Basic syntax
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````js
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const EventEmitter = require('events') // import the module
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// Raise an event
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const emitter = new EventEmitter('messageLogged')
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// Register a listener
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emitter.on('messagedLogged', function() {
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console.log('The listener was called.')
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})
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````
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* If we ran this file, we would see `The listener was called` logged to the console.
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* Without a listener (similar to a subscriber in Angular) nothing happens.
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* When the emission occurs the emitter works *synchronously* through each listener function that is attached to it.
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## Event arguments
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* Typically we would not just emit a string, we would attach an object to the emitter to pass more useful data. This data is called an **Event Argument**.
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* Refactoring the previous example:
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````js
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// Raise an event
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const emitter = new EventEmitter('messageLogged', function(eventArg) {
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console.log('Listener called', eventArg)
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})
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// Register a listener
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emitter.on('messagedLogged', {id: 1, url: 'http://www.example.com'})
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````
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## Extending the `EventEmitter` class
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* It's not best practice to call the EventEmitter class directly in `app.js`. If we want to use the capabilities of the class we should create our own module that extends `EventEmitter`, inheriting its functionality with specific additional features that we want to add.
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* So, refactoring the previous example:
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````js
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// File: Logger.js
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const EventEmitter = require('events')
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class Logger extends EventEmitter {
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log(message){
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console.log(message)
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this.emit('messageLogged', {id: 1, url: 'http://www.example.com'})
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}
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}
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````
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*The `this` in the `log` method refers to the properties and methods of `EventEmitter` which we have extended.*
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* We also need to refactor our listener code within `app.js` so that it calls the extended class rather than the `EventEmitter` class directly:
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````js
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// File app.js
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const Logger = require('./Logger')
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const logger = new Logger()
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logger.on('messageLogged', function(eventArg){
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console.log('Listener called', eventArg)
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}
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logger.log('message')
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````
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