Node.js learning and chatting about the Events module

This article will take you to understand the Events module inNode.jsand introduce the publish and subscribe model in Events. I hope it will be helpful to everyone!

Events module

Reference official website: events event trigger | Node.js

http ://nodejs.cn/api/events.html

Eventsmodule is the most important module of Node. It provides an attributeEventEmitter,# The core of ##EventEmitteris event emission and event listener. Most of the modules in

Nodeinherit from theEventsmodule.

• Eventsmodule is Node’s implementation ofpublish/subscribe mode(publish/subscribe). One object passes messages to another object through this module.
This module provides a constructor through the
• EventEmitterattribute. Instances of this constructor haveonmethods that can be used to listen to specified events and trigger callback functions.
Any object can publish specified events, which are monitored by the on method of the
• EventEmitterinstance.

Publish and subscribe model

For

publish and subscribe model, you can refer to my previous blog article.

Events, we must first understand several of its common methods.

• Subscription method:onmethod is used to subscribe to events. Subscription maps methods into a one-to-many relationship.
• Publishing method:emitUsed to execute subscribed events.
• Unsubscribe: Theoffmethod can remove the corresponding event listener.
• Subscribe once:onceThe bound event automatically deletes the subscribed event after execution.

on and emit

onThe first parameter of the method is used to set the class name, and the second parameter is also a Function, which can receive parameters passed in when publishing.

emitThe first parameter of the method is the class name, and the subsequent parameters are the parameters passed into theonmethod function.

onandemitFor specific applications, please refer to the simple Demo below.

const EventEmitter = require('events'); // 自定义一个 构造函数 function Cat() {} // 原型继承 需要通过实例来调用继承方法 Object.setPrototypeOf(Cat.prototype, EventEmitter.prototype); let cat = new Cat(); const sleep = (a, b) => { console.log(a, '睡'); }; const eat = (a, b) => { console.log(b, '吃'); }; cat.on('猫咪', sleep) cat.on('猫咪', eat) setTimeout(() => { // 小胡子 吃 // 小胖仙 睡 cat.emit('猫咪', '小胖仙', '小胡子') }, 1000);

onandemitmethods.

function EventEmitter() { this._event = {} } // on 方法 EventEmitter.prototype.on = function (eventName, callBack) { if (!this._event) { this._event = {} } if (this._event[eventName]) { this._event[eventName].push(callBack) // 相当于 {eventName:[fn1,fn2]} } else { this._event[eventName] = [callBack]; // 相当于 {eventName:[fn1]} } } // emit 方法 EventEmitter.prototype.emit = function (eventName, ...args) { this._event[eventName].forEach(fn => { fn(...args) }); }

off

offThe first parameter of the method is used to set the class name, and the second parameter passed in needs to be removed function callback.

// ... setTimeout(() => { // 小胡子 吃 // 小胖仙 睡 cat.emit('猫咪', '小胖仙', '小胡子') cat.off('猫咪', sleep); // 小胡子 吃 cat.emit('猫咪', '小胖仙', '小胡子') }, 1000);

filtermethod.

// off 方法 EventEmitter.prototype.off = function (eventName, callBack) { if (this._event && this._event[eventName]) { this._event[eventName] = this._event[eventName].filter( fn => fn !== callBack && fn.c !== callBack // fn.c参考下面的once方法实现 ) } }

once

onceThe first parameter of the method is used to set the class name. The second parameter is passed in and only needs to be executed once. function callback.

// ... const demolition =() => { console.log('拆家'); } cat.once('猫咪', demolition) setTimeout(() => { // ...... 拆家 cat.emit('猫咪', '小胖仙', '小胡子') }, 1000);

onandoff.

// once 方法 EventEmitter.prototype.once = function (eventName, callBack) { const one = () => { callBack(); this.off(eventName, one); } this.on(eventName, one); }

offmethod before executing theoncemethod.

oncemethod(theoffmethod has already been processed).

EventEmitter.prototype.once = function (eventName, callBack) { const one = () => { // ... } one.c = callBack; // 自定义一个属性 // ... }

oncemethod.

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