初探 node events模块

模块概述

在 node 中 events模块是核心模块之一, 几乎所有常用的 node 模块都继承了 events模块

例如，net.Server 会在每次有新连接时触发事件，fs.ReadStream 会在打开文件时触发事件，stream会在数据可读时触发事件等

events模块本身非常简单，从官网文档中我们发现该模块的API虽然很多，但是我们常用的也就那几个，下面我们来看一下几个简单例子

例子

例子1：单个事件监听

let EventEmitter = require('events')
class Animal extends EventEmitter {}
let dog = new Animal()

dog.on('eat', function() {
  console.log('a dog eating food!')
})

dog.emit('eat')

// a dog eating food!

例子2：同个事件，多个事件监听

let EventEmitter = require('events')
class Animal extends EventEmitter {}
let dog = new Animal()
dog.on('eat', function(){
  console.log('a dog eating food')
})
dog.on('eat', function(){
  console.log('a dog eating')
})
dog.emit('eat')

// a dog eating food
// a dog eating

从上面我们可以看到，事件触发时，事件监听器按照注册的顺序依次执行

例子3：只运行一次的事件监听

let EventEmitter = require('events')
class Animal extends EventEmitter {}
let dog = new Animal()
dog.on('eat',function(){
  console.log('a dog eating food')
})
dog.once('eat',function(){
  console.log('a dog eating food once')
})
dog.emit('eat')
dog.emit('eat')

// a dog eating food
// a dog eating food once
// a dog eating food

例子4： 注册时间监听器前，事件先触发

let EventEmitter = require('events')
class Animal extends EventEmitter {}
let dog = new Animal()
dog.emit('eat',1)
dog.on('eat', function(index){
  console.log('a dog eating food-'+ index)
})
dog.emit('eat',2)
// a dog eating food-2

从上面例子中我们可以看到，注册时间监听器前，事件先触发，则该事件会直接被忽略

例子5：执行顺序

let EventEmitter = require('events')
class Animal extends EventEmitter {}
let dog = new Animal()
dog.on('eat', function(){
  console.log('a dog eating food1')
})
dog.emit('eat')
dog.on('eat', function(){
  console.log('a dog eating food2')
})
// a dog eating food1
// a dog eating food1

例子6：移出事件监听

let EventEmitter = require('events')
class Animal extends EventEmitter {}

function eat(){
 console.log('a dog eating food')
}
let dog = new Animal()
dog.on('eat', eat)
dog.emit('eat')
dog.removeListener('eat', eat)
// a dog eating food