Nodejs学习笔记:事件机制-events

模块概览

events模块是node的核心模块之一,几乎所有常用的node模块都继承了events模块,比如http、fs等。

模块本身非常简单,API虽然也不少,但常用的就那么几个,这里举几个简单例子。

基础例子

下面一共是6个例子,都非常简单,可以直接拷贝出来运行。例子5比较有意思,虽然也并不复杂,但确实是容易记错的点,感兴趣的同学可以看下。

例子1:单个事件监听器

var EventEmitter = require('events');

class Man extends EventEmitter {}

var man = new Man();

man.on('wakeup', function(){
    console.log('man has woken up');
});

man.emit('wakeup');
// 输出如下:
// man has woken up

例子2:同个事件,多个事件监听器

可以看到,事件触发时,事件监听器按照注册的顺序执行。

var EventEmitter = require('events');

class Man extends EventEmitter {}

var man = new Man();

man.on('wakeup', function(){
    console.log('man has woken up');
});

man.on('wakeup', function(){
    console.log('man has woken up again');
});

man.emit('wakeup');

// 输出如下:
// man has woken up
// man has woken up again

例子3:只运行一次的事件监听器

var EventEmitter = require('events');

class Man extends EventEmitter {}

var man = new Man();

man.on('wakeup', function(){
    console.log('man has woken up');
});

man.once('wakeup', function(){
    console.log('man has woken up again');
});

man.emit('wakeup');
man.emit('wakeup');

// 输出如下:
// man has woken up
// man has woken up again
// man has woken up

例子4:注册事件监听器前,事件先触发

可以看到,注册事件监听器前,事件先触发,则该事件会直接被忽略。

var EventEmitter = require('events');

class Man extends EventEmitter {}

var man = new Man();

man.emit('wakeup', 1);

man.on('wakeup', function(index){
    console.log('man has woken up ->' + index);
});

man.emit('wakeup', 2);
// 输出如下:
// man has woken up ->2

例子5:异步执行,还是顺序执行

例子很简单,但非常重要。究竟是代码1先执行,还是代码2先执行,这点差异,无论对于我们理解别人的代码,还是自己编写node程序,都非常关键。

实践证明,代码1先执行了。(node v6.1.0)

var EventEmitter = require('events');

class Man extends EventEmitter {}

var man = new Man();

man.on('wakeup', function(){
    console.log('man has woken up'); // 代码1
});

man.emit('wakeup');

console.log('woman has woken up');  // 代码2

// 输出如下:
// man has woken up
// woman has woken up

例子6:移除事件监听器

var EventEmitter = require('events');

function wakeup(){
    console.log('man has woken up');
}

class Man extends EventEmitter {}

var man = new Man();

man.on('wakeup', wakeup);
man.emit('wakeup');

man.removeListener('wakeup', wakeup);
man.emit('wakeup');

// 输出如下:
// man has woken up

相关链接

https://nodejs.org/api/events.html

原文链接:https://github.com/chyingp/nodejs-learning-guide

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