Events
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图：Nguyen Nhut
Events
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。
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
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