Node核心模塊之Events

Node.js V6.9.4
Node.js核心API的大部份是圍繞1個(gè)異步事件驅(qū)動(dòng)架構(gòu)構(gòu)建的，因此Events模塊是很多模塊的基礎(chǔ)，很多模塊都是基于Events模塊建立的，

例如：net.Server對(duì)象每當(dāng)對(duì)等體連接到它時(shí)發(fā)出1個(gè)事件; fs.ReadStream在打開(kāi)文件時(shí)發(fā)失事件; 每當(dāng)數(shù)據(jù)可用于讀取時(shí)，流發(fā)失事件。

所有發(fā)失事件的對(duì)象都是EventEmitter類的實(shí)例。這些對(duì)象公然了1個(gè)EventEmitter.on()函數(shù)，它允許1個(gè)或多個(gè)函數(shù)附加到對(duì)象發(fā)出的命名空間。

我們可以為1個(gè)事件對(duì)象注冊(cè)多個(gè)事件，可以為1個(gè)事件注冊(cè)多個(gè)監(jiān)聽(tīng)器

1.Events模塊基本用法

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();

myEmitter.on('event', () => {       //注冊(cè)1個(gè)名為'event'的事件
  console.log('an event occurred!');
});

//觸發(fā)名為'event'的事件
myEmitter.emit('event');  // => 'an event occurred!'    

2.同步與異步
* EventEmitter依照事件的注冊(cè)順序同步的調(diào)用監(jiān)聽(tīng)器，如果想切換到異步模式，使用setImmediate()或process.nextTick() *

const myEmitter = new MyEmitter();

myEmitter.on('event', (a, b) => {

  setImmediate(() => {
    console.log('this happens asynchronously');     // 該事件會(huì)異步觸發(fā)
  });
});

myEmitter.emit('event', 'a', 'b');

3.Error events
* 當(dāng)事件隊(duì)列中產(chǎn)生毛病或沒(méi)有為毛病事件注冊(cè)監(jiān)聽(tīng)器，則利用會(huì)拋出1個(gè)’error’事件，并且程序會(huì)退出 *

const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));  // 拋出毛病并且程序退出

* 避免拋出毛病時(shí)，進(jìn)程崩潰，可以為進(jìn)程注冊(cè)1個(gè)uncaughtException事件，用來(lái)處理毛病，避免程序崩潰 *

const myEmitter = new MyEmitter();

process.on('uncaughtException', (err) => {
  console.log('whoops! there was an error');
});

//進(jìn)程不會(huì)崩潰
myEmitter.emit('error', new Error('whoops!'));  // => 'whoops! there was an error'

* 最好實(shí)踐：應(yīng)當(dāng)始終為監(jiān)聽(tīng)器注冊(cè)1個(gè)’error’事件 *

const myEmitter = new MyEmitter();

myEmitter.on('error', (err) => {
  console.log('whoops! there was an error');
});

myEmitter.emit('error', new Error('whoops!'));   // => 'whoops! there was an error'

4.Event經(jīng)常使用API介紹
* const EventEmitter = require(‘events’) *
* const myEE = new EventEmitter() *