/**
*事件和回调处理程序
*异步是基于事件的，不是顺序走的
*回调-》事件-》异步-》非阻塞
*/

//标准事件
const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);

const myEmitter = new MyEmitter();

//定义事件
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
//触发事件
myEmitter.emit('event');

//事件传参

const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
//添加参数
myEmitter.on('event', function(a, b) {
  console.log(a, b, this);
    // Prints:
    //   a b MyEmitter {
    //     domain: null,
    //     _events: { event: [Function] },
    //     _eventsCount: 1,
    //     _maxListeners: undefined }
});
myEmitter.emit('event', 'a', 'b');

//异步事件

const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
//异步的执行
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');

//事件只允许触发一次

const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
var m = 0;
//只触发一次
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
  // Prints: 1
myEmitter.emit('event');
  // Ignored

//标准错误事件

const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
//myEmitter.emit('error', new Error('whoops!'));
  // 抛出错误

//此处抛出的错误被捕获
/*process.on('uncaughtException', (err) => {
  console.log('whoops! there was an error');
});

myEmitter.emit('error', new Error('whoops!'));
//为了最佳实践，最好定义事件处理程序*/

myEmitter.on('error', (err) => {
  console.log('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
  // Prints: whoops! there was an error

//标准newListener监听事件的注册

const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
//event事件名，listener监听函数
//同样的还有removeListener
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
  // Prints:
  //   B
  //   A

//设置最大监听数，默认是10

const EventEmitter = require('events');
const util = require('util');

//通过继承实现
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

emitter.addListener(event, listener)//另一种用法emitter.on(event, listener).
emitter.getMaxListeners()
EventEmitter.defaultMaxListeners.
emitter.removeAllListeners([event])//删除listener
emitter.removeListener(event, listener)//删除具体的listener