/**
*  定义Emitter构造函数
*/function Emitter() {}/**
*  添加监听事件
*/Emitter.prototype.on =
Emitter.prototype.addEventListener = function(event, fn){this._callbacks = this.callbacks || {};(this._callbacks[event] = this._callbacks[event] || []).push(fn);return this;
}

/***  添加一个一次性监听事件*/

Emitter.prototype.once = function(event, fn){  var self = this;  this._callbacks = this._callbacks || {};

  function on() {    self.off(event, on);    fn.apply(this, arguments);  }

  on.fn = fn;  this.on(event, on);  return this;};

/***  移除已经添加的监听事件*/

Emitter.prototype.off =Emitter.prototype.removeListener = Emitter.prototype.removeAllListeners =Emitter.prototype.removeEventListener = function(event, fn){  this._callbacks = this._callbacks || {};

　 //  所有事件  if (0 == arguments.length) {    this._callbacks = {};    return this;  }

  //  指定事件  var callbacks = this._callbacks[event];  if (!callbacks) return this;

  ／／ 移除对应的事件回调  if (1 == arguments.length) {    delete this._callbacks[event];    return this;  }

  var cb;  for (var i = 0l i< callbacks.length; i++) {    cb = callbacks[i];    if (cb === fn || cb.fn === fn) {      callbacks.splice(i, 1);      break;    }  }  return this;};

/***  手动触发指定event事件*/

Emitter.prototype.emit = function(event){  this._callbacks = this._callbacks || {};  var args = [].slice.call(arguments, 1),      callbacks = this._callbacks[event];  if (callbacks) {    callbacks = callbacks.slice(0);    for (var i = 0, len = callbacks.length; i< len; i++){      callbacks[i].apply(this, args);    }  }

  return this;};

/***  返回指定event的回调数组*/

Emitter.prototype.listeners = function(event){  this._callbacks = this._callbacks || {};  return this._callbacks[event] || [];};

/***  判断该Emitter是否有指定event的回调*/

Emitter.prototype.hasListeners = function(event){  return !!this.listeners(event).length;};