/**

*  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;
};