var EventEmitter = {
  _events: {},
  dispatch: function (event, data) {
      if (!this._events[event]) { // 没有监听事件
        return;
      }
      
      for(var cb of this._events[event].values()) {
        cb(data);
      }
      //let arr = this._events[event].values();
      //arr.forEach(function (cb) {
      //    cb(data);
      //})
  },
  subscribe: function (event, source, callback) {
    // 创建一个新事件数组
    if (!this._events[event]) {
      this._events[event] = new Map();
    }

    this._events[event].set(source, callback);
    
  },
  unsubscribe: function (event, source) {
    if(this._events && this._events[event]) {
      this._events[event].delete(source);
    }
  }
};