docs: function (callbacks) {
                      var index = 0,
                          callback,
                          args = [].slice.call(arguments);
  
                      if (unique) {
                          while (callback = args[index]) {
                              if (list.indexOf(callback) === -1) {
                                  list.push(callback);
                              }
  
                              index++;
                          }
                      } else {
                          list.push.apply(list, args);
                      }
  
                      // 添加回调之后重新获取列表长度
                      fireLength = list.length;
  
                      return this;
                  }