/** 切换到正在刷新状态,会调用delegate的onBGARefreshLayoutBeginRefreshing方法 */
 public void beginRefreshing() {
   if (mCurrentRefreshStatus != RefreshStatus.REFRESHING && mDelegate != null) {
     mCurrentRefreshStatus = RefreshStatus.REFRESHING;
     changeRefreshHeaderViewToZero();
     handleRefreshStatusChanged();
     mDelegate.onBGARefreshLayoutBeginRefreshing(this);
   }
 }