code: public void onLoading(ImageLoadHandler handler) {
          mFlag = mFlag | STATUS_LOADING;
  
          if (null == handler) {
              return;
          }
  
          if (mFirstImageViewHolder == null) {
              handler.onLoading(this, null);
          } else {
              ImageViewHolder holder = mFirstImageViewHolder;
              do {
                  final CubeImageView imageView = holder.getImageView();
                  if (null != imageView) {
                      handler.onLoading(this, imageView);
                  }
              } while ((holder = holder.mNext) != null);
          }
      }
docs: When loading from network
  
  @param handler