code: function(id, optChunkIdx, xhr, optAjaxRequester) {
              var xhrsId = optChunkIdx == null ? -1 : optChunkIdx,
                  tempState = handler._getFileState(id).temp;
  
              tempState.xhrs = tempState.xhrs || {};
              tempState.ajaxRequesters = tempState.ajaxRequesters || {};
  
              tempState.xhrs[xhrsId] = xhr;
  
              if (optAjaxRequester) {
                  tempState.ajaxRequesters[xhrsId] = optAjaxRequester;
              }
  
              return xhr;
          }
docs: Registers an XHR transport instance created elsewhere.
  
  @param id ID of the associated file
  @param optChunkIdx The chunk index associated with this XHR, if applicable
  @param xhr XMLHttpRequest object instance
  @param optAjaxRequester `qq.AjaxRequester` associated with this request, if applicable.
  @returns {XMLHttpRequest}