code: function (ev) {
      var loaded = ev.loaded || ev.position;
      var totalSize = ev.total || ev.totalSize;
  
      if (totalSize) {
        this._dispatchProgress(loaded / totalSize);
      } else {
        this._dispatchProgress(0);
      }
    }
docs: This callback will be called when the XHR progresses in its load.
  
  @method _onProgress
  @protected
  @param  {XMLHttpRequestProgressEvent} ev This event contains data for the progress of the load