code: function(ev) {
          var $target = $(ev.target);
          var columnName;
  
          if (!this._triggerPublicMousedown(ev)) {
              return;
          }
  
          if ($target.hasClass(classNameConst.BTN_SORT)) {
              return;
          }
  
          columnName = $target.closest('th').attr(ATTR_COLUMN_NAME);
          if (columnName) {
              this.dragEmitter.start(ev, {
                  columnName: columnName
              });
          }
      }
docs: Mousedown event handler
  @param {jQuery.Event} ev - MouseDown event
  @private