/**
  * 渲染表头
  * @return string
  */
 protected function renderHeader()
 {
     if ($this->header) {
         $this->keys = array_keys($this->header);
         $renderContent = '';
         if ($this->checkbox) {
             $renderContent .= Html::tag('th', Html::input('checkbox', '', '', ['class' => 'group-checkable', 'data-set' => '#' . $this->id . ' .checkboxes']), ['class' => 'table-checkbox']);
         }
         foreach ($this->header as $key => $val) {
             $renderContent .= Html::tag('th', $val);
         }
         return Html::tag('thead', Html::tag('tr', $renderContent));
     }
     return '';
 }