protected function getHtmlBody(array $rows): string
  {
    $ret = '';
    $i   = 0;
    foreach ($rows as $row)
    {
      $class = static::$class;
      if ($class!==null) $class .= ' ';
      $class .= ($i % 2==0) ? 'even' : 'odd';

      $ret .= Html::generateTag('tr', ['class' => $class]);
      foreach ($this->columns as $column)
      {
        $ret .= $column->getHtmlCell($row);
      }
      $ret .= '</tr>';
      $i++;
    }

    return $ret;
  }