protected function get_direction_icon($output, $field) {
        $direction = self::DEFAULTSORTDIRECTION;
        if ($this->sort == $field) {
            $direction = $this->sortdirection;
        }
        if ($direction === 'ASC') {
            return $output->pix_icon('t/sort_asc', get_string('sort'));
        } else {
            return $output->pix_icon('t/sort_desc', get_string('sort'));
        }
    }