function is_link($field, $value, $fieldinfo) {
        $require = $fieldinfo['min_length']  ? 'required ' : '';
        if($value) {
            $url = $this->data['url'];
            $checked = 'checked';
            $readonly = '';
        } else {
            $url = $checked = '';
            $readonly = 'readonly';
        }
        $string =  '<div class="mb-3 row '.$require.'">';
        $string .= '<label class="col-md-2 col-form-label">'.$fieldinfo['name'].'</label>';
        $string .= '<div class="col-md-10">';
        $string .= '<input type="text" name="info[url]" '.$readonly.' id="info_url" value="'.$url.'"   autocomplete="off" class="form-control '.$require.'" >';
        $string .= '<div class="col-sm-auto  ">';
        $string .= '<input  value="1"   '.$checked.'  type="checkbox" switch="success" class="row-switch" name="info[is_link]" id="info_is_link"  >';
        $string .= '<label class="switch-check-label mt10" for="info_is_link" data-on-label="开启" data-off-label="关闭"></label>';
        $string .= '</div>';
        $string .= '</div>';
        $string .= '</div>';
        return $string;
    }