protected function renderExtraRow($model, $key, $index, $totals)
    {
        if ($this->extraRowValue instanceof Closure) {
            $content = call_user_func($this->extraRowValue, $model, $index, $totals);
        } else {
            $values = [];
            foreach ($this->extraRowColumns as $name) {
                $values[] = ArrayHelper::getValue($model, $name);
            }

            $content = '<strong>' . implode(' :: ', $values) . '</strong>';
        }

        $colspan = count($this->columns);

        $cell = Html::tag('td', $content, ['class' => $this->extraRowClass, 'colspan' => $colspan]);

        return Html::tag('tr', $cell);
    }