protected function displayTitle()
    {
        if ($this->title !== null) {
            $header_tag = new SwatHtmlTag('h' . $this->getHeaderLevel());
            $header_tag->class = 'swat-frame-title';
            $header_tag->setContent($this->title, $this->title_content_type);

            if ($this->subtitle === null) {
                $header_tag->display();
            } else {
                $span_tag = new SwatHtmlTag('span');
                $span_tag->class = 'swat-frame-subtitle';
                $span_tag->setContent(
                    $this->subtitle,
                    $this->title_content_type
                );

                $header_tag->open();
                $header_tag->displayContent();
                echo $this->title_separator;
                $span_tag->display();
                $header_tag->close();
            }
        }
    }