protected function getPreviewCode()
    {
        $file = file($this->stack['file']);
        $line = $this->stack['line'];
        $_line = $line - 1;
        if ($_line - 5 >= 0) {
            $start = $_line - 5;
            $end = $_line + 5;
        } else {
            $start = $_line;
            $end = $line;
        }
        for ($i = $start; $i <= $end; $i++) {
            if (!isset($file[$i])) {
                break;
            }
            $text = trim($file[$i]);
            if ($i === $_line) {
                $this->stack['previewCode'] .=
                    "<span style='background:red' class='line'>".($i + 1).'</span>'.
                    "<span style='background:red'>".$text.'</span><br>';
                continue;
            }
            $this->stack['previewCode'] .=
                "<span class='line'>".($i + 1).'</span>'.
                '<span>'.$text.'</span><br>';
        }
    }