public function getPageOrder(Page $parentPage = null)
    {
        if ($parentPage instanceof Page) {
            $parentPageString = $parentPage->getID();
        } else {
            $parentPageString = '';
        }
        if (!isset($this->pageOrder[$parentPageString])) {
            return [];
        }

        $retArray = [];
        foreach ($this->pageOrder[$parentPageString] as $id) {
            $retArray[] = $this->activePages[$id];
        }

        return $retArray;
    }