public function installedData(string $slug, string $key, bool $parent = false): string
    {
        if (!$slug || !$key) {
            return ''; // Not possible.
        }
        switch ($key) {
            case 'name':
                $key = 'Name';
                break;
            case 'version':
                $key = 'Version';
                break;
            case 'description':
                $key = 'Description';
                break;
            case 'url':
                $key = 'ThemeURI';
                break;
            case 'text_domain':
                $key = 'TextDomain';
                break;
            case 'author':
                $key = 'Author';
                break;
            case 'author_url':
                $key = 'AuthorURI';
                break;
            default: // Default case.
                return ''; // Unknown key.
        }
        if (($installed_theme = &$this->cacheKey(__FUNCTION__, $slug)) === null) {
            $installed_theme = wp_get_theme($slug);
        }
        if (!$installed_theme || !$installed_theme->exists()) {
            return ''; // Theme not installed.
        } elseif ($parent) { // Looking for parent theme data?
            return $installed_theme->parent() ? (string) $installed_theme->parent()->get($key) : '';
        } else {
            return (string) $installed_theme->get($key);
        }
    }