public function displayOptions()
    {
        if ($this->displayOptions === null) {
            $this->setDisplayOptions($this->defaultDisplayOptions());

            $ident    = $this->ident();
            $metadata = $this->adminSecondaryMenu();

            if (isset($metadata[$ident]['display_options'])) {
                $options = $metadata[$ident]['display_options'];

                if (!is_array($options)) {
                    throw new RuntimeException('The display options must be an associative array.');
                }

                $this->setDisplayOptions(array_merge($this->displayOptions, $options));
            }
        }

        return $this->displayOptions;
    }