class MenuDeleteForm

class MenuDeleteForm extends EntityDeleteForm {
    
    /**
     * The menu link manager.
     *
     * @var \Drupal\Core\Menu\MenuLinkManagerInterface
     */
    protected $menuLinkManager;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * Constructs a new MenuDeleteForm.
     *
     * @param \Drupal\Core\Menu\MenuLinkManagerInterface $menu_link_manager
     *   The menu link manager.
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection.
     */
    public function __construct(MenuLinkManagerInterface $menu_link_manager, Connection $connection) {
        $this->menuLinkManager = $menu_link_manager;
        $this->connection = $connection;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('plugin.manager.menu.link'), $container->get('database'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDescription() {
        $caption = '';
        $num_links = $this->menuLinkManager
            ->countMenuLinks($this->entity
            ->id());
        if ($num_links) {
            $caption .= '<p>' . $this->formatPlural($num_links, '<strong>Warning:</strong> There is currently 1 menu link in %title. It will be deleted (system-defined links will be reset).', '<strong>Warning:</strong> There are currently @count menu links in %title. They will be deleted (system-defined links will be reset).', [
                '%title' => $this->entity
                    ->label(),
            ]) . '</p>';
        }
        $caption .= '<p>' . $this->t('This action cannot be undone.') . '</p>';
        return $caption;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function logDeletionMessage() {
        $this->logger('menu')
            ->notice('Deleted custom menu %title and all its menu links.', [
            '%title' => $this->entity
                ->label(),
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Locked menus may not be deleted.
        if ($this->entity
            ->isLocked()) {
            return;
        }
        // Delete all links to the overview page for this menu.
        // @todo Add a more generic helper function to the menu link plugin
        //   manager to remove links to an entity or other ID used as a route
        //   parameter that is being removed. Also, consider moving this to
        //   menu_ui.module as part of a generic response to entity deletion.
        //   https://www.drupal.org/node/2310329
        $menu_links = $this->menuLinkManager
            ->loadLinksByRoute('entity.menu.edit_form', [
            'menu' => $this->entity
                ->id(),
        ], TRUE);
        foreach ($menu_links as $id => $link) {
            $this->menuLinkManager
                ->removeDefinition($id);
        }
        parent::submitForm($form, $form_state);
    }

}