class MenuLinkContentForm

class MenuLinkContentForm extends ContentEntityForm {
    
    /**
     * The content menu link.
     *
     * @var \Drupal\menu_link_content\MenuLinkContentInterface
     */
    protected $entity;
    
    /**
     * The parent form selector service.
     *
     * @var \Drupal\Core\Menu\MenuParentFormSelectorInterface
     */
    protected $menuParentSelector;
    
    /**
     * The path validator.
     *
     * @var \Drupal\Core\Path\PathValidatorInterface
     */
    protected $pathValidator;
    
    /**
     * Constructs a MenuLinkContentForm object.
     *
     * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
     *   The entity repository.
     * @param \Drupal\Core\Menu\MenuParentFormSelectorInterface $menu_parent_selector
     *   The menu parent form selector service.
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager.
     * @param \Drupal\Core\Path\PathValidatorInterface $path_validator
     *   The path validator.
     * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
     *   The entity type bundle service.
     * @param \Drupal\Component\Datetime\TimeInterface $time
     *   The time service.
     */
    public function __construct(EntityRepositoryInterface $entity_repository, MenuParentFormSelectorInterface $menu_parent_selector, LanguageManagerInterface $language_manager, PathValidatorInterface $path_validator, EntityTypeBundleInfoInterface $entity_type_bundle_info = NULL, TimeInterface $time = NULL) {
        parent::__construct($entity_repository, $entity_type_bundle_info, $time);
        $this->menuParentSelector = $menu_parent_selector;
        $this->pathValidator = $path_validator;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('entity.repository'), $container->get('menu.parent_form_selector'), $container->get('language_manager'), $container->get('path.validator'), $container->get('entity_type.bundle.info'), $container->get('datetime.time'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $form = parent::form($form, $form_state);
        $default = $this->entity
            ->getMenuName() . ':' . $this->entity
            ->getParentId();
        $id = $this->entity
            ->isNew() ? '' : $this->entity
            ->getPluginId();
        $form['menu_parent'] = $this->menuParentSelector
            ->parentSelectElement($default, $id);
        $form['menu_parent']['#weight'] = 10;
        $form['menu_parent']['#title'] = $this->t('Parent link');
        $form['menu_parent']['#description'] = $this->t('The maximum depth for a link and all its children is fixed. Some menu links may not be available as parents if selecting them would exceed this limit.');
        $form['menu_parent']['#attributes']['class'][] = 'menu-title-select';
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function actions(array $form, FormStateInterface $form_state) {
        $element = parent::actions($form, $form_state);
        $element['submit']['#button_type'] = 'primary';
        $element['delete']['#access'] = $this->entity
            ->access('delete');
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildEntity(array $form, FormStateInterface $form_state) {
        
        /** @var \Drupal\menu_link_content\MenuLinkContentInterface $entity */
        $entity = parent::buildEntity($form, $form_state);
        [
            $menu_name,
            $parent,
        ] = explode(':', $form_state->getValue('menu_parent'), 2);
        $entity->parent->value = $parent;
        $entity->menu_name->value = $menu_name;
        $entity->enabled->value = !$form_state->isValueEmpty([
            'enabled',
            'value',
        ]);
        $entity->expanded->value = !$form_state->isValueEmpty([
            'expanded',
            'value',
        ]);
        return $entity;
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(array $form, FormStateInterface $form_state) {
        // The entity is rebuilt in parent::submit().
        $menu_link = $this->entity;
        $menu_link->save();
        $this->messenger()
            ->addStatus($this->t('The menu link has been saved.'));
        $form_state->setRedirectUrl($menu_link->toUrl('canonical'));
    }

}