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'));
  }

}