class EntityReferenceEntityFormatter

class EntityReferenceEntityFormatter extends EntityReferenceFormatterBase {
  
  /**
   * The number of times this formatter allows rendering the same entity.
   *
   * @var int
   */
  const RECURSIVE_RENDER_LIMIT = 20;
  
  /**
   * The logger factory.
   *
   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
   */
  protected $loggerFactory;
  
  /**
   * The entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityTypeManager;
  
  /**
   * The entity display repository.
   *
   * @var \Drupal\Core\Entity\EntityDisplayRepositoryInterface
   */
  protected $entityDisplayRepository;
  
  /**
   * An array of counters for the recursive rendering protection.
   *
   * Each counter takes into account all the relevant information about the
   * field and the referenced entity that is being rendered.
   *
   * @see \Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceEntityFormatter::viewElements()
   *
   * @var array
   */
  protected static $recursiveRenderDepth = [];
  
  /**
   * Constructs an EntityReferenceEntityFormatter instance.
   *
   * @param string $plugin_id
   *   The plugin ID for the formatter.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the formatter is associated.
   * @param array $settings
   *   The formatter settings.
   * @param string $label
   *   The formatter label display setting.
   * @param string $view_mode
   *   The view mode.
   * @param array $third_party_settings
   *   Any third party settings.
   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $logger_factory
   *   The logger factory.
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   * @param \Drupal\Core\Entity\EntityDisplayRepositoryInterface $entity_display_repository
   *   The entity display repository.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, LoggerChannelFactoryInterface $logger_factory, EntityTypeManagerInterface $entity_type_manager, EntityDisplayRepositoryInterface $entity_display_repository) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
    $this->loggerFactory = $logger_factory;
    $this->entityTypeManager = $entity_type_manager;
    $this->entityDisplayRepository = $entity_display_repository;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('logger.factory'), $container->get('entity_type.manager'), $container->get('entity_display.repository'));
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'view_mode' => 'default',
      'link' => FALSE,
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $elements['view_mode'] = [
      '#type' => 'select',
      '#options' => $this->entityDisplayRepository
        ->getViewModeOptions($this->getFieldSetting('target_type')),
      '#title' => $this->t('View mode'),
      '#default_value' => $this->getSetting('view_mode'),
      '#required' => TRUE,
    ];
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $view_modes = $this->entityDisplayRepository
      ->getViewModeOptions($this->getFieldSetting('target_type'));
    $view_mode = $this->getSetting('view_mode');
    $summary[] = $this->t('Rendered as @mode', [
      '@mode' => $view_modes[$view_mode] ?? $view_mode,
    ]);
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $view_mode = $this->getSetting('view_mode');
    $elements = [];
    foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
      // Due to render caching and delayed calls, the viewElements() method
      // will be called later in the rendering process through a '#pre_render'
      // callback, so we need to generate a counter that takes into account
      // all the relevant information about this field and the referenced
      // entity that is being rendered.
      $recursive_render_id = $items->getFieldDefinition()
        ->getTargetEntityTypeId() . $items->getFieldDefinition()
        ->getTargetBundle() . $items->getName() . $items->getEntity()
        ->id() . $entity->getEntityTypeId() . $entity->id();
      if (isset(static::$recursiveRenderDepth[$recursive_render_id])) {
        static::$recursiveRenderDepth[$recursive_render_id]++;
      }
      else {
        static::$recursiveRenderDepth[$recursive_render_id] = 1;
      }
      // Protect ourselves from recursive rendering.
      if (static::$recursiveRenderDepth[$recursive_render_id] > static::RECURSIVE_RENDER_LIMIT) {
        $this->loggerFactory
          ->get('entity')
          ->error('Recursive rendering detected when rendering entity %entity_type: %entity_id, using the %field_name field on the %parent_entity_type:%parent_bundle %parent_entity_id entity. Aborting rendering.', [
          '%entity_type' => $entity->getEntityTypeId(),
          '%entity_id' => $entity->id(),
          '%field_name' => $items->getName(),
          '%parent_entity_type' => $items->getFieldDefinition()
            ->getTargetEntityTypeId(),
          '%parent_bundle' => $items->getFieldDefinition()
            ->getTargetBundle(),
          '%parent_entity_id' => $items->getEntity()
            ->id(),
        ]);
        return $elements;
      }
      $view_builder = $this->entityTypeManager
        ->getViewBuilder($entity->getEntityTypeId());
      $elements[$delta] = $view_builder->view($entity, $view_mode, $entity->language()
        ->getId());
      // Add a resource attribute to set the mapping property's value to the
      // entity's URL. Since we don't know what the markup of the entity will
      // be, we shouldn't rely on it for structured data.
      if (!empty($items[$delta]->_attributes) && !$entity->isNew() && $entity->hasLinkTemplate('canonical')) {
        $items[$delta]->_attributes += [
          'resource' => $entity->toUrl()
            ->toString(),
        ];
      }
    }
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    // This formatter is only available for entity types that have a view
    // builder.
    $target_type = $field_definition->getFieldStorageDefinition()
      ->getSetting('target_type');
    return \Drupal::entityTypeManager()->getDefinition($target_type)
      ->hasViewBuilderClass();
  }

}