class EntityReferenceFieldItemNormalizer

class EntityReferenceFieldItemNormalizer extends FieldItemNormalizer {
  use EntityReferenceFieldItemNormalizerTrait;
  
  /**
   * {@inheritdoc}
   */
  protected $supportedInterfaceOrClass = EntityReferenceItem::class;
  
  /**
   * The entity repository.
   *
   * @var \Drupal\Core\Entity\EntityRepositoryInterface
   */
  protected $entityRepository;
  
  /**
   * Constructs an EntityReferenceFieldItemNormalizer object.
   *
   * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
   *   The entity repository.
   */
  public function __construct(EntityRepositoryInterface $entity_repository) {
    $this->entityRepository = $entity_repository;
  }
  
  /**
   * {@inheritdoc}
   */
  public function normalize($field_item, $format = NULL, array $context = []) {
    $values = parent::normalize($field_item, $format, $context);
    $this->normalizeRootReferenceValue($values, $field_item);
    /** @var \Drupal\Core\Entity\EntityInterface $entity */
    if ($entity = $field_item->get('entity')
      ->getValue()) {
      $values['target_type'] = $entity->getEntityTypeId();
      // Add the target entity UUID to the normalized output values.
      $values['target_uuid'] = $entity->uuid();
      // Add a 'url' value if there is a reference and a canonical URL. Hard
      // code 'canonical' here as config entities override the default $rel
      // parameter value to 'edit-form.
      if ($entity->hasLinkTemplate('canonical') && !$entity->isNew() && $url = $entity->toUrl('canonical')
        ->toString(TRUE)) {
        $this->addCacheableDependency($context, $url);
        $values['url'] = $url->getGeneratedUrl();
      }
      elseif ($entity instanceof FileInterface) {
        $values['url'] = $entity->createFileUrl(FALSE);
      }
    }
    return $values;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function constructValue($data, $context) {
    if (isset($data['target_uuid'])) {
      /** @var \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $field_item */
      $field_item = $context['target_instance'];
      if (empty($data['target_uuid'])) {
        throw new InvalidArgumentException(sprintf('If provided "target_uuid" cannot be empty for field "%s".', $field_item->getName()));
      }
      $target_type = $field_item->getFieldDefinition()
        ->getSetting('target_type');
      if (!empty($data['target_type']) && $target_type !== $data['target_type']) {
        throw new UnexpectedValueException(sprintf('The field "%s" property "target_type" must be set to "%s" or omitted.', $field_item->getFieldDefinition()
          ->getName(), $target_type));
      }
      if ($entity = $this->entityRepository
        ->loadEntityByUuid($target_type, $data['target_uuid'])) {
        return [
          'target_id' => $entity->id(),
        ] + array_intersect_key($data, $field_item->getProperties());
      }
      else {
        // Unable to load entity by uuid.
        throw new InvalidArgumentException(sprintf('No "%s" entity found with UUID "%s" for field "%s".', $data['target_type'], $data['target_uuid'], $field_item->getName()));
      }
    }
    return parent::constructValue($data, $context);
  }

}