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);
    }

}