class FieldItemNormalizer

class FieldItemNormalizer extends NormalizerBase {
    use FieldableEntityNormalizerTrait;
    use SerializedColumnNormalizerTrait;
    
    /**
     * {@inheritdoc}
     */
    protected $supportedInterfaceOrClass = FieldItemInterface::class;
    
    /**
     * {@inheritdoc}
     */
    public function normalize($field_item, $format = NULL, array $context = []) {
        // The values are wrapped in an array, and then wrapped in another array
        // keyed by field name so that field items can be merged by the
        // FieldNormalizer. This is necessary for the EntityReferenceItemNormalizer
        // to be able to place values in the '_links' array.
        $field = $field_item->getParent();
        return [
            $field->getName() => [
                $this->normalizedFieldValues($field_item, $format, $context),
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function denormalize($data, $class, $format = NULL, array $context = []) {
        if (!isset($context['target_instance'])) {
            throw new InvalidArgumentException('$context[\'target_instance\'] must be set to denormalize with the FieldItemNormalizer');
        }
        if ($context['target_instance']->getParent() == NULL) {
            throw new InvalidArgumentException('The field item passed in via $context[\'target_instance\'] must have a parent set.');
        }
        $field_item = $context['target_instance'];
        $this->checkForSerializedStrings($data, $class, $field_item);
        // If this field is translatable, we need to create a translated instance.
        if (isset($data['lang'])) {
            $langcode = $data['lang'];
            unset($data['lang']);
            $field_definition = $field_item->getFieldDefinition();
            if ($field_definition->isTranslatable()) {
                $field_item = $this->createTranslatedInstance($field_item, $langcode);
            }
        }
        $field_item->setValue($this->constructValue($data, $context));
        return $field_item;
    }
    
    /**
     * Normalizes field values for an item.
     *
     * @param \Drupal\Core\Field\FieldItemInterface $field_item
     *   The field item instance.
     * @param string|null $format
     *   The normalization format.
     * @param array $context
     *   The context passed into the normalizer.
     *
     * @return array
     *   An array of field item values, keyed by property name.
     */
    protected function normalizedFieldValues(FieldItemInterface $field_item, $format, array $context) {
        $normalized = [];
        // We normalize each individual property, so each can do their own casting,
        // if needed.
        
        /** @var \Drupal\Core\TypedData\TypedDataInterface $property */
        $field_properties = !empty($field_item->getProperties(TRUE)) ? TypedDataInternalPropertiesHelper::getNonInternalProperties($field_item) : $field_item->getValue();
        foreach ($field_properties as $property_name => $property) {
            $normalized[$property_name] = $this->serializer
                ->normalize($property, $format, $context);
        }
        if (isset($context['langcode'])) {
            $normalized['lang'] = $context['langcode'];
        }
        return $normalized;
    }
    
    /**
     * Get a translated version of the field item instance.
     *
     * To indicate that a field item applies to one translation of an entity and
     * not another, the property path must originate with a translation of the
     * entity. This is the reason for using target_instances, from which the
     * property path can be traversed up to the root.
     *
     * @param \Drupal\Core\Field\FieldItemInterface $item
     *   The untranslated field item instance.
     * @param $langcode
     *   The langcode.
     *
     * @return \Drupal\Core\Field\FieldItemInterface
     *   The translated field item instance.
     */
    protected function createTranslatedInstance(FieldItemInterface $item, $langcode) {
        // Remove the untranslated item that was created for the default language
        // by FieldNormalizer::denormalize().
        $items = $item->getParent();
        $delta = $item->getName();
        unset($items[$delta]);
        // Instead, create a new item for the entity in the requested language.
        $entity = $item->getEntity();
        $entity_translation = $entity->hasTranslation($langcode) ? $entity->getTranslation($langcode) : $entity->addTranslation($langcode);
        $field_name = $item->getFieldDefinition()
            ->getName();
        return $entity_translation->get($field_name)
            ->appendItem();
    }

}