class FieldNormalizer

class FieldNormalizer extends NormalizerBase implements DenormalizerInterface {
    
    /**
     * The interface or class that this Normalizer supports.
     *
     * @var string
     */
    protected $supportedInterfaceOrClass = FieldItemListInterface::class;
    
    /**
     * {@inheritdoc}
     */
    public function normalize($field, $format = NULL, array $context = []) {
        
        /** @var \Drupal\Core\Field\FieldItemListInterface $field */
        $normalized_items = $this->normalizeFieldItems($field, $format, $context);
        assert($context['resource_object'] instanceof ResourceObject);
        return $context['resource_object']->getResourceType()
            ->getFieldByInternalName($field->getName())
            ->hasOne() ? array_shift($normalized_items) ?: CacheableNormalization::permanent(NULL) : CacheableNormalization::aggregate($normalized_items);
    }
    
    /**
     * {@inheritdoc}
     */
    public function denormalize($data, $class, $format = NULL, array $context = []) {
        $field_definition = $context['field_definition'];
        assert($field_definition instanceof FieldDefinitionInterface);
        $resource_type = $context['resource_type'];
        assert($resource_type instanceof ResourceType);
        // If $data contains items (recognizable by numerical array keys, which
        // Drupal's Field API calls "deltas"), then it already is itemized; it's not
        // using the simplified JSON structure that JSON:API generates.
        $is_already_itemized = is_array($data) && array_reduce(array_keys($data), function ($carry, $index) {
            return $carry && is_numeric($index);
        }, TRUE);
        $itemized_data = $is_already_itemized ? $data : [
            0 => $data,
        ];
        // Single-cardinality fields don't need itemization.
        $field_item_class = $field_definition->getItemDefinition()
            ->getClass();
        if (count($itemized_data) === 1 && $resource_type->getFieldByInternalName($field_definition->getName())
            ->hasOne()) {
            return $this->serializer
                ->denormalize($itemized_data[0], $field_item_class, $format, $context);
        }
        $data_internal = [];
        foreach ($itemized_data as $delta => $field_item_value) {
            $data_internal[$delta] = $this->serializer
                ->denormalize($field_item_value, $field_item_class, $format, $context);
        }
        return $data_internal;
    }
    
    /**
     * Helper function to normalize field items.
     *
     * @param \Drupal\Core\Field\FieldItemListInterface $field
     *   The field object.
     * @param string $format
     *   The format.
     * @param array $context
     *   The context array.
     *
     * @return \Drupal\jsonapi\Normalizer\Value\FieldItemNormalizerValue[]
     *   The array of normalized field items.
     */
    protected function normalizeFieldItems(FieldItemListInterface $field, $format, array $context) {
        $normalizer_items = [];
        if (!$field->isEmpty()) {
            foreach ($field as $field_item) {
                $normalizer_items[] = $this->serializer
                    ->normalize($field_item, $format, $context);
            }
        }
        return $normalizer_items;
    }
    
    /**
     * {@inheritdoc}
     */
    public function hasCacheableSupportsMethod() : bool {
        return TRUE;
    }

}