ListNormalizer.php

<?php

namespace Drupal\serialization\Normalizer;

use Drupal\Core\TypedData\ListInterface;

/**
 * Converts list objects to arrays.
 *
 * Ordinarily, this would be handled automatically by Serializer, but since
 * there is a TypedDataNormalizer and the Field class extends TypedData, any
 * Field will be handled by that Normalizer instead of being traversed. This
 * class ensures that TypedData classes that also implement ListInterface are
 * traversed instead of simply returning getValue().
 */
class ListNormalizer extends NormalizerBase {
    
    /**
     * {@inheritdoc}
     */
    protected $supportedInterfaceOrClass = ListInterface::class;
    
    /**
     * {@inheritdoc}
     */
    public function normalize($object, $format = NULL, array $context = []) {
        $attributes = [];
        foreach ($object as $fieldItem) {
            $attributes[] = $this->serializer
                ->normalize($fieldItem, $format, $context);
        }
        return $attributes;
    }

}