SerializedItem.php

<?php

namespace Drupal\entity_test\Plugin\Field\FieldType;

use Drupal\Core\Field\Attribute\FieldType;
use Drupal\Core\Field\FieldItemBase;
use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\TypedData\DataDefinition;

/**
 * Defines the 'serialized_item' entity field type.
 */
class SerializedItem extends FieldItemBase {
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['value'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Test serialized value'))
            ->setRequired(TRUE);
        return $properties;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        return [
            'columns' => [
                'value' => [
                    'type' => 'blob',
                    'size' => 'big',
                    'serialize' => TRUE,
                ],
            ],
        ];
    }

}