UuidItem.php

<?php

namespace Drupal\Core\Field\Plugin\Field\FieldType;

use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\Field\FieldDefinitionInterface;

/**
 * Defines the 'uuid' entity field type.
 *
 * The field uses a newly generated UUID as default value.
 *
 * @FieldType(
 *   id = "uuid",
 *   label = @Translation("UUID"),
 *   description = @Translation("An entity field containing a UUID."),
 *   no_ui = TRUE,
 *   default_formatter = "string"
 * )
 */
class UuidItem extends StringItem {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultStorageSettings() {
        return [
            'max_length' => 128,
            'is_ascii' => TRUE,
        ] + parent::defaultStorageSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function applyDefaultValue($notify = TRUE) {
        // Default to one field item with a generated UUID.
        $uuid = \Drupal::service('uuid');
        $this->setValue([
            'value' => $uuid->generate(),
        ], $notify);
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        $schema = parent::schema($field_definition);
        $schema['unique keys']['value'] = [
            'value',
        ];
        return $schema;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $values['value'] = \Drupal::service('uuid')->generate();
        return $values;
    }

}