class UuidItem

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;
  }

}