class StringItem

class StringItem extends StringItemBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultStorageSettings() {
    return [
      'max_length' => 255,
      'is_ascii' => FALSE,
    ] + parent::defaultStorageSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => $field_definition->getSetting('is_ascii') === TRUE ? 'varchar_ascii' : 'varchar',
          'length' => (int) $field_definition->getSetting('max_length'),
          'binary' => $field_definition->getSetting('case_sensitive'),
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getConstraints() {
    $constraints = parent::getConstraints();
    if ($max_length = $this->getSetting('max_length')) {
      $constraint_manager = \Drupal::typedDataManager()->getValidationConstraintManager();
      $constraints[] = $constraint_manager->create('ComplexData', [
        'value' => [
          'Length' => [
            'max' => $max_length,
            'maxMessage' => $this->t('%name: may not be longer than @max characters.', [
              '%name' => $this->getFieldDefinition()
                ->getLabel(),
              '@max' => $max_length,
            ]),
          ],
        ],
      ]);
    }
    return $constraints;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
    $random = new Random();
    $values['value'] = $random->word(mt_rand(1, $field_definition->getSetting('max_length')));
    return $values;
  }
  
  /**
   * {@inheritdoc}
   */
  public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
    $element = [];
    $element['max_length'] = [
      '#type' => 'number',
      '#title' => $this->t('Maximum length'),
      '#default_value' => $this->getSetting('max_length'),
      '#required' => TRUE,
      '#description' => $this->t('The maximum length of the field in characters.'),
      '#min' => 1,
      '#disabled' => $has_data,
    ];
    return $element;
  }

}