class TextItem

class TextItem extends TextItemBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultStorageSettings() {
    return [
      'max_length' => 255,
    ] + parent::defaultStorageSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'varchar',
          'length' => $field_definition->getSetting('max_length'),
        ],
        'format' => [
          'type' => 'varchar',
          'length' => 255,
        ],
      ],
      'indexes' => [
        'format' => [
          'format',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getConstraints() {
    $constraint_manager = \Drupal::typedDataManager()->getValidationConstraintManager();
    $constraints = parent::getConstraints();
    if ($max_length = $this->getSetting('max_length')) {
      $constraints[] = $constraint_manager->create('ComplexData', [
        'properties' => [
          'value' => [
            'Length' => [
              'max' => $max_length,
              'maxMessage' => $this->t('%name: the text may not be longer than @max characters.', [
                '%name' => $this->getFieldDefinition()
                  ->getLabel(),
                '@max' => $max_length,
              ]),
            ],
          ],
        ],
      ]);
    }
    return $constraints;
  }
  
  /**
   * {@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,
    ];
    $element += parent::storageSettingsForm($form, $form_state, $has_data);
    return $element;
  }

}