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', [
                '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;
    }

}