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();
            $options['max'] = $max_length;
            if ($label = $this->getFieldDefinition()
                ->getLabel()) {
                $options['maxMessage'] = $this->t('%name: may not be longer than @max characters.', [
                    '%name' => $label,
                    '@max' => $max_length,
                ]);
            }
            $constraints[] = $constraint_manager->create('ComplexData', [
                'value' => [
                    'Length' => $options,
                ],
            ]);
        }
        return $constraints;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $random = new Random();
        $max_length = $field_definition->getSetting('max_length');
        // When the maximum length is less than 15, or the field needs to be unique,
        // generate a random word using the maximum length.
        if ($max_length <= 15 || $field_definition->getConstraint('UniqueField')) {
            $values['value'] = ucfirst($random->word($max_length));
            return $values;
        }
        // The minimum length is either 10% of the maximum length, or 15 characters
        // long, whichever is greater.
        $min_length = max(ceil($max_length * 0.1), 15);
        // Reduce the max length to allow us to add a period.
        $max_length -= 1;
        // The random value is generated multiple times to create a slight
        // preference towards values that are closer to the minimum length of the
        // string. For values larger than 255 (which is the default maximum value),
        // the bias towards minimum length is increased. This is because the default
        // maximum length of 255 is often used for fields that include shorter
        // values (i.e. title).
        $length = mt_rand($min_length, mt_rand($min_length, $max_length >= 255 ? mt_rand($min_length, $max_length) : $max_length));
        $string = $random->sentences(1);
        while (mb_strlen($string) < $length) {
            $string .= " {$random->sentences(1)}";
        }
        if (mb_strlen($string) > $max_length) {
            $string = substr($string, 0, $length);
            $string = substr($string, 0, strrpos($string, ' '));
        }
        $string = rtrim($string, ' .');
        // Ensure that the string ends with a full stop if there are multiple
        // sentences.
        $values['value'] = $string . (str_contains($string, '.') ? '.' : '');
        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;
    }

}