class ListStringItem

class ListStringItem extends ListItemBase {
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Text value'))
      ->addConstraint('Length', [
      'max' => 255,
    ])
      ->setRequired(TRUE);
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'varchar',
          'length' => 255,
        ],
      ],
      'indexes' => [
        'value' => [
          'value',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  protected function allowedValuesDescription() {
    $description = '<p>' . $this->t('The possible values this field can contain. Enter one value per line, in the format key|label.');
    $description .= '<br/>' . $this->t('The key is the stored value. The label will be used in displayed values and edit forms.');
    $description .= '<br/>' . $this->t('The label is optional: if a line contains a single string, it will be used as key and label.');
    $description .= '</p>';
    $description .= '<p>' . $this->t('Allowed HTML tags in labels: @tags', [
      '@tags' => FieldFilteredMarkup::displayAllowedTags(),
    ]) . '</p>';
    return $description;
  }
  
  /**
   * {@inheritdoc}
   */
  protected static function validateAllowedValue($option) {
    if (mb_strlen($option) > 255) {
      return new TranslatableMarkup('Allowed values list: each key must be a string at most 255 characters long.');
    }
  }
  
  /**
   * {@inheritdoc}
   */
  protected static function castAllowedValue($value) {
    return (string) $value;
  }

}