class ListStringItem

class ListStringItem extends ListItemBase {
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['value'] = DataDefinition::create('string')->setLabel(t('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>' . t('The possible values this field can contain. Enter one value per line, in the format key|label.');
        $description .= '<br/>' . t('The key is the stored value. The label will be used in displayed values and edit forms.');
        $description .= '<br/>' . t('The label is optional: if a line contains a single string, it will be used as key and label.');
        $description .= '</p>';
        $description .= '<p>' . 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 t('Allowed values list: each key must be a string at most 255 characters long.');
        }
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function castAllowedValue($value) {
        return (string) $value;
    }

}