class ListItemBase

abstract class ListItemBase extends FieldItemBase implements OptionsProviderInterface {
    use AllowedTagsXssTrait;
    
    /**
     * {@inheritdoc}
     */
    public static function defaultStorageSettings() {
        return [
            'allowed_values' => [],
            'allowed_values_function' => '',
        ] + parent::defaultStorageSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getPossibleValues(AccountInterface $account = NULL) {
        // Flatten options firstly, because Possible Options may contain group
        // arrays.
        $flatten_options = OptGroup::flattenOptions($this->getPossibleOptions($account));
        return array_keys($flatten_options);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getPossibleOptions(AccountInterface $account = NULL) {
        return $this->getSettableOptions($account);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSettableValues(AccountInterface $account = NULL) {
        // Flatten options firstly, because Settable Options may contain group
        // arrays.
        $flatten_options = OptGroup::flattenOptions($this->getSettableOptions($account));
        return array_keys($flatten_options);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSettableOptions(AccountInterface $account = NULL) {
        $allowed_options = options_allowed_values($this->getFieldDefinition()
            ->getFieldStorageDefinition(), $this->getEntity());
        return $allowed_options;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $allowed_options = options_allowed_values($field_definition->getFieldStorageDefinition());
        $values['value'] = array_rand($allowed_options);
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    public function isEmpty() {
        return empty($this->value) && (string) $this->value !== '0';
    }
    
    /**
     * {@inheritdoc}
     */
    public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
        $allowed_values = $this->getSetting('allowed_values');
        $allowed_values_function = $this->getSetting('allowed_values_function');
        $element['allowed_values'] = [
            '#type' => 'textarea',
            '#title' => t('Allowed values list'),
            '#default_value' => $this->allowedValuesString($allowed_values),
            '#rows' => 10,
            '#access' => empty($allowed_values_function),
            '#element_validate' => [
                [
                    get_class($this),
                    'validateAllowedValues',
                ],
            ],
            '#field_has_data' => $has_data,
            '#field_name' => $this->getFieldDefinition()
                ->getName(),
            '#entity_type' => $this->getEntity()
                ->getEntityTypeId(),
            '#allowed_values' => $allowed_values,
        ];
        $element['allowed_values']['#description'] = $this->allowedValuesDescription();
        $element['allowed_values_function'] = [
            '#type' => 'item',
            '#title' => t('Allowed values list'),
            '#markup' => t('The value of this field is being determined by the %function function and may not be changed.', [
                '%function' => $allowed_values_function,
            ]),
            '#access' => !empty($allowed_values_function),
            '#value' => $allowed_values_function,
        ];
        return $element;
    }
    
    /**
     * Provides the field type specific allowed values form element #description.
     *
     * @return string
     *   The field type allowed values form specific description.
     */
    protected abstract function allowedValuesDescription();
    
    /**
     * #element_validate callback for options field allowed values.
     *
     * @param $element
     *   An associative array containing the properties and children of the
     *   generic form element.
     * @param $form_state
     *   The current state of the form for the form this element belongs to.
     *
     * @see \Drupal\Core\Render\Element\FormElement::processPattern()
     */
    public static function validateAllowedValues($element, FormStateInterface $form_state) {
        $values = static::extractAllowedValues($element['#value'], $element['#field_has_data']);
        if (!is_array($values)) {
            $form_state->setError($element, t('Allowed values list: invalid input.'));
        }
        else {
            // Check that keys are valid for the field type.
            foreach ($values as $key => $value) {
                if ($error = static::validateAllowedValue($key)) {
                    $form_state->setError($element, $error);
                    break;
                }
            }
            // Prevent removing values currently in use.
            if ($element['#field_has_data']) {
                $lost_keys = array_keys(array_diff_key($element['#allowed_values'], $values));
                if (_options_values_in_use($element['#entity_type'], $element['#field_name'], $lost_keys)) {
                    $form_state->setError($element, t('Allowed values list: some values are being removed while currently in use.'));
                }
            }
            $form_state->setValueForElement($element, $values);
        }
    }
    
    /**
     * Extracts the allowed values array from the allowed_values element.
     *
     * @param string $string
     *   The raw string to extract values from.
     * @param bool $has_data
     *   The current field already has data inserted or not.
     *
     * @return array|null
     *   The array of extracted key/value pairs, or NULL if the string is invalid.
     *
     * @see \Drupal\options\Plugin\Field\FieldType\ListItemBase::allowedValuesString()
     */
    protected static function extractAllowedValues($string, $has_data) {
        $values = [];
        $list = explode("\n", $string);
        $list = array_map('trim', $list);
        $list = array_filter($list, 'strlen');
        $generated_keys = $explicit_keys = FALSE;
        foreach ($list as $position => $text) {
            // Check for an explicit key.
            $matches = [];
            if (preg_match('/(.*)\\|(.*)/', $text, $matches)) {
                // Trim key and value to avoid unwanted spaces issues.
                $key = trim($matches[1]);
                $value = trim($matches[2]);
                $explicit_keys = TRUE;
            }
            elseif (!static::validateAllowedValue($text)) {
                $key = $value = $text;
                $explicit_keys = TRUE;
            }
            elseif (!$has_data) {
                $key = (string) $position;
                $value = $text;
                $generated_keys = TRUE;
            }
            else {
                return;
            }
            $values[$key] = $value;
        }
        // We generate keys only if the list contains no explicit key at all.
        if ($explicit_keys && $generated_keys) {
            return;
        }
        return $values;
    }
    
    /**
     * Checks whether a candidate allowed value is valid.
     *
     * @param string $option
     *   The option value entered by the user.
     *
     * @return string
     *   The error message if the specified value is invalid, NULL otherwise.
     */
    protected static function validateAllowedValue($option) {
    }
    
    /**
     * Generates a string representation of an array of 'allowed values'.
     *
     * This string format is suitable for edition in a textarea.
     *
     * @param array $values
     *   An array of values, where array keys are values and array values are
     *   labels.
     *
     * @return string
     *   The string representation of the $values array:
     *    - Values are separated by a carriage return.
     *    - Each value is in the format "value|label" or "value".
     */
    protected function allowedValuesString($values) {
        $lines = [];
        foreach ($values as $key => $value) {
            $lines[] = "{$key}|{$value}";
        }
        return implode("\n", $lines);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function storageSettingsToConfigData(array $settings) {
        if (isset($settings['allowed_values'])) {
            $settings['allowed_values'] = static::structureAllowedValues($settings['allowed_values']);
        }
        return $settings;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function storageSettingsFromConfigData(array $settings) {
        if (isset($settings['allowed_values'])) {
            $settings['allowed_values'] = static::simplifyAllowedValues($settings['allowed_values']);
        }
        return $settings;
    }
    
    /**
     * Simplifies allowed values to a key-value array from the structured array.
     *
     * @param array $structured_values
     *   Array of items with a 'value' and 'label' key each for the allowed
     *   values.
     *
     * @return array
     *   Allowed values were the array key is the 'value' value, the value is
     *   the 'label' value.
     *
     * @see \Drupal\options\Plugin\Field\FieldType\ListItemBase::structureAllowedValues()
     */
    protected static function simplifyAllowedValues(array $structured_values) {
        $values = [];
        foreach ($structured_values as $item) {
            if (is_array($item['label'])) {
                // Nested elements are embedded in the label.
                $item['label'] = static::simplifyAllowedValues($item['label']);
            }
            $values[$item['value']] = $item['label'];
        }
        return $values;
    }
    
    /**
     * Creates a structured array of allowed values from a key-value array.
     *
     * @param array $values
     *   Allowed values were the array key is the 'value' value, the value is
     *   the 'label' value.
     *
     * @return array
     *   Array of items with a 'value' and 'label' key each for the allowed
     *   values.
     *
     * @see \Drupal\options\Plugin\Field\FieldType\ListItemBase::simplifyAllowedValues()
     */
    protected static function structureAllowedValues(array $values) {
        $structured_values = [];
        foreach ($values as $value => $label) {
            if (is_array($label)) {
                $label = static::structureAllowedValues($label);
            }
            $structured_values[] = [
                'value' => static::castAllowedValue($value),
                'label' => $label,
            ];
        }
        return $structured_values;
    }
    
    /**
     * Converts a value to the correct type.
     *
     * @param mixed $value
     *   The value to cast.
     *
     * @return mixed
     *   The casted value.
     */
    protected static function castAllowedValue($value) {
        return $value;
    }

}