class ListItemBase

abstract class ListItemBase extends FieldItemBase implements OptionsProviderInterface {
    
    /**
     * {@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());
        if (empty($allowed_options)) {
            $values['value'] = NULL;
            return $values;
        }
        $values['value'] = array_rand($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) {
        if (!array_key_exists('allowed_values', $form_state->getStorage())) {
            $form_state->set('allowed_values', $this->getFieldDefinition()
                ->getSetting('allowed_values'));
        }
        $form['field_storage_submit']['#submit'][] = [
            static::class,
            'submitFieldStorageUpdate',
        ];
        $form['field_storage_submit']['#limit_validation_errors'] = [];
        $allowed_values = $form_state->getStorage()['allowed_values'];
        $allowed_values_function = $this->getSetting('allowed_values_function');
        if (!$form_state->get('items_count')) {
            $form_state->set('items_count', max(count($allowed_values), 0));
        }
        $wrapper_id = Html::getUniqueId('allowed-values-wrapper');
        $element['allowed_values'] = [
            '#element_validate' => [
                [
                    static::class,
                    'validateAllowedValues',
                ],
            ],
            '#field_has_data' => $has_data,
            '#allowed_values' => $allowed_values,
            '#required' => TRUE,
            '#prefix' => '<div id="' . $wrapper_id . '">',
            '#suffix' => '</div>',
            '#access' => empty($allowed_values_function),
            'help_text' => [
                '#markup' => $this->allowedValuesDescription(),
            ],
        ];
        $element['allowed_values']['table'] = [
            '#type' => 'table',
            '#header' => [
                $this->t('Allowed values'),
                $this->t('Delete'),
                $this->t('Weight'),
            ],
            '#attributes' => [
                'id' => 'allowed-values-order',
                'data-field-list-table' => TRUE,
                'class' => [
                    'allowed-values-table',
                ],
            ],
            '#tabledrag' => [
                [
                    'action' => 'order',
                    'relationship' => 'sibling',
                    'group' => 'weight',
                ],
            ],
            '#attached' => [
                'library' => [
                    'core/drupal.fieldListKeyboardNavigation',
                    'field_ui/drupal.field_ui',
                ],
            ],
        ];
        $max = $form_state->get('items_count');
        $entity_type_id = $this->getFieldDefinition()
            ->getTargetEntityTypeId();
        $field_name = $this->getFieldDefinition()
            ->getName();
        $current_keys = array_keys($allowed_values);
        for ($delta = 0; $delta <= $max; $delta++) {
            $element['allowed_values']['table'][$delta] = [
                '#attributes' => [
                    'class' => [
                        'draggable',
                    ],
                ],
                '#weight' => $delta,
            ];
            $element['allowed_values']['table'][$delta]['item'] = [
                'label' => [
                    '#type' => 'textfield',
                    '#title' => $this->t('Name'),
                    '#weight' => -30,
                    '#default_value' => isset($current_keys[$delta]) ? $allowed_values[$current_keys[$delta]] : '',
                    '#required' => $delta === 0,
                ],
                'key' => [
                    '#type' => 'textfield',
                    '#maxlength' => 255,
                    '#title' => $this->t('Value'),
                    '#default_value' => $current_keys[$delta] ?? '',
                    '#weight' => -20,
                    '#required' => $delta === 0,
                ],
            ];
            $element['allowed_values']['table'][$delta]['delete'] = [
                '#type' => 'submit',
                '#value' => $this->t('Remove'),
                '#name' => "remove_row_button__{$delta}",
                '#id' => "remove_row_button__{$delta}",
                '#delta' => $delta,
                '#submit' => [
                    [
                        static::class,
                        'deleteSubmit',
                    ],
                ],
                '#limit_validation_errors' => [],
                '#ajax' => [
                    'callback' => [
                        static::class,
                        'deleteAjax',
                    ],
                    'wrapper' => $wrapper_id,
                    'effect' => 'fade',
                ],
            ];
            $element['allowed_values']['table'][$delta]['weight'] = [
                '#type' => 'weight',
                '#title' => $this->t('Weight for row @number', [
                    '@number' => $delta + 1,
                ]),
                '#title_display' => 'invisible',
                '#delta' => 50,
                '#default_value' => 0,
                '#attributes' => [
                    'class' => [
                        'weight',
                    ],
                ],
            ];
            // Disable the remove button if there is only one row in the table.
            if ($max === 0) {
                $element['allowed_values']['table'][0]['delete']['#attributes']['disabled'] = 'disabled';
            }
            if ($delta < count($allowed_values)) {
                $query = \Drupal::entityQuery($entity_type_id)->accessCheck(FALSE)
                    ->condition($field_name, $current_keys[$delta]);
                $entity_ids = $query->execute();
                if (!empty($entity_ids)) {
                    $element['allowed_values']['table'][$delta]['item']['key']['#attributes']['disabled'] = 'disabled';
                    $element['allowed_values']['table'][$delta]['delete']['#attributes']['disabled'] = 'disabled';
                    $element['allowed_values']['table'][$delta]['delete'] += [
                        'message' => [
                            '#type' => 'item',
                            '#markup' => $this->t('Cannot be removed: option in use.'),
                        ],
                    ];
                }
            }
        }
        $element['allowed_values']['table']['#max_delta'] = $max;
        $element['allowed_values']['add_more_allowed_values'] = [
            '#type' => 'submit',
            '#name' => 'add_more_allowed_values',
            '#value' => $this->t('Add another item'),
            '#attributes' => [
                'class' => [
                    'field-add-more-submit',
                ],
                'data-field-list-button' => TRUE,
            ],
            // Allow users to add another row without requiring existing rows to have
            // values.
'#limit_validation_errors' => [],
            '#submit' => [
                [
                    static::class,
                    'addMoreSubmit',
                ],
            ],
            '#ajax' => [
                'callback' => [
                    static::class,
                    'addMoreAjax',
                ],
                'wrapper' => $wrapper_id,
                'effect' => 'fade',
                'progress' => [
                    'type' => 'throbber',
                    'message' => $this->t('Adding a new item...'),
                ],
            ],
        ];
        $element['allowed_values_function'] = [
            '#type' => 'item',
            '#title' => $this->t('Allowed values list'),
            '#markup' => $this->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;
    }
    
    /**
     * Adds a new option.
     *
     * @param array $form
     *   The form array to add elements to.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public static function addMoreSubmit(array $form, FormStateInterface $form_state) {
        $form_state->set('items_count', $form_state->get('items_count') + 1);
        $form_state->setRebuild();
    }
    
    /**
     * Ajax callback for the "Add another item" button.
     */
    public static function addMoreAjax(array $form, FormStateInterface $form_state) {
        $button = $form_state->getTriggeringElement();
        // Go one level up in the form.
        $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -1));
        $delta = $element['table']['#max_delta'];
        $element['table'][$delta]['item']['#prefix'] = '<div class="ajax-new-content" data-drupal-selector="field-list-add-more-focus-target">' . ($element['table'][$delta]['item']['#prefix'] ?? '');
        $element['table'][$delta]['item']['#suffix'] = ($element['table'][$delta]['item']['#suffix'] ?? '') . '</div>';
        // Enable the remove button for the first row if there are more rows.
        if ($delta > 0 && isset($element['table'][0]['delete']['#attributes']['disabled']) && !isset($element['table'][0]['item']['key']['#attributes']['disabled'])) {
            unset($element['table'][0]['delete']['#attributes']['disabled']);
        }
        $response = new AjaxResponse();
        $response->addCommand(new InsertCommand(NULL, $element));
        $response->addCommand(new FocusFirstCommand('[data-drupal-selector="field-list-add-more-focus-target"]'));
        return $response;
    }
    
    /**
     * Deletes a row/option.
     *
     * @param array $form
     *   The form array to add elements to.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public static function deleteSubmit(array $form, FormStateInterface $form_state) {
        $allowed_values = $form_state->getStorage()['allowed_values'];
        $button = $form_state->getTriggeringElement();
        $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -1));
        $item_to_be_removed = $element['item']['label']['#default_value'];
        $remaining_allowed_values = array_diff($allowed_values, [
            $item_to_be_removed,
        ]);
        $form_state->set('allowed_values', $remaining_allowed_values);
        // The user input is directly modified to preserve the rest of the data on
        // the page as it cannot be rebuilt from a fresh form state.
        $user_input = $form_state->getUserInput();
        NestedArray::unsetValue($user_input, $element['#parents']);
        // Reset the keys in the array.
        $table_parents = $element['#parents'];
        array_pop($table_parents);
        $new_values = array_values(NestedArray::getValue($user_input, $table_parents));
        NestedArray::setValue($user_input, $table_parents, $new_values);
        $form_state->setUserInput($user_input);
        $form_state->set('items_count', $form_state->get('items_count') - 1);
        $form_state->setRebuild();
    }
    
    /**
     * Ajax callback for per row delete button.
     */
    public static function deleteAjax(array $form, FormStateInterface $form_state) {
        $button = $form_state->getTriggeringElement();
        return NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -3));
    }
    
    /**
     * 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 $form_state
     *   The current state of the form for the form this element belongs to.
     *
     * @see \Drupal\Core\Render\Element\FormElementBase::processPattern()
     */
    public static function validateAllowedValues($element, FormStateInterface $form_state) {
        $items = array_filter(array_map(function ($item) use ($element) {
            $current_element = $element['table'][$item];
            if ($current_element['item']['key']['#value'] !== NULL && $current_element['item']['label']['#value']) {
                return $current_element['item']['key']['#value'] . '|' . $current_element['item']['label']['#value'];
            }
            elseif ($current_element['item']['key']['#value']) {
                return $current_element['item']['key']['#value'];
            }
            elseif ($current_element['item']['label']['#value']) {
                return $current_element['item']['label']['#value'];
            }
            return NULL;
        }, Element::children($element['table'])), function ($item) {
            return $item;
        });
        if ($reordered_items = $form_state->getValue([
            $element['#parents'],
            'table',
        ])) {
            uksort($items, function ($a, $b) use ($reordered_items) {
                $a_weight = $reordered_items[$a]['weight'] ?? 0;
                $b_weight = $reordered_items[$b]['weight'] ?? 0;
                return $a_weight <=> $b_weight;
            });
        }
        $values = static::extractAllowedValues($items, $element['#field_has_data']);
        if (!is_array($values)) {
            $form_state->setError($element, new TranslatableMarkup('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;
                }
            }
            $form_state->setValueForElement($element, $values);
        }
    }
    
    /**
     * Extracts the allowed values array from the allowed_values element.
     *
     * @param array $list
     *   The raw string or array 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(array $list, bool $has_data) {
        $values = [];
        $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;
    }
    
    /**
     * Resets the static variable on field storage update.
     */
    public static function submitFieldStorageUpdate() {
        drupal_static_reset('options_allowed_values');
    }

}