class OptionsWidgetBase

abstract class OptionsWidgetBase extends WidgetBase {
  
  /**
   * Abstract over the actual field columns.
   *
   * Allows different field types to reuse those widgets.
   *
   * @var string
   */
  protected $column;
  
  /**
   * Tracks whether the field is required.
   */
  protected bool $required;
  
  /**
   * Tracks whether the data is multi-valued.
   */
  protected bool $multiple;
  
  /**
   * Tracks whether the field has a value.
   */
  // phpcs:ignore Drupal.NamingConventions.ValidVariableName.LowerCamelName
  protected bool $has_value;
  
  /**
   * The array of options for the widget.
   */
  protected array $options;
  
  /**
   * {@inheritdoc}
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
    $property_names = $this->fieldDefinition
      ->getFieldStorageDefinition()
      ->getPropertyNames();
    $this->column = $property_names[0];
  }
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    // Prepare some properties for the child methods to build the actual form
    // element.
    $this->required = $element['#required'];
    $this->multiple = $this->fieldDefinition
      ->getFieldStorageDefinition()
      ->isMultiple();
    $this->has_value = isset($items[0]->{$this->column});
    // Add our custom validator.
    $element['#element_validate'][] = [
      static::class,
      'validateElement',
    ];
    $element['#key_column'] = $this->column;
    // The rest of the $element is built by child method implementations.
    return $element;
  }
  
  /**
   * Form validation handler for widget elements.
   *
   * @param array $element
   *   The form element.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   */
  public static function validateElement(array $element, FormStateInterface $form_state) {
    if ($element['#required'] && $element['#value'] == '_none') {
      if (isset($element['#required_error'])) {
        $form_state->setError($element, $element['#required_error']);
      }
      else {
        $form_state->setError($element, new TranslatableMarkup('@name field is required.', [
          '@name' => $element['#title'],
        ]));
      }
    }
    // Massage submitted form values.
    // Drupal\Core\Field\WidgetBase::submit() expects values as
    // an array of values keyed by delta first, then by column, while our
    // widgets return the opposite.
    if (is_array($element['#value'])) {
      $values = array_values($element['#value']);
    }
    else {
      $values = [
        $element['#value'],
      ];
    }
    // Filter out the 'none' option. Use a strict comparison, because
    // 0 == 'any string'.
    $index = array_search('_none', $values, TRUE);
    if ($index !== FALSE) {
      unset($values[$index]);
    }
    // Transpose selections from field => delta to delta => field.
    $items = [];
    foreach ($values as $value) {
      $items[] = [
        $element['#key_column'] => $value,
      ];
    }
    $form_state->setValueForElement($element, $items);
  }
  
  /**
   * Returns the array of options for the widget.
   *
   * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
   *   The entity for which to return options.
   *
   * @return array
   *   The array of options for the widget.
   */
  protected function getOptions(FieldableEntityInterface $entity) {
    if (!isset($this->options)) {
      // Limit the settable options for the current user account.
      $options = $this->fieldDefinition
        ->getFieldStorageDefinition()
        ->getOptionsProvider($this->column, $entity)
        ->getSettableOptions(\Drupal::currentUser());
      // Add an empty option if the widget needs one.
      if ($empty_label = $this->getEmptyLabel()) {
        $options = [
          '_none' => $empty_label,
        ] + $options;
      }
      $module_handler = \Drupal::moduleHandler();
      $context = [
        'fieldDefinition' => $this->fieldDefinition,
        'entity' => $entity,
        'widget' => $this,
      ];
      $module_handler->alter('options_list', $options, $context);
      array_walk_recursive($options, [
        $this,
        'sanitizeLabel',
      ]);
      // Options might be nested ("optgroups"). If the widget does not support
      // nested options, flatten the list.
      if (!$this->supportsGroups()) {
        $options = OptGroup::flattenOptions($options);
      }
      $this->options = $options;
    }
    return $this->options;
  }
  
  /**
   * Determines selected options from the incoming field values.
   *
   * @param \Drupal\Core\Field\FieldItemListInterface $items
   *   The field values.
   *
   * @return array
   *   The array of corresponding selected options.
   */
  protected function getSelectedOptions(FieldItemListInterface $items) {
    // We need to check against a flat list of options.
    $flat_options = OptGroup::flattenOptions($this->getOptions($items->getEntity()));
    $selected_options = [];
    foreach ($items as $item) {
      $value = $item->{$this->column};
      // Keep the value if it actually is in the list of options (needs to be
      // checked against the flat list).
      if (isset($flat_options[$value])) {
        $selected_options[] = $value;
      }
    }
    return $selected_options;
  }
  
  /**
   * Indicates whether the widgets support optgroups.
   *
   * @return bool
   *   TRUE if the widget supports optgroups, FALSE otherwise.
   */
  protected function supportsGroups() {
    return FALSE;
  }
  
  /**
   * Sanitizes a string label to display as an option.
   *
   * @param string $label
   *   The label to sanitize.
   */
  protected function sanitizeLabel(&$label) {
    // Allow a limited set of HTML tags.
    $label = FieldFilteredMarkup::create($label);
  }
  
  /**
   * Returns the empty option label to add to the list of options, if any.
   *
   * @return string|null
   *   Either a label of the empty option, or NULL.
   */
  protected function getEmptyLabel() {
  }

}