getOptionsProvider example

/** * {@inheritdoc} */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];

    // Only collect allowed options if there are actually items to display.     if ($items->count()) {
      $provider = $items->getFieldDefinition()
        ->getFieldStorageDefinition()
        ->getOptionsProvider('value', $items->getEntity());
      // Flatten the possible options, to support opt groups.       $options = OptGroup::flattenOptions($provider->getPossibleOptions());

      foreach ($items as $delta => $item) {
        $value = $item->value;
        // If the stored value is in the current set of allowed values, display         // the associated label, otherwise just display the raw value.         $output = $options[$value] ?? $value;
        $elements[$delta] = [
          '#markup' => $output,
          '#allowed_tags' => FieldFilteredMarkup::allowedTags(),
        ];

  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,