AttachmentTestFormatter.php

<?php

namespace Drupal\views_test_formatter\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldFormatter\NumericUnformattedFormatter;

/**
 * Plugin implementation of the 'number_unformatted_with_attachment' formatter.
 *
 * @FieldFormatter(
 *   id = "number_unformatted_with_attachment",
 *   label = @Translation("Unformatted, with attachments"),
 *   field_types = {
 *     "integer",
 *     "decimal",
 *     "float"
 *   }
 * )
 */
class AttachmentTestFormatter extends NumericUnformattedFormatter {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = parent::viewElements($items, $langcode);
        // Add dummy attachments.
        $entity_id = $items->getEntity()
            ->id();
        $elements['#attached']['library'][] = 'foo/fake_library';
        $elements['#attached']['drupalSettings']['AttachmentIntegerFormatter'][$entity_id] = $entity_id;
        return $elements;
    }

}