DummyAjaxFormatter.php

<?php

namespace Drupal\image_module_test\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;

/**
 * Empty renderer for a dummy field with an AJAX handler.
 *
 * @FieldFormatter(
 *   id = "image_module_test_dummy_ajax_formatter",
 *   module = "image_module_test",
 *   label = @Translation("Dummy AJAX"),
 *   field_types= {
 *     "image_module_test_dummy_ajax"
 *   }
 * )
 */
class DummyAjaxFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $summary[] = t('Renders nothing');
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $element = [];
        return $element;
    }

}