ResponsiveImageTestFormatter.php

<?php

namespace Drupal\responsive_image_test_module\Plugin\Field\FieldFormatter;

use Drupal\responsive_image\Plugin\Field\FieldFormatter\ResponsiveImageFormatter;
use Drupal\Core\Field\FieldItemListInterface;

/**
 * Plugin to test responsive image formatter.
 *
 * @FieldFormatter(
 *   id = "responsive_image_test",
 *   label = @Translation("Responsive image test"),
 *   field_types = {
 *     "image",
 *   }
 * )
 */
class ResponsiveImageTestFormatter extends ResponsiveImageFormatter {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = parent::viewElements($items, $langcode);
        // Unset #item_attributes to test that the theme function can handle that.
        foreach ($elements as &$element) {
            if (isset($element['#item_attributes'])) {
                unset($element['#item_attributes']);
            }
        }
        return $elements;
    }

}