AggregatorXSSFormatter.php

<?php

namespace Drupal\aggregator\Plugin\Field\FieldFormatter;

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

/**
 * Plugin implementation of the 'aggregator_xss' formatter.
 *
 * @FieldFormatter(
 *   id = "aggregator_xss",
 *   label = @Translation("Aggregator XSS"),
 *   description = @Translation("Filter output for aggregator items"),
 *   field_types = {
 *     "string",
 *     "string_long",
 *   }
 * )
 */
class AggregatorXSSFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            $elements[$delta] = [
                '#type' => 'markup',
                '#markup' => $item->value,
                '#allowed_tags' => _aggregator_allowed_tags(),
            ];
        }
        return $elements;
    }

}