FilterTestPlaceholders.php

<?php

namespace Drupal\filter_test\Plugin\Filter;

use Drupal\Component\Render\FormattableMarkup;
use Drupal\Core\Security\TrustedCallbackInterface;
use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;

/**
 * Provides a test filter to use placeholders.
 *
 * @Filter(
 *   id = "filter_test_placeholders",
 *   title = @Translation("Testing filter"),
 *   description = @Translation("Appends a placeholder to the content; associates #lazy_builder callback."),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_TRANSFORM_REVERSIBLE
 * )
 */
class FilterTestPlaceholders extends FilterBase implements TrustedCallbackInterface {
    
    /**
     * {@inheritdoc}
     */
    public function process($text, $langcode) {
        $result = new FilterProcessResult($text);
        $placeholder = $result->createPlaceholder('\\Drupal\\filter_test\\Plugin\\Filter\\FilterTestPlaceholders::renderDynamicThing', [
            'llama',
        ]);
        $result->setProcessedText($text . '<p>' . $placeholder . '</p>');
        return $result;
    }
    
    /**
     * #lazy_builder callback; builds a render array containing the dynamic thing.
     *
     * @param string $thing
     *   A "thing" string.
     *
     * @return array
     *   A renderable array.
     */
    public static function renderDynamicThing($thing) {
        return [
            '#markup' => new FormattableMarkup('This is a dynamic @thing.', [
                '@thing' => $thing,
            ]),
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function trustedCallbacks() {
        return [
            'renderDynamicThing',
        ];
    }

}