FilterImageLazyLoad.php

<?php

declare (strict_types=1);
namespace Drupal\filter\Plugin\Filter;

use Drupal\Component\Utility\Html;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\filter\Attribute\Filter;
use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;
use Drupal\filter\Plugin\FilterInterface;

/**
 * Provides a filter to lazy load tracked images.
 */
final class FilterImageLazyLoad extends FilterBase {
  
  /**
   * {@inheritdoc}
   */
  public function process($text, $langcode) : FilterProcessResult {
    $result = new FilterProcessResult($text);
    // If there are no images, return early.
    if (stripos($text, '<img ') === FALSE && stripos($text, 'data-entity-type="file"') === FALSE) {
      return $result;
    }
    return $result->setProcessedText($this->transformImages($text));
  }
  
  /**
   * Transform markup of images to include loading="lazy".
   *
   * @param string $text
   *   The markup to transform.
   *
   * @return string
   *   The transformed text with loading attribute added.
   */
  private function transformImages(string $text) : string {
    $dom = Html::load($text);
    $xpath = new \DOMXPath($dom);
    // Only set loading="lazy" if no existing loading attribute is specified and
    // dimensions are specified.
    foreach ($xpath->query('//img[not(@loading="eager") and @width and @height]') as $element) {
      assert($element instanceof \DOMElement);
      $element->setAttribute('loading', 'lazy');
    }
    return Html::serialize($dom);
  }

}