FilterAlign.php

<?php

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 align elements.
 */
class FilterAlign extends FilterBase {
    
    /**
     * {@inheritdoc}
     */
    public function process($text, $langcode) {
        $result = new FilterProcessResult($text);
        if (stristr($text, 'data-align') !== FALSE) {
            $dom = Html::load($text);
            $xpath = new \DOMXPath($dom);
            foreach ($xpath->query('//*[@data-align]') as $node) {
                // Read the data-align attribute's value, then delete it.
                $align = $node->getAttribute('data-align');
                $node->removeAttribute('data-align');
                // If one of the allowed alignments, add the corresponding class.
                if (in_array($align, [
                    'left',
                    'center',
                    'right',
                ])) {
                    $classes = $node->getAttribute('class');
                    $classes = strlen($classes) > 0 ? explode(' ', $classes) : [];
                    $classes[] = 'align-' . $align;
                    $node->setAttribute('class', implode(' ', $classes));
                }
            }
            $result->setProcessedText(Html::serialize($dom));
        }
        return $result;
    }
    
    /**
     * {@inheritdoc}
     */
    public function tips($long = FALSE) {
        if ($long) {
            return $this->t('
        <p>You can align images, videos, blockquotes and so on to the left, right or center. Examples:</p>
        <ul>
          <li>Align an image to the left: <code>&lt;img src="" data-align="left" /&gt;</code></li>
          <li>Align an image to the center: <code>&lt;img src="" data-align="center" /&gt;</code></li>
          <li>Align an image to the right: <code>&lt;img src="" data-align="right" /&gt;</code></li>
          <li>… and you can apply this to other elements as well: <code>&lt;video src="" data-align="center" /&gt;</code></li>
        </ul>');
        }
        else {
            return $this->t('You can align images (<code>data-align="center"</code>), but also videos, blockquotes, and so on.');
        }
    }

}