preprocessors-8.x-1.0-beta8/modules/preprocessors_example/src/Plugin/preprocessors/AnnotationNodePreprocessor.php

<?php

namespace Drupal\preprocessors_example\Plugin\preprocessors;

use Drupal\preprocessors\PreprocessorPluginBase;

/**
 * Provide plugin to preprocess variables for nodes.
 *
 * @Preprocessor(
 *   id = "preprocessors_example.preprocessor.annotation_node",
 *   hooks = {
 *     "node",
 *   },
 *   themes = "*",
 *   weight = 0,
 * )
 */
final class AnnotationNodePreprocessor extends PreprocessorPluginBase {

  /**
   * Preprocess your variables.
   *
   * This method works just like a 'hook_preprocess_HOOK()' function.
   *
   * {@inheritdoc}
   */
  public function preprocess(array &$variables, string $hook, array $info) : void {
    $variables['foo'] = "bar";
  }

}