Services can now execute cron tasks

Additionally to hooks, it is now possible that services are called directly when cron tasks are executed.

Before

/**
 * Implements hook_cron().
 *
 * Fires updateIndex() in the plugins for all indexable active search pages,
 * and cleans up dirty words.
 */
-function search_cron() {
  /** @var \Drupal\search\SearchPageRepositoryInterface $search_page_repository */
  $search_page_repository = \Drupal::service('search.search_page_repository');
  foreach ($search_page_repository->getIndexableSearchPages() as $entity) {
    $entity->getPlugin()->updateIndex();
  }
}

After

1. Implement the runCron() method of Drupal\Core\CronSubscriberInterface in the chosen service:

class SearchPageRepository implements SearchPageRepositoryInterface, CronSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public function runCron(): void {
    // Fires updateIndex() in the plugins for all indexable active search pages,
    // and cleans up dirty words.
    foreach ($this->getIndexableSearchPages() as $entity) {
      $entity->getPlugin()->updateIndex();
    }
  }

}

2. Add the cron tag to the service definition:

   search.search_page_repository:
     class: Drupal\search\SearchPageRepository
     arguments: ['@config.factory', '@entity_type.manager']
     tags:
      - { name: cron }

Hooks are not yet deprecated, although that may happen in a future update. A single module can have have multiple services that all have their own cron task. Note that alternative implementations of cron runners like the ultimate_cron module will need to be updated to support discovery of such cron services.