class Pager

class Pager extends RenderElementBase {
  
  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    return [
      '#pre_render' => [
        static::class . '::preRenderPager',
      ],
      '#theme' => 'pager',
      // The pager ID, to distinguish between multiple pagers on the same page.
'#element' => 0,
      // The heading level to use for the pager.
'#pagination_heading_level' => 'h4',
      // An associative array of query string parameters to append to the pager
      // links.
'#parameters' => [],
      // The number of pages in the list.
'#quantity' => 9,
      // An array of labels for the controls in the pager.
'#tags' => [],
      // The name of the route to be used to build pager links. By default no
      // path is provided, which will make links relative to the current URL.
      // This makes the page more effectively cacheable.
'#route_name' => '<none>',
    ];
  }
  
  /**
   * #pre_render callback to associate the appropriate cache context.
   *
   *
   * @param array $pager
   *   A renderable array of #type => pager.
   *
   * @return array
   */
  public static function preRenderPager(array $pager) {
    // Note: the default pager theme process function
    // template_preprocess_pager() also calls
    // \Drupal\Core\Pager\PagerManagerInterface::getUpdatedParameters(), which
    // maintains the existing query string. Therefore
    // template_preprocess_pager() adds the 'url.query_args' cache context,
    // which causes the more specific cache context below to be optimized away.
    // In other themes, however, that may not be the case.
    $pager['#cache']['contexts'][] = 'url.query_args.pagers:' . $pager['#element'];
    return $pager;
  }

}