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;
    }

}