SessionExistsCacheContextTestHooks.php

<?php

declare (strict_types=1);
namespace Drupal\session_exists_cache_context_test\Hook;

use Drupal\Core\Hook\Attribute\Hook;

/**
 * Hook implementations for session_exists_cache_context_test.
 */
class SessionExistsCacheContextTestHooks {
    
    /**
     * Implements hook_page_top().
     */
    public function pageTop(array &$page_top) {
        // Ensure this hook is invoked on every page load.
        $page_top['#cache']['max-age'] = 0;
        $request = \Drupal::request();
        $session_exists = \Drupal::service('session_configuration')->hasSession($request);
        $page_top['session_exists_cache_context_test'] = [
            'label' => [
                '#markup' => '<p>' . ($session_exists ? 'Session exists!' : 'Session does not exist!') . '</p>',
            ],
            'cache_context_value' => [
                '#markup' => '<code>[session.exists]=' . \Drupal::service('cache_context.session.exists')->getContext() . '</code>',
            ],
        ];
        $request = \Drupal::request();
        if ($request->query
            ->get('trigger_session')) {
            $request->getSession()
                ->set('session_exists_cache_context_test', TRUE);
        }
    }

}