AdminNegotiator.php

<?php

namespace Drupal\user\Theme;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\DependencyInjection\DeprecatedServicePropertyTrait;
use Drupal\Core\Routing\AdminContext;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;
use Symfony\Component\DependencyInjection\Attribute\Autowire;

/**
 * Sets the active theme on admin pages.
 */
class AdminNegotiator implements ThemeNegotiatorInterface {
  use DeprecatedServicePropertyTrait;
  
  /**
   * The service properties that should raise a deprecation error.
   */
  private array $deprecatedProperties = [
    'entityTypeManager' => 'entity_type.manager',
  ];
  public function __construct(protected AccountInterface $user, protected ConfigFactoryInterface $configFactory, #[Autowire(service: 'router.admin_context')] protected AdminContext $adminContext) {
  }
  
  /**
   * {@inheritdoc}
   */
  public function applies(RouteMatchInterface $route_match) {
    $is_admin_route = $this->adminContext
      ->isAdminRoute($route_match->getRouteObject());
    return $is_admin_route && $this->user
      ->hasPermission('view the administration theme');
  }
  
  /**
   * {@inheritdoc}
   */
  public function determineActiveTheme(RouteMatchInterface $route_match) {
    return $this->configFactory
      ->get('system.theme')
      ->get('admin') ?: NULL;
  }

}