class MaintenanceMode

class MaintenanceMode implements MaintenanceModeInterface {
  
  /**
   * The state.
   *
   * @var \Drupal\Core\State\StateInterface
   */
  protected $state;
  
  /**
   * Constructs a new maintenance mode service.
   *
   * @param \Drupal\Core\State\StateInterface $state
   *   The state.
   */
  public function __construct(StateInterface $state) {
    $this->state = $state;
  }
  
  /**
   * {@inheritdoc}
   */
  public function applies(RouteMatchInterface $route_match) {
    if (!$this->state
      ->get('system.maintenance_mode')) {
      return FALSE;
    }
    if ($route = $route_match->getRouteObject()) {
      if ($route->getOption('_maintenance_access')) {
        return FALSE;
      }
    }
    return TRUE;
  }
  
  /**
   * {@inheritdoc}
   */
  public function exempt(AccountInterface $account) {
    return $account->hasPermission('access site in maintenance mode');
  }

}