DDD\Domain\DomainEventPublisher::instance PHP Метод

    public static function instance()
    {
        if (null === static::$instance) {
            static::$instance = new self();
        }
        return static::$instance;
    }

 /**
  * @test
  */
 public function itShouldPublishUserRegisteredEvent()
 {
     $id = DomainEventPublisher::instance()->subscribe($subscriber = new SpySubscriber());
     new User($userId = new UserId(), '*****@*****.**', 'password');
     DomainEventPublisher::instance()->unsubscribe($id);
     $this->assertUserRegisteredEventPublished($subscriber, $userId);
 }