Symfony\Component\EventDispatcher\EventDispatcher::hasListeners PHP Method

    public function hasListeners($eventName = null)
    {
        return (Boolean) count($this->getListeners($eventName));
    }

 public function testDispatcherHasListener()
 {
     $this->assertTrue($this->dispatcher->hasListeners(ServiredEvent::PRE_PAYMENT_SUCCESS));
     $this->assertTrue($this->dispatcher->hasListeners(ServiredEvent::PRE_PAYMENT_FAILED));
     $this->assertTrue($this->dispatcher->hasListeners(ServiredEvent::POST_PAYMENT_SUCCESS));
     $this->assertTrue($this->dispatcher->hasListeners(ServiredEvent::POST_PAYMENT_FAILED));
 }