City Subscriber for AddressType when select States

AddCityFieldSubscriber.php

<?php

namespace AppBundle\Form\EventListener;

use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\PropertyAccess\PropertyAccess;
use Doctrine\ORM\EntityRepository;

class AddCityFieldSubscriber implements EventSubscriberInterface {

    private $propertyPathToCity;

    public function __construct($propertyPathToCity) {
        $this->propertyPathToCity = $propertyPathToCity;
    }

    public static function getSubscribedEvents() {
        return array(
            FormEvents::POST_SET_DATA => 'preSetData',
            FormEvents::PRE_SUBMIT => 'preSubmit'
        );
    }

    private function addCityForm($form, $state_id) {
        $formOptions = array(
            'class' => 'AppBundle:City',
            'empty_value' => 'City',
            'label' => 'City',
            'attr' => array(
                'class' => 'city_selector form-control',
            ),
            
            'query_builder' => function (EntityRepository $repository) use ($state_id) {
        $qb = $repository->createQueryBuilder('c')
                ->innerJoin('c.state', 's')
                ->where('s.id = :state')
                ->setParameter('state', $state_id)
        ;
        return $qb;
    }
        );
        $form->add($this->propertyPathToCity, 'entity', $formOptions);
    }

    public function preSetData(FormEvent $event) {
        $data = $event->getData();
        $form = $event->getForm();
        
        if (null === $data) {
            return;
        }
        
        $accessor = PropertyAccess::createPropertyAccessor();
        
        $city = $accessor->getValue($data, $this->propertyPathToCity);
        $state_id = ($city) ? $city->getState()->getId() : null;
        
        $this->addCityForm($form, $state_id);
    }

    public function preSubmit(FormEvent $event) {
        
        $data = $event->getData();
        $form = $event->getForm();
        
        $state_id = array_key_exists('state', $data) ? $data['state'] : null;
        
        $this->addCityForm($form, $state_id);
    }

}