ApplianceTypeListener.php

<?php

namespace Ibms\AppBundle\Form\EventListener;

use Doctrine\ORM\QueryBuilder;
use Ibms\SupplierBundle\Entity\Supplier;
use Ibms\SupplierBundle\Entity\SupplierManager;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormFactoryInterface;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class ApplianceTypeListener implements EventSubscriberInterface
{
    private $factory;
    private $fieldName;
    private $supplierManager;

    public function __construct($fieldName, FormFactoryInterface $factory, SupplierManager $supplierManager)
    {
        $this->factory         = $factory;
        $this->fieldName       = $fieldName;
        $this->supplierManager = $supplierManager;
    }

    public function preBind(FormEvent $event)
    {
        $form = $event->getForm();
        $data = $event->getData();

        $qb = $this->supplierManager->queryApplianceTypes();
        $qb->setParameter('supplierId', $data['supplier']);

        $this->addApplianceType($form, $qb);
    }

    public function preSetData(FormEvent $event)
    {
        $form = $event->getForm();
        $data = $event->getData();

        if (null === $data or null === $data->getSupplier()) {
            return;
        }

        $qb = $this->supplierManager->queryApplianceTypes($data->getSupplier());
        $this->addApplianceType($form, $qb);
    }

    protected function addApplianceType(FormInterface $form, QueryBuilder $qb)
    {
        $form->add($this->factory->createNamed($this->fieldName, 'entity', null, array(
            'class'         => 'Ibms\\SupplierBundle\\Entity\\ApplianceType',
            'empty_value'   => 'Pick one',
            'property'      => 'type',
            'required'      => false,
            'query_builder' => $qb,
        )));
    }

    public static function getSubscribedEvents()
    {
        return array(
            FormEvents::PRE_BIND     => 'preBind',
            FormEvents::PRE_SET_DATA => 'preSetData'
        );
    }
}

ConsumingType.php

<?php

class ConsumingType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        // ...

        // Handles adding a dropdown for Type given the forms set data
        $builder->addEventSubscriber(new ApplianceTypeListener('type', $builder->getFormFactory()));
    }
}