How does it changed this syntax in Symfony 3.0.4 - symfony-forms

Question

This syntax work correctly in symfony2 but it changed in Symfony3.
How could I retrieve parameter $em in FormType with symfony3?
$em=$this->getDoctrine()->getManager();
$form=$this->createForm(EtMenusType($em),$menu);
class EtMenusType extends AbstractType
{
/**
* #param FormBuilderInterface $builder
* #param array $options
*/
public function __construct($em)
{
$this->em=$em;
}

Answer 1

http://symfony.com/doc/current/book/forms#defining-your-forms-as-services
Your form type might have some external dependencies. You can define your form type as a service, and inject all dependencies you need.
You might want to use a service defined as app.my_service in your form type. Create a constructor to your form type to receive the service:
// src/AppBundle/Form/Type/TaskType.php
namespace AppBundle\Form\Type;
use App\Utility\MyService;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
class TaskType extends AbstractType
{
private $myService;
public function __construct(MyService $myService)
{
$this->myService = $myService;
}
public function buildForm(FormBuilderInterface $builder, array $options)
{
// You can now use myService.
$builder
->add('task')
->add('dueDate', null, array('widget' => 'single_text'))
->add('save', SubmitType::class)
;
}
}
Define your form type as a service.
# src/AppBundle/Resources/config/services.yml
services:
app.form.type.task:
class: AppBundle\Form\Type\TaskType
arguments: ["#app.my_service"]
tags:
- { name: form.type }