What does exactly Dependency Injection in Symfony2 do? - dependency-injection

Question

I registered my services.yml file like below :
services:
PMI.form.users_tasks:
class: PMI\UserBundle\Form\UsersTasksType
arguments:
EntityManager: "#doctrine.orm.default_entity_manager"
I can list it by php app/console container:debug, so that mean my service is registered properly.
In my UsersTasksType class I have like below :
class UsersTasksType extends AbstractType
{
protected $ur;
public function __construct(EntityManager $ur )
{
$this->setUr($ur);
}
// Get and setters
}
Does Dependency Injection mean that I don't have to pass the EntityManager to the class constructor anymore? Or what ?
Because when I have to run the code below :
$form = $this->createForm(new UsersTasksType(), $entity);
I get this error:
Catchable Fatal Error: Argument 1 passed to PMI\UserBundle\Form\UsersTasksType::__construct() must be an instance of Doctrine\ORM\EntityManager, none given, called in C:\wamp\www\PMI_sf2\src\PMI\UserBundle\Controller\UsersTasksController.php on line 74 and defined in C:\wamp\www\PMI_sf2\src\PMI\UserBundle\Form\UsersTasksType.php line 19
And I have to do something below :
$em = $this->container->get('doctrine.orm.entity_manager');
$form = $this->createForm(new UsersTasksType($em), $entity);
So what would be the whole purpose of Dependency Injection ?

Answer 1

Dependency Injection basically gives one service (in this case, your UserTasksType) access to another service (in this case, your the entity manager).
arguments:
EntityManager: "#doctrine.orm.default_entity_manager"
These two lines tell Symfony to expect the entity manager service to be passed into the constructor when you instantiate a new UserTasksType object, which effectively gives your UserTasksType access to the entity manager.
If you aren't using the entity manager in your UserTasksType, there is no need to inject it in the constructor and you could get rid of the two lines above and the __construct() / setUr() methods in your UserTasksType.
A better example to help you understand DIC might be that you have a service that is written specifically to send emails (Swiftmail, for e.g.) and you need to inject it into another service so that service can send emails.
By adding
arguments: [ #mailer ]
to your service definition, your services constructor will expect your mailer service
__construct ($mailer)
{
$this->mailer = $mailer;
}
which will give it access to send emails
someFunction()
{
//do something useful, then send an email using the swift mailer service
$this->mailer->sendEmail();
}
Check out the latest Symfony docs for more of an explanation.
http://symfony.com/doc/current/book/service_container.html