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How does the Controller Access the Container?

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Our controller is a beautiful, boring service. I love boring things. This means that, if we need to access some other service from here, we need to "inject" it - either through the constructor or by autowiring it as an argument to the controller method - a special superpower of controllers that we'll talk about soon.

The point is: we can't just "grab" a service out of thin air that we haven't injected.... which is why I'm wondering: how the heck does this render() shortcut method work? Certainly that uses the twig service... but we have not injected that into our class

Let's go digging! Hold command or control and click render() to jump into our parent class: AbstractController. This method basically just calls renderView(), which is right above us.

Hmm: renderView() apparently fetches the twig service directly from the container. But, hold on a second. How did our controller service get access to the container? Because, it's not like we're injecting it via autowiring or any other way. So... who is populating the $this->container property?

Oh, but it's even more mysterious than this. Search for parameter_bag in AbstractController to find a method called getParameter(). This method fetches a service directly from the container called parameter_bag.

Let's get some info on this service. Find your terminal and run:

php bin/console debug:container parameter_bag

Woh. It's public false! This is not a service that you should be able to fetch out of the container directly by saying $this->container->get('parameter_bag'). It should give us an error! So what the heck is going on?

Service Subscriber Magic

Here's the answer: our controller extends AbstractController. And AbstractController implements a special interface called ServiceSubscriberInterface. This is actually something we talk about in one of our Doctrine tutorials.

When you implement ServiceSubscriberInterface, it forces you to have a method called getSubscribedServices() where you return an array that says which services you need inside of this class. Then, Symfony will pass you a "mini" container that holds all of these services.

At the top of AbstractController, see this setContainer() method with a ContainerInterface type-hint? That will not be the real container. Nope, that will be the "mini-container" that holds all the services from our getSubscribedServices() method. And because our controller is an autowired service... and this method has @required above it, Symfony knows to call setContainer() immediately after instantiating this object.

This is what gives our controller the ability to fetch all those services that we didn't inject directly. It also fetches them lazily: none of the services are instantiated unless we need to use them.

So... our controller is not just a boring, normal service: it has this extra superpower. But... this is actually something that... any service in our system can implement - it's not special to controllers. So once again, our controller is beautifully boring and normal.

Next: we now have a callable controller! Let's keep going through HttpKernel to see what happens next. Because... one big thing we're still missing is what arguments we should pass to the controller.