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09.

Making an Argument Available to All Controllers

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Making an Argument Available to All Controllers

Here’s our next challenge: pretend that it’s really important to know if the user is on a Mac or not. We’re really wanting to start measuring how hipster our user base is. In fact, it’s so important, that we want to be able to have an $isMac argument in any controller function anywhere in the system. This won’t come from a routing wildcard like normal - we’ll figure this out by reading the User-Agent.

As an example, I’m going to put $isMac into indexAction:

// src/AppBundle/Controller/DinosaurController.php
// ...

public function indexAction($isMac)
{
    // ...

    return $this->render('dinosaurs/index.html.twig', [
        'dinos' => $dinos,
        'isMac' => $isMac
    ]);
}

We’ll pass that into our template, and inside there, use it to print out a threatening message if the user is on a Mac:

{# app/Resources/views/dinosaurs/index.html.twig #}
{# ... #}

{% if isMac %}
    <h3>We love eating Mac's...RAWR!</h3>
{% endif %}

Back to the homepage! If we try this now, we know what’s going to happen. We get a huge error because there is no {isMac} in the route. Symfony has no idea what to pass to the argument. And up until now that’s been the rule: whatever we have in our routing curly brace is available as an argument and there are no exceptions to that rule, except for the Request object.

Allowing Other Controller Arguments

But guys, that’s not true! And we know it. We know that it’s not really about the routing layer. The arguments to the controller come from the request attributes. And sure, the only thing that normally modifies those is the routing layer. But there’s nothing stopping someone else from adding some extra stuff.

In our subscriber, let’s first get an $isMac variable. We’ll look for the User-Agent header and we’ll look for the word mac. Check to see if stripos doesn’t equal false:

// src/AppBundle/EventListener/UserAgentSubscriber.php
// ...

public function onKernelRequest(GetResponseEvent $event)
{
    // ...

    $isMac = stripos($userAgent, 'Mac') !== false;
}

To make this available as an argument, all we need to do is put it in the request attributes:

// src/AppBundle/EventListener/UserAgentSubscriber.php
// ...

public function onKernelRequest(GetResponseEvent $event)
{
    // ...

    $isMac = stripos($userAgent, 'Mac') !== false;
    $request->attributes->set('isMac', $isMac);
}

Seriously, that’s it. When we refresh, it works!

And since I never trust when things work on the first try, let’s change our code to look for mac2. I’m on a mac, but the message hides since we changed that. Go ahead and change that back

The Person Behind the Curtain (Event Listeners)

So why is this important? Because understanding the core of Symfony is letting you do things that previously looked impossible. You’re also going to be able to figure out how magic from outside libraries is working.

For example look at the SensioFrameworkExtraBundle. This is basically a bundle of shortcuts that work via magic. Now that you’ve journeyed to the center of Symfony and back, if you look at each shortcut, you should be able to explain the magic behind each of these.

The one I want to look at now is the ParamConverter:

/**
 * @Route("/blog/{id}")
 * @ParamConverter("post", class="SensioBlogBundle:Post")
 */
public function showAction(Post $post)
{
}

In the example, you can see that the controller has a $post argument, but also that there’s no {post} in the routing. So this should throw an error. The ParamConverter, which works via a listener to kernel.controller, grabs the id off of the request attributes, queries for a Post object via Doctrine with that id, and then adds a new request attribute called post that’s set to that object:

// Summarized version of ParamConverterListener
public function onKernelController(FilterControllerEvent $event)
{
    $request = $event->getRequest();
    $id = $request->attributes->get('id');

    $entity = $this->em->getRepository('SensioBlogBundle:Post')
        ->find($id);
    if (!$entity) {
        throw new NotFoundHttpException('No Post found for '.$id);
    }

    $request->attributes->set('post', $entity);
}

And just by doing that, the showAction can have that $post argument.

If that makes any sense at all, you’re on the verge of really mastering a big part of Symfony.

Before we talk about sub requests, I want to point something out. If you’re playing with things, inside the profiler, there is a Request tab, which is interesting because it shows you the request attributes. You can see the _controller, the routes stuff and the isMac key. By the way, not that it’s necessarily useful, but the fact that there is an _route key does mean that you can have a $_route argument to any controller.