OMG: You just made a Sub-Request!¶

Why did I show this? Why is this important to understanding the core? Open up the profiler and go back to the Timeline. Hmm, it looks normal at first and we can see where our controller and template are run. But hey, what’s that darker bar behind the template? Scroll way down to find a second request called a sub-request. OMG!

The sub-request is totally independent: it goes through the entire same process we just learned. It executes the kernel.request listeners, it dispatches the kernel.controller event, it calls the _latestTweetsAction controller and has kernel.response on the end. It really is like we are handling two totally separate request-response cycles.

Heck, you can even click to see the profiler for just the sub-request.

Request Attributes in Sub-Request Controllers¶

Remember that in UserAgentSubscriber, we’re adding isMac to the request attributes and that means it’s available as an argument to any controller. That’s no different with our sub request controller, since this listener is called for that request too. To prove it, I’m going to add the $isMac argument. Let’s pass this into our template:

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

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

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

Let’s print it out to make sure it’s working:

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

<p class="text-center">{{ isMac ? 'on a Mac' : 'Not on a Mac' }}</p>
{# ... #}

When we go back and refresh we see that on the top it shows that we’re on a Mac, and on the bottom inside the Tweets area, we’re on a Mac too! Yay, no surprises!

A Disturbance in the Request¶

Here is where things get crazy. Go back to UserAgentSubscriber. Let’s add an override so it’s easier for us to play with this “is Mac” stuff, since I’m pretty permanently using one.

If there’s a query parameter, called notMac, that’s set to some value like 1, then let’s always set $isMac to false:

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

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

    $isMac = stripos($userAgent, 'Mac') !== false;
    if ($request->query->get('notMac')) {
        $isMac = false;
    }
    $request->attributes->set('isMac', $isMac);
}

Back on the browser, when I refresh, I’m still on a Mac. But if I add a ?notMac=1 to the URL, it goes away. The override correctly makes it look like I’m not on a Mac.

Now scroll down. Woh! The sub request still thinks we’re on a Mac. Something just short circuited in the system. But before we fix it, let’s dive one level deeper and see how sub-requests really work.