How to Set Up Before and After Filters

3.3 version

How to Set Up Before and After Filters

It is quite common in web application development to need some logic to be executed just before or just after your controller actions acting as filters or hooks.

Some web frameworks define methods like preExecute() and postExecute(), but there is no such thing in Symfony. The good news is that there is a much better way to interfere with the Request -> Response process using the EventDispatcher component.

Token Validation Example

Imagine that you need to develop an API where some controllers are public but some others are restricted to one or some clients. For these private features, you might provide a token to your clients to identify themselves.

So, before executing your controller action, you need to check if the action is restricted or not. If it is restricted, you need to validate the provided token.

Note

Please note that for simplicity in this recipe, tokens will be defined in config and neither database setup nor authentication via the Security component will be used.

Before Filters with the kernel.controller Event

First, store some basic token configuration using config.yml and the parameters key:

  • YAML
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    # app/config/config.yml
    parameters:
        tokens:
            client1: pass1
            client2: pass2
    
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    <!-- app/config/config.xml -->
    <?xml version="1.0" encoding="UTF-8" ?>
    <container xmlns="http://symfony.com/schema/dic/services"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/dic/services
            http://symfony.com/schema/dic/services/services-1.0.xsd">
    
        <parameters>
            <parameter key="tokens" type="collection">
                <parameter key="client1">pass1</parameter>
                <parameter key="client2">pass2</parameter>
            </parameter>
        </parameters>
    </container>
    
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    // app/config/config.php
    $container->setParameter('tokens', array(
        'client1' => 'pass1',
        'client2' => 'pass2',
    ));
    

Tag Controllers to Be Checked

A kernel.controller (aka KernelEvents::CONTROLLER) listener gets notified on every request, right before the controller is executed. So, first, you need some way to identify if the controller that matches the request needs token validation.

A clean and easy way is to create an empty interface and make the controllers implement it:

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namespace AppBundle\Controller;

interface TokenAuthenticatedController
{
    // ...
}

A controller that implements this interface simply looks like this:

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namespace AppBundle\Controller;

use AppBundle\Controller\TokenAuthenticatedController;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;

class FooController extends Controller implements TokenAuthenticatedController
{
    // An action that needs authentication
    public function barAction()
    {
        // ...
    }
}

Creating an Event Subscriber

Next, you'll need to create an event listener, which will hold the logic that you want to be executed before your controllers. If you're not familiar with event listeners, you can learn more about them at Events and Event Listeners:

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// src/AppBundle/EventSubscriber/TokenSubscriber.php
namespace AppBundle\EventSubscriber;

use AppBundle\Controller\TokenAuthenticatedController;
use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
use Symfony\Component\HttpKernel\Event\FilterControllerEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\KernelEvents;

class TokenSubscriber implements EventSubscriberInterface
{
    private $tokens;

    public function __construct($tokens)
    {
        $this->tokens = $tokens;
    }

    public function onKernelController(FilterControllerEvent $event)
    {
        $controller = $event->getController();

        /*
         * $controller passed can be either a class or a Closure.
         * This is not usual in Symfony but it may happen.
         * If it is a class, it comes in array format
         */
        if (!is_array($controller)) {
            return;
        }

        if ($controller[0] instanceof TokenAuthenticatedController) {
            $token = $event->getRequest()->query->get('token');
            if (!in_array($token, $this->tokens)) {
                throw new AccessDeniedHttpException('This action needs a valid token!');
            }
        }
    }

    public static function getSubscribedEvents()
    {
        return array(
            KernelEvents::CONTROLLER => 'onKernelController',
        );
    }
}

That's it! Your services.yml file should already be setup to load services from the EventSubscriber directory. Symfony takes care of the rest. Your TokenSubscriber onKernelController() method will be executed on each request. If the controller that is about to be executed implements TokenAuthenticatedController, token authentication is applied. This lets you have a "before" filter on any controller you want.

Tip

If your subscriber is not called on each request, double-check that you're loading services from the EventSubscriber directory and have autoconfigure enabled. You can also manually add the kernel.event_subscriber tag.

After Filters with the kernel.response Event

In addition to having a "hook" that's executed before your controller, you can also add a hook that's executed after your controller. For this example, imagine that you want to add a sha1 hash (with a salt using that token) to all responses that have passed this token authentication.

Another core Symfony event - called kernel.response (aka KernelEvents::RESPONSE) - is notified on every request, but after the controller returns a Response object. Creating an "after" listener is as easy as creating a listener class and registering it as a service on this event.

For example, take the TokenSubscriber from the previous example and first record the authentication token inside the request attributes. This will serve as a basic flag that this request underwent token authentication:

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public function onKernelController(FilterControllerEvent $event)
{
    // ...

    if ($controller[0] instanceof TokenAuthenticatedController) {
        $token = $event->getRequest()->query->get('token');
        if (!in_array($token, $this->tokens)) {
            throw new AccessDeniedHttpException('This action needs a valid token!');
        }

        // mark the request as having passed token authentication
        $event->getRequest()->attributes->set('auth_token', $token);
    }
}

Now, configure the subscriber to listen to another event and add onKernelResponse(). This will look for the auth_token flag on the request object and set a custom header on the response if it's found:

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// add the new use statement at the top of your file
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;

public function onKernelResponse(FilterResponseEvent $event)
{
    // check to see if onKernelController marked this as a token "auth'ed" request
    if (!$token = $event->getRequest()->attributes->get('auth_token')) {
        return;
    }

    $response = $event->getResponse();

    // create a hash and set it as a response header
    $hash = sha1($response->getContent().$token);
    $response->headers->set('X-CONTENT-HASH', $hash);
}

public static function getSubscribedEvents()
{
    return array(
        KernelEvents::CONTROLLER => 'onKernelController',
        KernelEvents::RESPONSE => 'onKernelResponse',
    );
}

That's it! The TokenSubscriber is now notified before every controller is executed (onKernelController()) and after every controller returns a response (onKernelResponse()). By making specific controllers implement the TokenAuthenticatedController interface, your listener knows which controllers it should take action on. And by storing a value in the request's "attributes" bag, the onKernelResponse() method knows to add the extra header. Have fun!

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.