Events and Event Listeners

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Events and Event Listeners

During the execution of a Symfony application, lots of event notifications are triggered. Your application can listen to these notifications and respond to them by executing any piece of code.

Symfony triggers several events related to the kernel while processing the HTTP Request. Third-party bundles may also dispatch events, and you can even dispatch custom events from your own code.

All the examples shown in this article use the same KernelEvents::EXCEPTION event for consistency purposes. In your own application, you can use any event and even mix several of them in the same subscriber.

Creating an Event Listener

The most common way to listen to an event is to register an event listener:

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// src/EventListener/ExceptionListener.php
namespace App\EventListener;

use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;

class ExceptionListener
{
    public function __invoke(ExceptionEvent $event): void
    {
        // You get the exception object from the received event
        $exception = $event->getThrowable();
        $message = sprintf(
            'My Error says: %s with code: %s',
            $exception->getMessage(),
            $exception->getCode()
        );

        // Customize your response object to display the exception details
        $response = new Response();
        $response->setContent($message);

        // HttpExceptionInterface is a special type of exception that
        // holds status code and header details
        if ($exception instanceof HttpExceptionInterface) {
            $response->setStatusCode($exception->getStatusCode());
            $response->headers->replace($exception->getHeaders());
        } else {
            $response->setStatusCode(Response::HTTP_INTERNAL_SERVER_ERROR);
        }

        // sends the modified response object to the event
        $event->setResponse($response);
    }
}

Now that the class is created, you need to register it as a service and notify Symfony that it is an event listener by using a special "tag":

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# config/services.yaml
services:
    App\EventListener\ExceptionListener:
        tags: [kernel.event_listener]

Symfony follows this logic to decide which method to call inside the event listener class:

  1. If the kernel.event_listener tag defines the method attribute, that's the name of the method to be called;
  2. If no method attribute is defined, try to call the __invoke() magic method (which makes event listeners invokable);
  3. If the __invoke() method is not defined either, throw an exception.

Note

There is an optional attribute for the kernel.event_listener tag called priority, which is a positive or negative integer that defaults to 0 and it controls the order in which listeners are executed (the higher the number, the earlier a listener is executed). This is useful when you need to guarantee that one listener is executed before another. The priorities of the internal Symfony listeners usually range from -256 to 256 but your own listeners can use any positive or negative integer.

Note

There is an optional attribute for the kernel.event_listener tag called event which is useful when listener $event argument is not typed. If you configure it, it will change type of $event object. For the kernel.exception event, it is ExceptionEvent. Check out the Symfony events reference to see what type of object each event provides.

With this attribute, Symfony follows this logic to decide which method to call inside the event listener class:

  1. If the kernel.event_listener tag defines the method attribute, that's the name of the method to be called;
  2. If no method attribute is defined, try to call the method whose name is on + "PascalCased event name" (e.g. onKernelException() method for the kernel.exception event);
  3. If that method is not defined either, try to call the __invoke() magic method (which makes event listeners invokable);
  4. If the __invoke() method is not defined either, throw an exception.

Defining Event Listeners with PHP Attributes

An alternative way to define an event listener is to use the AsEventListener PHP attribute. This allows to configure the listener inside its class, without having to add any configuration in external files:

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namespace App\EventListener;

use Symfony\Component\EventDispatcher\Attribute\AsEventListener;

#[AsEventListener]
final class MyListener
{
    public function __invoke(CustomEvent $event): void
    {
        // ...
    }
}

You can add multiple #[AsEventListener()] attributes to configure different methods:

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namespace App\EventListener;

use Symfony\Component\EventDispatcher\Attribute\AsEventListener;

#[AsEventListener(event: CustomEvent::class, method: 'onCustomEvent')]
#[AsEventListener(event: 'foo', priority: 42)]
#[AsEventListener(event: 'bar', method: 'onBarEvent')]
final class MyMultiListener
{
    public function onCustomEvent(CustomEvent $event): void
    {
        // ...
    }

    public function onFoo(): void
    {
        // ...
    }

    public function onBarEvent(): void
    {
        // ...
    }
}

AsEventListener can also be applied to methods directly:

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namespace App\EventListener;

use Symfony\Component\EventDispatcher\Attribute\AsEventListener;

final class MyMultiListener
{
    #[AsEventListener()]
    public function onCustomEvent(CustomEvent $event): void
    {
        // ...
    }

    #[AsEventListener(event: 'foo', priority: 42)]
    public function onFoo(): void
    {
        // ...
    }

    #[AsEventListener(event: 'bar')]
    public function onBarEvent(): void
    {
        // ...
    }
}

Note

Note that the attribute doesn't require its event parameter to be set if the method already type-hints the expected event.

Creating an Event Subscriber

Another way to listen to events is via an event subscriber, which is a class that defines one or more methods that listen to one or various events. The main difference with the event listeners is that subscribers always know the events to which they are listening.

If different event subscriber methods listen to the same event, their order is defined by the priority parameter. This value is a positive or negative integer which defaults to 0. The higher the number, the earlier the method is called. Priority is aggregated for all listeners and subscribers, so your methods could be called before or after the methods defined in other listeners and subscribers. To learn more about event subscribers, read The EventDispatcher Component.

The following example shows an event subscriber that defines several methods which listen to the same kernel.exception event:

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// src/EventSubscriber/ExceptionSubscriber.php
namespace App\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class ExceptionSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents(): array
    {
        // return the subscribed events, their methods and priorities
        return [
            KernelEvents::EXCEPTION => [
                ['processException', 10],
                ['logException', 0],
                ['notifyException', -10],
            ],
        ];
    }

    public function processException(ExceptionEvent $event): void
    {
        // ...
    }

    public function logException(ExceptionEvent $event): void
    {
        // ...
    }

    public function notifyException(ExceptionEvent $event): void
    {
        // ...
    }
}

That's it! Your services.yaml file should already be setup to load services from the EventSubscriber directory. Symfony takes care of the rest.

Tip

If your methods are not called when an exception is thrown, 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.

Request Events, Checking Types

A single page can make several requests (one main request, and then multiple sub-requests - typically when embedding controllers in templates). For the core Symfony events, you might need to check to see if the event is for a "main" request or a "sub request":

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// src/EventListener/RequestListener.php
namespace App\EventListener;

use Symfony\Component\HttpKernel\Event\RequestEvent;

class RequestListener
{
    public function onKernelRequest(RequestEvent $event): void
    {
        if (!$event->isMainRequest()) {
            // don't do anything if it's not the main request
            return;
        }

        // ...
    }
}

Certain things, like checking information on the real request, may not need to be done on the sub-request listeners.

Listeners or Subscribers

Listeners and subscribers can be used in the same application indistinctly. The decision to use either of them is usually a matter of personal taste. However, there are some minor advantages for each of them:

  • Subscribers are easier to reuse because the knowledge of the events is kept in the class rather than in the service definition. This is the reason why Symfony uses subscribers internally;
  • Listeners are more flexible because bundles can enable or disable each of them conditionally depending on some configuration value.

Event Aliases

When configuring event listeners and subscribers via dependency injection, Symfony's core events can also be referred to by the fully qualified class name (FQCN) of the corresponding event class:

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// src/EventSubscriber/RequestSubscriber.php
namespace App\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\RequestEvent;

class RequestSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents(): array
    {
        return [
            RequestEvent::class => 'onKernelRequest',
        ];
    }

    public function onKernelRequest(RequestEvent $event): void
    {
        // ...
    }
}

Internally, the event FQCN are treated as aliases for the original event names. Since the mapping already happens when compiling the service container, event listeners and subscribers using FQCN instead of event names will appear under the original event name when inspecting the event dispatcher.

This alias mapping can be extended for custom events by registering the compiler pass AddEventAliasesPass:

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// src/Kernel.php
namespace App;

use App\Event\MyCustomEvent;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\EventDispatcher\DependencyInjection\AddEventAliasesPass;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel
{
    protected function build(ContainerBuilder $container): void
    {
        $container->addCompilerPass(new AddEventAliasesPass([
            MyCustomEvent::class => 'my_custom_event',
        ]));
    }
}

The compiler pass will always extend the existing list of aliases. Because of that, it is safe to register multiple instances of the pass with different configurations.

Debugging Event Listeners

You can find out what listeners are registered in the event dispatcher using the console. To show all events and their listeners, run:

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$ php bin/console debug:event-dispatcher

You can get registered listeners for a particular event by specifying its name:

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$ php bin/console debug:event-dispatcher kernel.exception

or can get everything which partial matches the event name:

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$ php bin/console debug:event-dispatcher kernel // matches "kernel.exception", "kernel.response" etc.
$ php bin/console debug:event-dispatcher Security // matches "Symfony\Component\Security\Http\Event\CheckPassportEvent"

The security system uses an event dispatcher per firewall. Use the --dispatcher option to get the registered listeners for a particular event dispatcher:

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$ php bin/console debug:event-dispatcher --dispatcher=security.event_dispatcher.main

How to Set Up Before and After Filters

It is quite common in web application development to need some logic to be performed right before or directly 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, define some token configuration as parameters:

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# config/services.yaml
parameters:
    tokens:
        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 App\Controller;

interface TokenAuthenticatedController
{
    // ...
}

A controller that implements this interface looks like this:

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namespace App\Controller;

use App\Controller\TokenAuthenticatedController;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Response;

class FooController extends AbstractController implements TokenAuthenticatedController
{
    // An action that needs authentication
    public function bar(): Response
    {
        // ...
    }
}

Creating an Event Subscriber

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

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

use App\Controller\TokenAuthenticatedController;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ControllerEvent;
use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
use Symfony\Component\HttpKernel\KernelEvents;

class TokenSubscriber implements EventSubscriberInterface
{
    public function __construct(
        private array $tokens
    ) {
    }

    public function onKernelController(ControllerEvent $event): void
    {
        $controller = $event->getController();

        // when a controller class defines multiple action methods, the controller
        // is returned as [$controllerInstance, 'methodName']
        if (is_array($controller)) {
            $controller = $controller[0];
        }

        if ($controller 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(): array
    {
        return [
            KernelEvents::CONTROLLER => 'onKernelController',
        ];
    }
}

That's it! Your services.yaml 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. To create an "after" listener, create a listener class and register 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(ControllerEvent $event): void
{
    // ...

    if ($controller 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\ResponseEvent;

public function onKernelResponse(ResponseEvent $event): void
{
    // check to see if onKernelController marked this as a token "auth'ed" request
    if (!$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(): array
{
    return [
        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!

How to Customize a Method Behavior without Using Inheritance

If you want to do something right before, or directly after a method is called, you can dispatch an event respectively at the beginning or at the end of the method:

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class CustomMailer
{
    // ...

    public function send(string $subject, string $message): mixed
    {
        // dispatch an event before the method
        $event = new BeforeSendMailEvent($subject, $message);
        $this->dispatcher->dispatch($event, 'mailer.pre_send');

        // get $subject and $message from the event, they may have been modified
        $subject = $event->getSubject();
        $message = $event->getMessage();

        // the real method implementation is here
        $returnValue = ...;

        // do something after the method
        $event = new AfterSendMailEvent($returnValue);
        $this->dispatcher->dispatch($event, 'mailer.post_send');

        return $event->getReturnValue();
    }
}

In this example, two events are dispatched:

  1. mailer.pre_send, before the method is called,
  2. and mailer.post_send after the method is called.

Each uses a custom Event class to communicate information to the listeners of the two events. For example, BeforeSendMailEvent might look like this:

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// src/Event/BeforeSendMailEvent.php
namespace App\Event;

use Symfony\Contracts\EventDispatcher\Event;

class BeforeSendMailEvent extends Event
{
    public function __construct(
        private string $subject,
        private string $message,
    ) {
    }

    public function getSubject(): string
    {
        return $this->subject;
    }

    public function setSubject(string $subject): string
    {
        $this->subject = $subject;
    }

    public function getMessage(): string
    {
        return $this->message;
    }

    public function setMessage(string $message): void
    {
        $this->message = $message;
    }
}

And the AfterSendMailEvent even like this:

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// src/Event/AfterSendMailEvent.php
namespace App\Event;

use Symfony\Contracts\EventDispatcher\Event;

class AfterSendMailEvent extends Event
{
    public function __construct(
        private mixed $returnValue,
    ) {
    }

    public function getReturnValue(): mixed
    {
        return $this->returnValue;
    }

    public function setReturnValue(mixed $returnValue): void
    {
        $this->returnValue = $returnValue;
    }
}

Both events allow you to get some information (e.g. getMessage()) and even change that information (e.g. setMessage()).

Now, you can create an event subscriber to hook into this event. For example, you could listen to the mailer.post_send event and change the method's return value:

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// src/EventSubscriber/MailPostSendSubscriber.php
namespace App\EventSubscriber;

use App\Event\AfterSendMailEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class MailPostSendSubscriber implements EventSubscriberInterface
{
    public function onMailerPostSend(AfterSendMailEvent $event): void
    {
        $returnValue = $event->getReturnValue();
        // modify the original $returnValue value

        $event->setReturnValue($returnValue);
    }

    public static function getSubscribedEvents(): array
    {
        return [
            'mailer.post_send' => 'onMailerPostSend',
        ];
    }
}

That's it! Your subscriber should be called automatically (or read more about event subscriber configuration).

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