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);
// the exception message can contain unfiltered user input;
// set the content-type to text to avoid XSS issues
$response->headers->set('Content-Type', 'text/plain; charset=utf-8');
// HttpExceptionInterface is a special type of exception that
// holds status code and header details
if ($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]
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<!-- config/services.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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\EventListener\ExceptionListener">
<tag name="kernel.event_listener"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\EventListener\ExceptionListener;
return function(ContainerConfigurator $container): void {
$services = $container->services();
$services->set(ExceptionListener::class)
->tag('kernel.event_listener')
;
};
Symfony follows this logic to decide which method to call inside the event listener class:
- If the
kernel.event_listener
tag defines themethod
attribute, that's the name of the method to be called; - If no
method
attribute is defined, try to call the__invoke()
magic method (which makes event listeners invokable); - 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:
- If the
kernel.event_listener
tag defines themethod
attribute, that's the name of the method to be called; - If no
method
attribute is defined, try to call the method whose name ison
+ "PascalCased event name" (e.g.onKernelException()
method for thekernel.exception
event); - If that method is not defined either, try to call the
__invoke()
magic method (which makes event listeners invokable); - 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.
The method
property is optional, and when not defined, it defaults to
on
+ uppercased event name. In the example below, the 'foo'
event listener
doesn't explicitly define its method, so the onFoo()
method will be called:
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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
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<!-- config/services.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
https://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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// config/services.php
$container->setParameter('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:
mailer.pre_send
, before the method is called,- 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).