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 a 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
    {
        // ...
    }
}

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()
    {
        // return the subscribed events, their methods and priorities
        return [
            KernelEvents::EXCEPTION => [
                ['processException', 10],
                ['logException', 0],
                ['notifyException', -10],
            ],
        ];
    }

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

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

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

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 $event)
    {
        if (!$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)
    {
        // ...
    }
}

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 $containerBuilder)
    {
        $containerBuilder->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

Learn more

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