The EventDispatcher Component

Events

When an event is dispatched, it's identified by a unique name (e.g. kernel.response), which any number of listeners might be listening to. An Event instance is also created and passed to all of the listeners. As you'll see later, the Event object itself often contains data about the event being dispatched.

The Dispatcher

The dispatcher is the central object of the event dispatcher system. In general, a single dispatcher is created, which maintains a registry of listeners. When an event is dispatched via the dispatcher, it notifies all listeners registered with that event:

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use Symfony\Component\EventDispatcher\EventDispatcher;

$dispatcher = new EventDispatcher();

Connecting Listeners

To take advantage of an existing event, you need to connect a listener to the dispatcher so that it can be notified when the event is dispatched. A call to the dispatcher's addListener() method associates any valid PHP callable to an event:

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$listener = new AcmeListener();
$dispatcher->addListener('acme.foo.action', [$listener, 'onFooAction']);

The addListener() method takes up to three arguments:

1. The event name (string) that this listener wants to listen to;
2. A PHP callable that will be executed when the specified event is dispatched;
3. An optional priority, defined as a positive or negative integer (defaults to 0). The higher the number, the earlier the listener is called. If two listeners have the same priority, they are executed in the order that they were added to the dispatcher.

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use Symfony\Component\EventDispatcher\Event;

$dispatcher->addListener('acme.foo.action', function (Event $event) {
    // will be executed when the acme.foo.action event is dispatched
});

Once a listener is registered with the dispatcher, it waits until the event is notified. In the above example, when the acme.foo.action event is dispatched, the dispatcher calls the AcmeListener::onFooAction() method and passes the Event object as the single argument:

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use Symfony\Component\EventDispatcher\Event;

class AcmeListener
{
    // ...

    public function onFooAction(Event $event)
    {
        // ... do something
    }
}

The $event argument is the event object that was passed when dispatching the event. In many cases, a special event subclass is passed with extra information. You can check the documentation or implementation of each event to determine which instance is passed.

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use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\ParameterBag\ParameterBag;
use Symfony\Component\DependencyInjection\Reference;
use Symfony\Component\EventDispatcher\DependencyInjection\RegisterListenersPass;
use Symfony\Component\EventDispatcher\EventDispatcher;

$containerBuilder = new ContainerBuilder(new ParameterBag());
// register the compiler pass that handles the 'kernel.event_listener'
// and 'kernel.event_subscriber' service tags
$containerBuilder->addCompilerPass(new RegisterListenersPass());

$containerBuilder->register('event_dispatcher', EventDispatcher::class);

// registers an event listener
$containerBuilder->register('listener_service_id', \AcmeListener::class)
    ->addTag('kernel.event_listener', [
        'event' => 'acme.foo.action',
        'method' => 'onFooAction',
    ]);

// registers an event subscriber
$containerBuilder->register('subscriber_service_id', \AcmeSubscriber::class)
    ->addTag('kernel.event_subscriber');

Creating and Dispatching an Event

In addition to registering listeners with existing events, you can create and dispatch your own events. This is useful when creating third-party libraries and also when you want to keep different components of your own system flexible and decoupled.

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namespace Acme\Store\Event;

use Acme\Store\Order;
use Symfony\Component\EventDispatcher\Event;

/**
 * The order.placed event is dispatched each time an order is created
 * in the system.
 */
class OrderPlacedEvent extends Event
{
    const NAME = 'order.placed';

    protected $order;

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

    public function getOrder()
    {
        return $this->order;
    }
}

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use Acme\Store\Event\OrderPlacedEvent;
use Acme\Store\Order;

// the order is somehow created or retrieved
$order = new Order();
// ...

// creates the OrderPlacedEvent and dispatches it
$event = new OrderPlacedEvent($order);
$dispatcher->dispatch(OrderPlacedEvent::NAME, $event);

Using Event Subscribers

The most common way to listen to an event is to register an event listener with the dispatcher. This listener can listen to one or more events and is notified each time those events are dispatched.

Another way to listen to events is via an event subscriber. An event subscriber is a PHP class that's able to tell the dispatcher exactly which events it should subscribe to. It implements the EventSubscriberInterface interface, which requires a single static method called getSubscribedEvents(). Take the following example of a subscriber that subscribes to the kernel.response and order.placed events:

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namespace Acme\Store\Event;

use Acme\Store\Event\OrderPlacedEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class StoreSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::RESPONSE => [
                ['onKernelResponsePre', 10],
                ['onKernelResponsePost', -10],
            ],
            OrderPlacedEvent::NAME => 'onStoreOrder',
        ];
    }

    public function onKernelResponsePre(FilterResponseEvent $event)
    {
        // ...
    }

    public function onKernelResponsePost(FilterResponseEvent $event)
    {
        // ...
    }

    public function onStoreOrder(OrderPlacedEvent $event)
    {
        // ...
    }
}

This is very similar to a listener class, except that the class itself can tell the dispatcher which events it should listen to. To register a subscriber with the dispatcher, use the addSubscriber() method:

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use Acme\Store\Event\StoreSubscriber;
// ...

$subscriber = new StoreSubscriber();
$dispatcher->addSubscriber($subscriber);

The dispatcher will automatically register the subscriber for each event returned by the getSubscribedEvents() method. This method returns an array indexed by event names and whose values are either the method name to call or an array composed of the method name to call and a priority (a positive or negative integer that defaults to 0).

The example above shows how to register several listener methods for the same event in subscriber and also shows how to pass the priority of each listener method. The higher the number, the earlier the method is called. In the above example, when the kernel.response event is triggered, the methods onKernelResponsePre() and onKernelResponsePost() are called in that order.

Stopping Event Flow/Propagation

In some cases, it may make sense for a listener to prevent any other listeners from being called. In other words, the listener needs to be able to tell the dispatcher to stop all propagation of the event to future listeners (i.e. to not notify any more listeners). This can be accomplished from inside a listener via the stopPropagation() method:

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use Acme\Store\Event\OrderPlacedEvent;

public function onStoreOrder(OrderPlacedEvent $event)
{
    // ...

    $event->stopPropagation();
}

Now, any listeners to order.placed that have not yet been called will not be called.

It is possible to detect if an event was stopped by using the isPropagationStopped() method which returns a boolean value:

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// ...
$dispatcher->dispatch('foo.event', $event);
if ($event->isPropagationStopped()) {
    // ...
}

EventDispatcher Aware Events and Listeners

The EventDispatcher always passes the dispatched event, the event's name and a reference to itself to the listeners. This can lead to some advanced applications of the EventDispatcher including dispatching other events inside listeners, chaining events or even lazy loading listeners into the dispatcher object.

Dispatcher Shortcuts

If you do not need a custom event object, you can simply rely on a plain Event object. You do not even need to pass this to the dispatcher as it will create one by default unless you specifically pass one:

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$dispatcher->dispatch('order.placed');

Moreover, the event dispatcher always returns whichever event object that was dispatched, i.e. either the event that was passed or the event that was created internally by the dispatcher. This allows for nice shortcuts:

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if (!$dispatcher->dispatch('foo.event')->isPropagationStopped()) {
    // ...
}

Or:

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$event = new OrderPlacedEvent($order);
$order = $dispatcher->dispatch('bar.event', $event)->getOrder();

and so on.

Event Name Introspection

The EventDispatcher instance, as well as the name of the event that is dispatched, are passed as arguments to the listener:

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use Symfony\Component\EventDispatcher\Event;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;

class Foo
{
    public function myEventListener(Event $event, $eventName, EventDispatcherInterface $dispatcher)
    {
        // ... do something with the event name
    }
}