Event Dispatcher

Syntax#

$dispatcher->dispatch(string $eventName, Event $event);
$dispatcher->addListener(string $eventName, callable $listener, int $priority = 0);
$dispatcher->addSubscriber(EventSubscriberInterface $subscriber);

Remarks#

It is often best to use a single instance of EventDispatcher in your application that you inject into the objects that need to fire events.
It is best practice to have a single location where you manage the configuration of, and add event listeners to, your EventDispatcher. The Symfony framework uses the Dependency Injection Container.
These patterns will allow you to easily change your event listeners without needing to change the code of any module that is dispatching events.
The decoupling of event dispatch from event listener configuration is what makes the Symfony EventDispatcher so powerful
The EventDispatcher helps you satisfy the Open/Closed Principle.

Event Dispatcher Quick Start

use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Component\EventDispatcher\Event;
use Symfony\Component\EventDispatcher\GenericEvent;

// you may store this in a dependency injection container for use as a service
$dispatcher = new EventDispatcher();

// you can attach listeners to specific events directly with any callable
$dispatcher->addListener('an.event.occurred', function(Event $event) {
    // process $event
});

// somewhere in your system, an event happens
$data = // some important object
$event = new GenericEvent($data, ['more' => 'event information']);

// dispatch the event
// our listener on "an.event.occurred" above will be called with $event
// we could attach many more listeners to this event, and they too would be called
$dispatcher->dispatch('an.event.occurred', $event);

Event Subscribers

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

$dispatcher = new EventDispatcher();

// you can attach event subscribers, which allow a single object to subscribe 
// to many events at once
$dispatcher->addSubscriber(new class implements EventSubscriberInterface {
    public static function getSubscribedEvents()
    {
        // here we subscribe our class methods to listen to various events
        return [
            // when anything fires a "an.event.occurred" call "onEventOccurred"
            'an.event.occurred' => 'onEventOccurred',
            // an array of listeners subscribes multiple methods to one event
            'another.event.happened' => ['whenAnotherHappened', 'sendEmail'],
        ];
    }

    function onEventOccurred(Event $event) {
        // process $event
    }

    function whenAnotherHappened(Event $event) {
        // process $event
    }

    function sendEmail(Event $event) {
        // process $event
    }
});