Use events in symfony

The other day I had to add some logic right after an user was saved to the database. I ended up using events to get this done without cluttering the entity.

I first added the logic in the User-entity but I then realised this logic was not really related to the user entity itself. Or the application, for that matters.

In order to seperate concerns, I decided to create a hook after the User save-action. That would allow me to add logic at that particular time without cluttering the User entity with nonrelevant code.

This has a downside though. When you want to debug what the heck happens after the User is saved to the database, you won’t find it in the User entity. This might send you down a long code-hunt. But you will see the dispatching of the event though. So if you’re new to this, remind yourself that there can be a whole different world behind the dispatching of an event.

Setting it up consists of 3 steps:

1. create an Event class (the one that will be dispatched)
2. dispatch the event at the right time and place (after the User is saved to the database)
3. create (and subscribe) the subscriber which will take action upon the dispatched event

Create Event

The event is nothing more than a class.

The event is the object which is passed around. Therefore you want to populate the event with all the information the subscriber(s) need.

For that purpose I create a setter and a getter. The code which will dispatch the event will use the setter, the event subscriber will use the getter.

# file src/AppBundle/Event/UserCreatedEvent.php
<?php
namespace AppBundle\Event;

use AppBundle\Entity\User;
use Symfony\Component\EventDispatcher\Event;

class UserCreatedEvent extends Event
{
    private $user;

    public function setUser(User $user)
    {
        $this->user = $user;
    }

    public function getUser()
    {
        return $this->user;
    }
}

Dispatch Event

Now we decide in what moment of time we’ll dispatch (fire) the event. In our example this will be right after the user is saved to the database.

# file src/AppBundle/Entity/User.php
/* other code */

public function createUser(User $user)
{

    /* more code */

    // save the user to the database
    $this->em->persist($user);
    $this->em->flush();

    // dispatch an event where others can work with the newly created user
    $eventDispatcher = $this->container->get('event_dispatcher');
    $event = new UserCreatedEvent();
    $event->setUser($user);
    $eventDispatcher->dispatch('user.event.created', $event);

    return $user;
}

Act on event with the subscriber

Create subscriber

The subscriber is the class with the method which gets called once the event gets dispatched.

# file src/AppBundle/EventSubscriber/UserCreatedSubscriber.php
<?php
namespace AppBundle\EventSubscriber;

class UserCreatedSubscriber
{
    public function newUserCreated(UserCreatedEvent $event)
    {
        var_dump($event->getUser());
    }
}

Subscribe the subscriber

Now that we have the code for the subscriber, we need to actually subscribe the subcriber to the event. This is the glue between dispatching an event and acting upon it.

You do this in services.yml as you register it as a service.

# create a listener for the UserCreatedEvent
valuation.event.created:
  class: AppBundle\EventSubscriber\UserCreatedSubscriber
  tags:
    - { name: kernel.event_listener, event: valuation.event.created, method: newUserCreated }

That’s it!

Sources

If you want to read more:

• Symfony documentation:
  https://symfony.com/doc/current/event_dispatcher.html
• Nice dense setup: https://stackoverflow.com/a/34162603