The Event System

Note: using Kernel event with API Platform should be mostly limited to tweaking the generated HTTP response. Also, GraphQL is not supported. For most use cases, better extension points, working both with REST and GraphQL, are available.

API Platform Core implements the Action-Domain-Responder pattern. This implementation is covered in depth in the Creating custom operations and controllers chapter.

Basically, API Platform Core executes an action class that will return an entity or a collection of entities. Then a series of event listeners are executed which validate the data, persist it in database, serialize it (typically in a JSON-LD document) and create an HTTP response that will be sent to the client.

To do so, API Platform Core leverages events triggered by the Symfony HTTP Kernel. You can also hook your own code to those events. There are handy and powerful extension points available at all points of the request lifecycle.

If you are using Doctrine, lifecycle events (ORM, MongoDB ODM) are also available if you want to hook into the persistence layer’s object lifecycle.

These built-in event listeners are registered for routes managed by API Platform:

Some of these built-in listeners can be enabled/disabled by setting operation attributes:

Some of these built-in listeners can be enabled/disabled by setting request attributes (for instance in the defaults attribute of an operation):

Registering your own event listeners to add extra logic is convenient.

The ApiPlatform\Core\EventListener\EventPriorities class comes with a convenient set of class constants corresponding to commonly used priorities:

In the following example, we will send a mail each time a new book is created using the API:

<?php
// api/src/EventSubscriber/BookMailSubscriber.php

namespace App\EventSubscriber;

use ApiPlatform\Core\EventListener\EventPriorities;
use App\Entity\Book;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Event\ViewEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\Mime\Email;
use Symfony\Component\Mailer\MailerInterface;

final class BookMailSubscriber implements EventSubscriberInterface
{
    private $mailer;

    public function __construct(MailerInterface $mailer)
    {
        $this->mailer = $mailer;
    }

    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::VIEW => ['sendMail', EventPriorities::POST_WRITE],
        ];
    }

    public function sendMail(ViewEvent $event): void
    {
        $book = $event->getControllerResult();
        $method = $event->getRequest()->getMethod();

        if (!$book instanceof Book || Request::METHOD_POST !== $method) {
            return;
        }

        $message = (new Email())
            ->from('[email protected]')
            ->to('[email protected]')
            ->subject('A new book has been added')
            ->text(sprintf('The book #%d has been added.', $book->getId()));

        $this->mailer->send($message);
    }
}

If you use the official API Platform distribution, creating the previous class is enough. The Symfony DependencyInjection component will automatically register this subscriber as a service and will inject its dependencies thanks to the autowiring feature.

Alternatively, the subscriber must be registered manually.