The Event System

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 execute 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. They are handy and powerful extension points available at all points of the request lifecycle.

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\GetResponseForControllerResultEvent;
use Symfony\Component\HttpKernel\KernelEvents;

final class BookMailSubscriber implements EventSubscriberInterface
{
    private $mailer;

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

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

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

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

        $message = (new \Swift_Message('A new book has been added'))
            ->setFrom('[email protected]')
            ->setTo('[email protected]')
            ->setBody(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 Dependency Injection 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.

Doctrine events are also available (if you use it) if you want to hook at the object lifecycle events.

Built-in event listeners are:

Those built-in listeners are always executed for routes managed by API Platform. Registering your own event listeners to add extra logic is convenient.

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

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