JMS Serializer Subscriber example

AppBundle\Resources\config\services.yml

services:
    serializer.subscriber.news:
        class: AppBundle\Serializer\Subscriber\NewsSubscriber
        arguments: [@vich_uploader.storage, @liip_imagine.cache.manager, @request_stack]
        tags:
            - { name: jms_serializer.event_subscriber }

AppBundle\Serializer\Subscriber\NewsSubscriber.php

<?php

namespace AppBundle\Serializer\Subscriber;

use AppBundle\Entity\News;
use JMS\Serializer\EventDispatcher\EventSubscriberInterface;
use JMS\Serializer\EventDispatcher\ObjectEvent;
use Liip\ImagineBundle\Imagine\Cache\CacheManager;
use Symfony\Component\HttpFoundation\RequestStack;
use Vich\UploaderBundle\Storage\StorageInterface;

class NewsSubscriber implements EventSubscriberInterface
{
    /**
     * @var CacheManager
     */
    private $imagineCacheManager;
    /**
     * @var StorageInterface
     */
    private $storage;
    /**
     * @var RequestStack
     */
    private $requestStack;

    /**
     * @param StorageInterface $storage
     * @param CacheManager     $imagineCacheManager
     */
    public function __construct(StorageInterface $storage, CacheManager $imagineCacheManager, RequestStack $requestStack)
    {
        $this->imagineCacheManager = $imagineCacheManager;
        $this->storage = $storage;
        $this->requestStack = $requestStack;
    }

    public static function getSubscribedEvents()
    {
        return [
            [
                'event' => 'serializer.post_serialize',
                'method' => 'onPostSerialize',
                'class' => News::class,
            ],
        ];
    }

    public function onPostSerialize(ObjectEvent $event)
    {
        /** @var News $object */
        $object = $event->getObject();
        $uri = $this->storage->resolveUri($object, 'imageFile');
        $request = $this->requestStack->getCurrentRequest();

        $thumbUrl = null;
        $url = null;

        if ($uri) {
            $thumbUrl = $this->imagineCacheManager->getBrowserPath($uri, 'news_image_api');
            $url = $this->imagineCacheManager->getBrowserPath($uri, 'news_image');
        }

        /** @var \JMS\Serializer\JsonSerializationVisitor $visitor */
        $visitor = $event->getVisitor();
        $visitor->addData('image_thumb', $thumbUrl);
        $visitor->addData('image', $url);
    }
}

Thank you very much! It's important because there are no examples in documentation!

$visitor->addData('image_thumb', $thumbUrl);
is an old version of JMS, now use
$visitor->visitProperty(new StaticPropertyMetadata('', 'image_thumb', null), $thumbUrl);

Thanks.
I'm agree with @NikitaKharkov => "no examples in documentation" -> no easy to use JMS Serializer !

To complete @yfrommelt, for JMS serializer 3.x (3.4 for me) :
Into \JMS\Serializer\JsonSerializationVisitor.php class (vendor\JMS\src folder), one method exists called "setData".
However, this method is deprecated, and with this comment :

@deprecated Use `::visitProperty(new StaticPropertyMetadata('', 'name', 'value'), 'value')` instead
     *
     * Allows you to replace existing data on the current object element.