JMS serializer bundle visitor example - how to add another field into serialized entity

ExampleVisitor.php

<?php

namespace Acme\DemoBundle\Serializer;

use JMS\Serializer\EventDispatcher\EventSubscriberInterface;
use JMS\Serializer\EventDispatcher\ObjectEvent;

class ExampleVisitor implements EventSubscriberInterface
{
    static public function getSubscribedEvents()
    {
        return array(
            array('event' => 'serializer.post_serialize', 'class' => 'Acme\DemoBundle\Entity\Example', 'method' => 'onPostSerialize'),
        );
    }

    public function onPostSerialize(ObjectEvent $event)
    {
        $event->getVisitor()->addData('someKey','someValue');
    }
}

service.yml

treewer.serializer.company:
    class: Acme\DemoBundle\Serializer\ExampleVisitor
    tags:
        - { name: jms_serializer.event_subscriber }