AppState.php from https://www.symfony.fi/entry/sharing-state-in-a-symfony-hybrid-app-with-twig-react-etc

AppState.php

<?php

namespace AppBundle\State;
use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

class AppState implements \JsonSerializable {

    public $fetchMore = false;
    public $sortBy = false;
    public $selectedCountry = false;
    public $apartments = [];

    public function jsonSerialize() {

        $encoders = array(new JsonEncoder());
        $normalizers = array(new ObjectNormalizer());
        $serializer = new Serializer($normalizers, $encoders);

        return $serializer->serialize($this, 'json');

    }

    /**
     * @return string
     */
    public function getSortBy()
    {
        return $this->sortBy;
    }

...