Add extra information for the list of objects while serializing via JMS

DTO\ItemListDTO.php

<?php

declare(strict_types=1);

namespace AppBundle\DTO;

use AppBundle\Entity\Item;
use JMS\Serializer\Annotation as JMS;

/**
 * @JMS\ExclusionPolicy("all")
 */
class ItemListDTO
{
    /**
     * @JMS\Expose
     * @JMS\Type("array<AppBundle\Entity\Item>")
     */
    private array $items;

    public function __construct(array $items)
    {
        $this->items = $items;
    }

    /**
     * @return Item[]
     */
    public function getItems(): array
    {
        return $this->items;
    }
}

Entity\Item.php

<?php

declare(strict_types=1);

namespace AppBundle\Entity;

use JMS\Serializer\Annotation as JMS;

/**
 * @JMS\ExclusionPolicy("all")
 */
class Item
{
    /**
     * @JMS\Expose
     * @JMS\Type("integer")
     */
    private ?int $id = null;

    /**
     * @JMS\Expose
     * @JMS\Type("string")
     */
    private ?string $title = null;
}

Listener\ItemListSubscribingHandler.php

<?php

declare(strict_types=1);

namespace AppBundle\Listener;

use AppBundle\DTO\ItemListDTO;
use JMS\Serializer\Context;
use JMS\Serializer\GraphNavigator;
use JMS\Serializer\Handler\SubscribingHandlerInterface;
use JMS\Serializer\JsonSerializationVisitor;

class ItemListSubscribingHandler implements SubscribingHandlerInterface
{
    public static function getSubscribingMethods(): array
    {
        return [
            [
                'direction' => GraphNavigator::DIRECTION_SERIALIZATION,
                'format' => 'json',
                'type' => ItemListDTO::class,
                /** @see ItemListSubscribingHandler::serializeItemListToJson() */
                'method' => 'serializeItemListToJson',
            ],
        ];
    }

    public function serializeItemListToJson(
        JsonSerializationVisitor $visitor,
        ItemListDTO $itemList,
        array $type,
        Context $context
    ) {
        return $itemList->getItems();
    }
}

Listener\ItemSerializationSubscriber.php

<?php

declare(strict_types=1);

namespace AppBundle\Listener;

use AppBundle\DTO\ItemListDTO;
use AppBundle\Entity\Item;
use JMS\Serializer\Context;
use JMS\Serializer\EventDispatcher\Events;
use JMS\Serializer\EventDispatcher\EventSubscriberInterface;
use JMS\Serializer\EventDispatcher\ObjectEvent;
use JMS\Serializer\JsonSerializationVisitor;
use JMS\Serializer\Metadata\StaticPropertyMetadata;

class ItemSerializationSubscriber implements EventSubscriberInterface
{
    private array $extraDataList;

    public static function getSubscribedEvents(): array
    {
        return [
            [
                'event' => Events::PRE_SERIALIZE,
                /** @see ItemSerializationSubscriber::onItemListPreSerialize() */
                'method' => 'onItemListPreSerialize',
                'class' => ItemListDTO::class,
            ],
            [
                'event' => Events::POST_SERIALIZE,
                /** @see ItemSerializationSubscriber::onPostSerialize() */
                'method' => 'onPostSerialize',
                'class' => Item::class,
            ],
        ];
    }

    public function onItemListPreSerialize(ObjectEvent $event): void
    {
        $visitor = $event->getVisitor();

        if (!$visitor instanceof JsonSerializationVisitor) {
            return;
        }

        $itemList = $event->getObject();

        if (!$itemList instanceof ItemListDTO) {
            return;
        }

        $itemIdList = array_map(fn (Item $item) => $item->getId(), $itemList->getItems());

        $this->extraDataList = $this->statsService->getItemSize($itemIdList);
    }

    public function onPostSerialize(ObjectEvent $event): void
    {
        $visitor = $event->getVisitor();

        if (!$visitor instanceof JsonSerializationVisitor) {
            return;
        }

        $item = $event->getObject();

        if (!$item instanceof Item) {
            return;
        }

        $itemId = $item->getId();
        $extraData = $this->extraDataList[$itemId] ?? null;
        $context = $event->getContext();

        $visitor->visitProperty(new StaticPropertyMetadata('', 'extra', $extraData), $extraData, $context);
    }
}