first implementation (wip) of an ajax admin controller. (for review).

Update.php

<?php


namespace SebSept\Adminproductquantities\Controller;

use PrestaShopBundle\Api\Stock\Movement;
use PrestaShopBundle\Controller\Admin\FrameworkBundleAdminController;
use PrestaShopBundle\Entity\ProductIdentity;
use PrestaShopBundle\Entity\Repository\StockRepository;
use PrestaShopBundle\Service\ProductService;
use Symfony\Component\HttpFoundation\JsonResponse;

class Update extends FrameworkBundleAdminController
{
    /**
     * Update Product stock
     *
     * (pas de gestion des déclinaisons, c'est normal)
     *
     * @param int $quantity new quantity (absolute value)
     * @param int $id_product
     *
     * @return JsonResponse
     */
    public function update(int $quantity, int $id_product)
    {
        try {
            $product_provider = $this->get('prestashop.adapter.data_provider.product');
            $stock_repository = $this->get('prestashop.core.api.stock.repository');

            $delta = $quantity - (int)$product_provider->getQuantity($id_product);

            $stock_data = $stock_repository->updateStock(new Movement(new ProductIdentity($id_product), $delta));
            $fetched_quantity = $stock_data['product_available_quantity'] ?? 'no-data';
            if ($quantity !== $fetched_quantity) 
            {
                throw new \Exception("Echec mise à jour stock : quantité voulue et recupérée differentes. [$quantity] vs [$fetched_quantity]");
            }
        } 
        catch (\Exception $exception) 
        {
            return new JsonResponse(['error' => $exception->getMessage()]);
        }

        // @todo implement reponse positive
        return new JsonResponse(['q' => $quantity, 'id' => $id_product]);
    }
}

Hello,
Today, to my knowledge, there is no way to access services of the backoffice in the front office.
Sorry.
This will come later, I suppose.

Hello friend, I found this article https://blog.floriancourgey.com/2018/05/how-to-work-with-the-symfony-kernel-anywhere-in-prestashop-1-7/, which indicates how to instantiate a symfony service container inside a module front controller (legacy). With the symfony service container I just need to run MyModule::getContainer()->get('prestashop.core.api.stock.repository') to get an instance of StockRepository. Greetings :)

Thanks for the feedback 😄
👍 For sure, booting the kernel and grab it, but I suppose performances is going to suffer from that.