Magento 2: Add a REST API for bulk updating category-product relationships

/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace DMTQ\Catalog\Api\Data;

use Magento\Framework\Api\ExtensibleDataInterface;

/**
 * @api
 * @since 100.0.2
 */
interface CategoryProductIdLinkInterface extends ExtensibleDataInterface
{
    /**
     * @return int|null
     */
    public function getProductId();

    /**
     * @param int $productId
     * @return $this
     */
    public function setProductId($productId);

    /**
     * @return int|null
     */
    public function getPosition();

    /**
     * @param int $position
     * @return $this
     */
    public function setPosition($position);

}

/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace DMTQ\Catalog\Api;

use DMTQ\Catalog\Api\Data\CategoryProductIdLinkInterface;

/**
 * @api
 * @since 100.0.2
 */
interface CategoryProductLinkManagementInterface
{
    /**
     * Assign products link to category
     *
     * @param int $categoryId
     * @param CategoryProductIdLinkInterface [] $items
     * @return bool
     */
    public function setCategoryProductLinks(int $categoryId, array $items);
}

/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace DMTQ\Catalog\Model\Api;

/**
 * @codeCoverageIgnore
 */
class CategoryProductIdLink extends \Magento\Framework\Api\AbstractExtensibleObject implements
    \DMTQ\Catalog\Api\Data\CategoryProductIdLinkInterface
{
    /**#@+
     * Constant for confirmation status
     */
    const KEY_PRODUCT_ID = 'product_id';
    const KEY_POSITION = 'position';
    /**#@-*/

    /**
     * {@inheritdoc}
     */
    public function getProductId()
    {
        return $this->_get(self::KEY_PRODUCT_ID);
    }

    /**
     * {@inheritdoc}
     */
    public function getPosition()
    {
        return $this->_get(self::KEY_POSITION);
    }


    /**
     * @param int $productId
     * @return $this
     */
    public function setProductId($productId)
    {
        return $this->setData(self::KEY_PRODUCT_ID, $productId);
    }

    /**
     * @param int $position
     * @return $this
     */
    public function setPosition($position)
    {
        return $this->setData(self::KEY_POSITION, $position);
    }

}

/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

namespace DMTQ\Catalog\Model\Api;

use Magento\Catalog\Api\CategoryRepositoryInterface;
use Magento\Catalog\Model\Indexer\Product\Category;
use Magento\Framework\Exception\CouldNotSaveException;
use Magento\Framework\Exception\NoSuchEntityException;
use Magento\Framework\Indexer\IndexerRegistry;
use DMTQ\Catalog\Api\CategoryProductLinkManagementInterface;

/**
 * Represents Category Product Link Management class
 */
class CategoryProductLinkManagement implements CategoryProductLinkManagementInterface
{
    /**
     * @var CategoryRepositoryInterface
     */
    protected CategoryRepositoryInterface $categoryRepository;

    /**
     * @var IndexerRegistry
     */
    protected IndexerRegistry $indexerRegistry;

    /**
     * CategoryLinkManagement constructor.
     *
     * @param CategoryRepositoryInterface $categoryRepository
     * @param IndexerRegistry $indexerRegistry
     */
    public function __construct(
        CategoryRepositoryInterface $categoryRepository,
        IndexerRegistry $indexerRegistry,
    ) {
        $this->categoryRepository = $categoryRepository;
        $this->indexerRegistry = $indexerRegistry;
    }

    /**
     * @param $categoryId
     * @param array $items
     * @return true
     * @throws CouldNotSaveException|NoSuchEntityException
     */
    public function setCategoryProductLinks($categoryId, array $items): true
    {
        $category = $this->categoryRepository->get($categoryId);
        //$productPositions = $category->getProductsPosition();
        $productPositions = [];
        foreach ($items as $item) {
            $productPositions[$item->getProductId()] = $item->getPosition();
        }
        $category->setPostedProducts($productPositions);
        try {
            $category->save();
        } catch (\Exception $e) {
            throw new CouldNotSaveException(
                __(
                    'Could not save product to category %1',
                    $category->getId()
                ),
                $e
            );
        }

        $productCategoryIndexer = $this->indexerRegistry->get(Category::INDEXER_ID);
        if (!$productCategoryIndexer->isScheduled()) {
            foreach ($items as $item) {
                $productCategoryIndexer->reindexRow($item->getProductId());
            }
        }else {
            $productCategoryIndexer->invalidate();
        }
        return true;
    }

}

{
    "items": [
        {
            "product_id": 201,
            "position": 1
        },
        {
            "product_id": 202,
            "position": 2
        },
        {
            "product_id": 203,
            "position": 3
        },
        {
            "product_id": 204,
            "position": 4
        }
    ]
}