Added a separate resolver for returning category products count

app/code/Magento/CatalogGraphQl/Model/Category/Hydrator.php

@@ -52,11 +52,11 @@ public function hydrateCategory(Category $category, $basicFieldsOnly = false) :
             $categoryData = $category->getData();
         } else {
             $categoryData = $this->dataObjectProcessor->buildOutputDataArray($category, CategoryInterface::class);
-            $categoryData['product_count'] = $category->getProductCount();
         }
         $categoryData['id'] = $category->getId();
         $categoryData['children'] = [];
         $categoryData['available_sort_by'] = $category->getAvailableSortBy();
+        $categoryData['model'] = $category;
         return $this->flattener->flatten($categoryData);
     }
 }

app/code/Magento/CatalogGraphQl/Model/Resolver/Category/ProductsCount.php

@@ -0,0 +1,70 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\CatalogGraphQl\Model\Resolver\Category;
+
+use Magento\Catalog\Model\Category;
+use Magento\Catalog\Model\Product\Visibility;
+use Magento\CatalogGraphQl\Model\Resolver\Products\DataProvider\Product\CollectionProcessor\StockProcessor;
+use Magento\Framework\Api\SearchCriteriaInterface;
+use Magento\Framework\GraphQl\Exception\GraphQlInputException;
+use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
+use Magento\Framework\GraphQl\Config\Element\Field;
+use Magento\Framework\GraphQl\Query\ResolverInterface;
+
+/**
+ * Retrieves products count for a category
+ */
+class ProductsCount implements ResolverInterface
+{
+    /**
+     * @var Visibility
+     */
+    private $catalogProductVisibility;
+
+    /**
+     * @var StockProcessor
+     */
+    private $stockProcessor;
+
+    /**
+     * @var SearchCriteriaInterface
+     */
+    private $searchCriteria;
+
+    /**
+     * @param Visibility $catalogProductVisibility
+     * @param SearchCriteriaInterface $searchCriteria
+     * @param StockProcessor $stockProcessor
+     */
+    public function __construct(
+        Visibility $catalogProductVisibility,
+        SearchCriteriaInterface $searchCriteria,
+        StockProcessor $stockProcessor
+    ) {
+        $this->catalogProductVisibility = $catalogProductVisibility;
+        $this->searchCriteria = $searchCriteria;
+        $this->stockProcessor = $stockProcessor;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function resolve(Field $field, $context, ResolveInfo $info, array $value = null, array $args = null)
+    {
+        if (!isset($value['model'])) {
+            throw new GraphQlInputException(__('"model" value should be specified'));
+        }
+        /** @var Category $category */
+        $category = $value['model'];
+        $productsCollection = $category->getProductCollection();
+        $productsCollection->setVisibility($this->catalogProductVisibility->getVisibleInSiteIds());
+        $productsCollection = $this->stockProcessor->process($productsCollection, $this->searchCriteria, []);
+
+        return $productsCollection->getSize();
+    }
+}

app/code/Magento/CatalogGraphQl/etc/schema.graphqls

@@ -379,7 +379,7 @@ interface CategoryInterface @typeResolver(class: "Magento\\CatalogGraphQl\\Model
     level: Int @doc(description: "Indicates the depth of the category within the tree")
     created_at: String @doc(description: "Timestamp indicating when the category was created")
     updated_at: String @doc(description: "Timestamp indicating when the category was updated")
-    product_count: Int @doc(description: "The number of products in the category")
+    product_count: Int @doc(description: "The number of products in the category") @resolver(class: "Magento\\CatalogGraphQl\\Model\\Resolver\\Category\\ProductsCount")
     default_sort_by: String @doc(description: "The attribute to use for sorting")
     products(
         pageSize: Int = 20 @doc(description: "Specifies the maximum number of results to return at once. This attribute is optional."),

dev/tests/api-functional/testsuite/Magento/GraphQl/Catalog/CategoryProductsCountTest.php

@@ -0,0 +1,143 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GraphQl\Catalog;
+
+use Magento\TestFramework\TestCase\GraphQlAbstract;
+use Magento\Catalog\Api\ProductRepositoryInterface;
+use Magento\TestFramework\Helper\Bootstrap;
+use Magento\Catalog\Model\Product\Visibility;
+use Magento\Catalog\Model\Product\Attribute\Source\Status as productStatus;
+
+/**
+ * Class CategoryProductsCountTest
+ *
+ * Test for Magento\CatalogGraphQl\Model\Resolver\Category\ProductsCount resolver
+ */
+class CategoryProductsCountTest extends GraphQlAbstract
+{
+    /**
+     * @var ProductRepositoryInterface
+     */
+    private $productRepository;
+
+    protected function setUp()
+    {
+        $objectManager = Bootstrap::getObjectManager();
+        $this->productRepository = $objectManager->create(ProductRepositoryInterface::class);
+    }
+
+    /**
+     * @magentoApiDataFixture Magento/Catalog/_files/product_simple.php
+     */
+    public function testCategoryWithSaleableProduct()
+    {
+        $categoryId = 2;
+
+        $query = <<<QUERY
+{
+  category(id: {$categoryId}) {
+      id
+      product_count
+    }
+}
+QUERY;
+        $response = $this->graphQlQuery($query);
+
+        self::assertEquals(1, $response['category']['product_count']);
+    }
+
+    /**
+     * @magentoApiDataFixture Magento/Catalog/_files/category_product.php
+     */
+    public function testCategoryWithInvisibleProduct()
+    {
+        $categoryId = 333;
+        $sku = 'simple333';
+
+        $product = $this->productRepository->get($sku);
+        $product->setVisibility(Visibility::VISIBILITY_NOT_VISIBLE);
+        $this->productRepository->save($product);
+
+        $query = <<<QUERY
+{
+  category(id: {$categoryId}) {
+      id
+      product_count
+    }
+}
+QUERY;
+        $response = $this->graphQlQuery($query);
+
+        self::assertEquals(0, $response['category']['product_count']);
+    }
+
+    /**
+     * @magentoApiDataFixture Magento/Catalog/_files/product_simple_out_of_stock.php
+     */
+    public function testCategoryWithOutOfStockProductManageStockEnabled()
+    {
+        $categoryId = 2;
+
+        $query = <<<QUERY
+{
+  category(id: {$categoryId}) {
+      id
+      product_count
+    }
+}
+QUERY;
+        $response = $this->graphQlQuery($query);
+
+        self::assertEquals(0, $response['category']['product_count']);
+    }
+
+    /**
+     * @magentoApiDataFixture Magento/Catalog/_files/category_product.php
+     */
+    public function testCategoryWithOutOfStockProductManageStockDisabled()
+    {
+        $categoryId = 333;
+
+        $query = <<<QUERY
+{
+  category(id: {$categoryId}) {
+      id
+      product_count
+    }
+}
+QUERY;
+        $response = $this->graphQlQuery($query);
+
+        self::assertEquals(1, $response['category']['product_count']);
+    }
+
+    /**
+     * @magentoApiDataFixture Magento/Catalog/_files/category_product.php
+     */
+    public function testCategoryWithDisabledProduct()
+    {
+        $categoryId = 333;
+        $sku = 'simple333';
+
+        $product = $this->productRepository->get($sku);
+        $product->setStatus(ProductStatus::STATUS_DISABLED);
+        $this->productRepository->save($product);
+
+        $query = <<<QUERY
+{
+  category(id: {$categoryId}) {
+      id
+      product_count
+    }
+}
+QUERY;
+        $response = $this->graphQlQuery($query);
+
+        self::assertEquals(0, $response['category']['product_count']);
+    }
+}