Document injection of tagged extensions

Author: abluchet

core/data-providers.md

@@ -161,3 +161,78 @@ final class BlogPostItemDataProvider implements ItemDataProviderInterface, Seria
     }
 }
 ```
+
+## Injecting Extensions (Pagination, Filter, EagerLoading etc.)
+
+ApiPlatform provides a few extensions that you can reuse in your custom DataProvider.
+Note that there are a few kind of extensions which are detailed in [their own chapter of the documentation](extensions.md).
+Because extensions are tagged services, you can use the [injection of tagged services](https://symfony.com/blog/new-in-symfony-3-4-simpler-injection-of-tagged-services):
+
+```yaml
+services:
+    'App\DataProvider\BlogPostItemDataProvider':
+        arguments:
+          $itemExtensions: !tagged api_platform.doctrine.orm.query_extension.item
+```
+
+Or in XML:
+
+```xml
+<services>
+    <service id="App\DataProvider\BlogPostItemDataProvider">
+        <argument key="$itemExtensions" type="tagged" tag="api_platform.doctrine.orm.query_extension.item" />
+    </service>
+</services>
+```
+
+Your data provider will now have access to the core extensions, here is an example on how to use them:
+
+```
+<?php
+// api/src/DataProvider/BlogPostItemDataProvider.php
+
+namespace App\DataProvider;
+
+use ApiPlatform\Core\Bridge\Doctrine\Orm\Util\QueryNameGenerator;
+use ApiPlatform\Core\DataProvider\ItemDataProviderInterface;
+use ApiPlatform\Core\DataProvider\RestrictedDataProviderInterface;
+use ApiPlatform\Core\Exception\ResourceClassNotSupportedException;
+use App\Entity\BlogPost;
+use Doctrine\Common\Persistence\ManagerRegistry;
+
+final class BlogPostItemDataProvider implements ItemDataProviderInterface, RestrictedDataProviderInterface
+{
+    private $itemExtensions;
+    private $managerRegistry;
+
+    public function __construct(ManagerRegistry $managerRegistry, iterable $itemExtensions)
+    {
+      $this->managerRegistry = $managerRegistry;
+      $this->itemExtensions = $itemExtensions;
+    }
+
+    public function supports(string $resourceClass, string $operationName = null, array $context = []): bool
+    {
+        return BlogPost::class === $resourceClass;
+    }
+
+    public function getItem(string $resourceClass, $id, string $operationName = null, array $context = []): ?BlogPost
+    {
+        $manager = $this->managerRegistry->getManagerForClass($resourceClass);
+        $repository = $manager->getRepository($resourceClass);
+        $queryBuilder = $repository->createQueryBuilder('o');
+        $queryNameGenerator = new QueryNameGenerator();
+        $identifiers = ['id' => $id];
+
+        foreach ($this->itemExtensions as $extension) {
+            $extension->applyToItem($queryBuilder, $queryNameGenerator, $resourceClass, $identifiers, $operationName, $context);
+            if ($extension instanceof QueryResultItemExtensionInterface && $extension->supportsResult($resourceClass, $operationName, $context))                 {
+                return $extension->getResult($queryBuilder, $resourceClass, $operationName, $context);
+            }
+        }
+
+        return $queryBuilder->getQuery()->getOneOrNullResult();
+    }
+}
+
+```