Add use of service in dynamic validation group (#259)

Author: pierre-H

core/validation.md

@@ -95,6 +95,70 @@ class Book
 }
 ```
 
+Alternatively, you can use a service to retrieve the groups to use:
+
+```php
+<?php
+
+// src/AppBundle/Validator/AdminGroupsGenerator.php
+
+namespace AppBundle\Validator;
+
+use AppBundle\Entity\Book;
+use Symfony\Component\Security\Core\Authorization\AuthorizationCheckerInterface;
+
+final class AdminGroupsGenerator
+{
+    private $authorizationChecker;
+
+    public function __construct(AuthorizationCheckerInterface $authorizationChecker)
+    {
+        $this->authorizationChecker = $authorizationChecker;
+    }
+
+    public function __invoke(Book $book): array
+    {
+        return $this->authorizationChecker->isGranted('ROLE_ADMIN', $book) ? ['a', 'b'] : ['a'];
+    }
+}
+```
+
+This class selects the groups to apply regarding the role of the current user: if the current user has the `ROLE_ADMIN` role, groups `a` and `b` are returned. In other cases, just `a` is returned.
+
+This class is automatically registered as a service thanks to [the autowiring feature of the Symfony Dependency Injection Component](https://symfony.com/doc/current/service_container/autowiring.html).
+
+Then, configure the entity class to use this service to retrieve validation groups:
+
+```php
+<?php
+
+// src/AppBundle/Entity/Book.php
+
+namespace AppBundle\Entity;
+
+use ApiPlatform\Core\Annotation\ApiResource;
+use AppBundle\Validator\AdminGroupsGenerator;
+use Symfony\Component\Validator\Constraints as Assert;
+
+/**
+ * @ApiResource(attributes={"validation_groups"=AdminGroupsGenerator::class})
+ */
+class Book
+{
+    /**
+     * @Assert\NotBlank(groups={"a"})
+     */
+    private $name;
+
+    /**
+     * @Assert\NotNull(groups={"b"})
+     */
+    private $author;
+
+    // ...
+}
+```
+
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