Add dependency injection page for 9.5

Releases: 9.5, 8.7, 7.6, 6.2
Related: TYPO3-Documentation/T3DocTeam#142

Documentation/ApiOverview/DependencyInjection/Index.rst

@@ -0,0 +1,142 @@
+.. include:: ../../Includes.txt
+
+.. _DependencyInjection:
+
+====================
+Dependency Injection
+====================
+
+Since TYPO3 10.0, Symfony dependency injection is supported
+(see :doc:`Changelog/10.0/Feature-84112-SymfonyDependencyInjectionForCoreAndExtbase`).
+
+This page handles dependency injection in TYPO3 version 9.5.
+
+Dependency injection is not a specific TYPO3 concept. It is a
+
+   one form of the broader technique of inversion of control. A client who
+   wants to call some services should not have to know how to construct
+   those services. Instead, the client delegates the responsibility of
+   providing its services to external code (the injector).
+
+- `Wikipedia: Dependency injection <https://en.wikipedia.org/wiki/Dependency_injection>`__
+
+For more information about the general (not TYPO3 specific) concepts, see
+
+* `Thorben Janssen: "Design Patterns Explained - Dependency Injection with
+  Code Examples" <https://stackify.com/dependency-injection/>`__
+* `Martin Fowler: Inversion of Control Containers and the Dependency
+  Injection pattern <https://martinfowler.com/articles/injection.html>`__
+
+Methods of dependency injection
+===============================
+
+Constructor injection
+---------------------
+
+`Constructor injection <https://en.wikipedia.org/wiki/Dependency_injection#Constructor_injection>`__
+is a method of passing an object in the constructor.
+
+Example::
+
+   use Vendor\Foo\Service\MyServiceInterface;
+
+   class MySomething
+   {
+
+      /**
+       * @var MyServiceInterface
+       */
+      protected $myService;
+
+      /**
+       * @param $myService
+       */
+       public function __construct(MyService $myService)
+       {
+           $this->myService = $myService;
+       }
+
+
+It is now possible to pass any service object implementing the
+:php:`MyServiceInterface` in the constructor of the class :php:`MySomething`.
+
+This method can be used in TYPO3 and is recommended.
+
+Setter injection
+----------------
+
+Setter injection supplies a method to pass the object to be injected.
+
+Example::
+
+   use Vendor\Foo\Service\MyServiceInterface;
+
+   class MySomething
+   {
+
+       /**
+        * @var MyService
+        */
+       protected $myService;
+
+       /**
+        * @param FooRepository
+        */
+        public function setMyService(MyServiceInterface $myServiceInterface)
+        {
+            $this->myServiceInterface = $myServiceInterface;
+        }
+
+Using objects without dependency injection
+------------------------------------------
+
+Similar to the constructor injection, you could also do::
+
+
+    use Vendor\Foo\Service\MyService;
+
+    class MyController
+    {
+
+        /**
+        * @var MyService
+        */
+        protected $myService;
+
+        public function __construct()
+        {
+            $this->myService = GeneralUtility::makeInstance(MyService::class);
+        }
+
+This would hardwire usage of the :php:`MyService` in the controller class.
+Even if this class is intended to always use :php:`MyService`, it will
+be more difficult to write tests for it (because it is for example not
+possible to pass a different mockup object to be used as service object).
+
+
+Not recommended methods
+=======================
+
+`@inject` annotations
+---------------------
+
+.. important::
+
+   This method is not recommended!
+
+Since TYPO3 4.7, it is possible to use the `@inject` annotation.
+
+Example::
+
+   /**
+    * @var \Vendor\Foo\Domain\Repository\FooRepository
+    * @inject
+    */
+    protected $fooRepository;
+
+Mostly for performance reasons, it is no longer recommended to use `@inject`.
+
+
+.. seealso::
+
+   * `Why you should never use @inject in TYPO3 Extbase <https://gist.github.com/NamelessCoder/3b2e5931a6c1af19f9c3f8b46e74f837>`__

Documentation/Index.rst

@@ -121,6 +121,7 @@ address the task at hand.
    ApiOverview/Context/Index
    ApiOverview/ContextSensitiveHelp/Index
    ApiOverview/Database/Index
+   ApiOverview/DependencyInjection/Index
    ApiOverview/Fal/Index
    ApiOverview/DirectoryStructure/Index
    ApiOverview/Enumerations/Index