[TASK] Create stub for DTO explanation (TYPO3-Documentation#4823)

* [TASK] Create stub for DTO explanation

This is a basic first step definition of a DTO

Addresses TYPO3-Documentation#4822

* [TASK] CGL

Documentation/ExtensionArchitecture/BestPractises/SoftwareDesignPrinciples.rst

@@ -19,3 +19,40 @@ extensions for TYPO3.
 
 We also recommend to study common `software design
 patterns <https://designpatternsphp.readthedocs.io/en/latest/>`__.
+
+.. _concept-dto:
+
+DTO / Data Transfer Objects
+----------------------------
+
+A very common pattern in Extbase extensions is a "DTO" ("Data Transfer Object").
+
+A DTO is an instance of a basic class that usually only has a constructor,
+getters and setters. It is not meant to be an extension of an Extbase `AbstractEntity`.
+
+This DTO serves as pure data storage. You can use it to receive and retrieve
+data in a `<f:form>` fluid
+`CRUD <https://en.wikipedia.org/wiki/Create,_read,_update_and_delete>`__
+("Create Read Update Delete") setup.
+
+Later on, the DTO can be accessed and converted into a "proper" Extbase domain model
+entity: The DTO getters retrieve the data, and the Extbase domain model entity's setters
+receive that data:
+
+..  literalinclude:: _dto.php
+    :language: php
+    :caption: Example of DTO and AbstractEntity used in an Extbase controller
+
+DTOs are helpful because:
+
+*   They allow to decouple pure data from processed/validated data in entities.
+*   They allow to structure data into distinct data models.
+*   They allow to use good type hinting and using setters/getters instead of using
+    untyped and unstructured PHP arrays for data storage.
+*   They can be used to hide implementation details of your actual entities by transferring
+    data like filter settings that internally gets applied to actual data models.
+
+
+Some more reading:
+
+*   `<https://usetypo3.com/dtos-in-extbase/>`__

Documentation/ExtensionArchitecture/BestPractises/_dto.php

@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+// The actual "plain" DTO, just setters and getters
+class PersonDTO
+{
+    protected string $first_name;
+    protected string $last_name;
+
+    public function getFirstName(): string
+    {
+        return $this->first_name;
+    }
+
+    public function setFirstName(string $first_name): void
+    {
+        $this->first_name = $first_name;
+    }
+
+    public function getLastName(): string
+    {
+        return $this->last_name;
+    }
+
+    public function setLastName(string $last_name): void
+    {
+        $this->last_name = $last_name;
+    }
+}
+
+// The Extbase domain model entity.
+// Note that the getters and setters can easily be mapped
+// to the DTO due to their same names!
+class Person extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity
+{
+    protected string $first_name;
+    protected string $last_name;
+
+    public function getFirstName(): string
+    {
+        return $this->first_name;
+    }
+
+    public function setFirstName(string $first_name): void
+    {
+        $this->first_name = $first_name;
+    }
+
+    public function getLastName(): string
+    {
+        return $this->last_name;
+    }
+
+    public function setLastName(string $last_name): void
+    {
+        $this->last_name = $last_name;
+    }
+}
+
+// An Extbase controller utilizing DTO-to-entity transfer
+class DtoController extends TYPO3\CMS\Extbase\Mvc\Controller\ActionController
+{
+    public function __construct(protected MyVendor\MyExtension\Domain\Repository\PersonRepository $personRepository) {}
+
+    public function createAction(): Psr\Http\Message\ResponseInterface
+    {
+        // Set up a DTO to be filled with input data.
+        // The Fluid template would use <f:form> and its helpers.
+        $this->view->assign('personDTO', new PersonDTO());
+    }
+
+    public function saveAction(PersonDTO $personDTO): Psr\Http\Message\ResponseInterface
+    {
+        // Transfer all data to a proper Extbase entity.
+        // Create an empty entity first:
+        $person = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(Person::class);
+
+        // Use setters/getters for propagation
+        $person->setFirstName($personDTO->getFirstName());
+        $person->setLastName($personDTO->getLastName());
+
+        // Persist the extbase entity
+        $this->personRepository->add($person);
+
+        // The "old" DTO needs to further processing.
+    }
+}