Add working usage example.

Author: BooleanType

service_container/factories.rst

@@ -301,5 +301,138 @@ previous examples takes the ``templating`` service as an argument:
                 ->args([service('templating')])
             ;
         };
+        
+Usage example
+--------------------
+
+The following example is intended to show how to create and use a factory method in Symfony framework.
+Suppose you want to realize the factory method pattern for services, that describe two delivery methods - DPD and SmartPost.
+
+Services (subclasses) definition::
+
+    // src/Deliveries/DPD.php
+    namespace App\Deliveries;
+
+    class DPD
+    {
+        public $costLabel;
+    
+        public function cost() {
+            return 100;
+        }
+    }
+    
+    // src/Deliveries/SmartPost.php
+    namespace App\Deliveries;
+
+    class SmartPost
+    {
+        public $costLabel;
+    
+        public function cost() {
+            return 200;
+        }
+    }
+
+Factory definition::
+
+    // src/Factories/DeliveryFactory.php
+    namespace App\Factories;
+    
+    abstract class DeliveryFactory
+    {
+        public static function create($deliveryMethod)
+        {
+            $delivery = new $deliveryMethod;
+            $delivery->costLabel = 'Delivery cost is: ';
+    
+            return $delivery;
+        }
+        
+        abstract public function price();
+    }
+    
+As you can see, ``DeliveryFactory`` doesn't specify the exact class of the object that will be created.
+    
+Next, use settings similar to those in the sections above. These settings allow you to define a factory method for subclasses without explicitly extending the abstract class (i.e., without ``class DPD extends DeliveryFactory``)!
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/services.yaml
+        services:
+            # ...
+
+            App\Deliveries\DPD:
+                factory:   ['@App\Factories\DeliveryFactory', create]
+                arguments:
+                    $deliveryMethod: 'App\Deliveries\DPD'
+            App\Deliveries\SmartPost:
+                factory:   ['@App\Factories\DeliveryFactory', create]
+                arguments:
+                    $deliveryMethod: 'App\Deliveries\SmartPost'
+            
+
+    .. code-block:: xml
+
+        <!-- config/services.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                https://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <services>
+                <!-- ... -->
+
+                <service id="App\Deliveries\DPD">
+                    <factory service="App\Factories\DeliveryFactory" method="create"/>
+                    <argument key="$deliveryMethod">App\Deliveries\DPD</argument>
+                </service>
+                <service id="App\Deliveries\SmartPost">
+                    <factory service="App\Factories\DeliveryFactory" method="create"/>
+                    <argument key="$deliveryMethod">App\Deliveries\SmartPost</argument>
+                </service>
+            </services>
+        </container>
+
+    .. code-block:: php
+
+        // config/services.php
+        namespace Symfony\Component\DependencyInjection\Loader\Configurator;
+
+        use App\Deliveries\DPD;
+        use App\Deliveries\SmartPost;
+        use App\Factories\DeliveryFactory;
+
+        return function(ContainerConfigurator $configurator) {
+            $services = $configurator->services();
+
+            $services->set(DPD::class)
+                ->factory([ref(DeliveryFactory::class), 'create'])
+                ->arg('$deliveryMethod', 'App\Deliveries\DPD')
+            ;
+            $services->set(SmartPost::class)
+                ->factory([ref(DeliveryFactory::class), 'create'])
+                ->arg('$deliveryMethod', 'App\Deliveries\SmartPost')
+            ;
+        };
+
+Now we can use our services as usual (via dependency injection). The only difference is that object instances are created in the factory. Let's get those services in controller::
+
+    /**
+     * @Route("/get-deliveries-cost", methods={"GET"})
+     */
+    public function getDeliveriesCost(DPD $dpd, SmartPost $smartPost)
+    {
+        // ...
+        
+        // $dpd->costLabel is fulfilled in factory method.
+        $dpdCost = $dpd->costLabel . $dpd->cost();
+        $smartPostCost = $smartPost->costLabel . $smartPost->cost();
+        
+        // ...
+    }
 
 .. _`factory design pattern`: https://en.wikipedia.org/wiki/Factory_(object-oriented_programming)