[DependencyInjection] Fixed public service use case

Author: HeahDude

service_container.rst

@@ -815,24 +815,36 @@ loss, enable the compiler pass in your application.
 Public Versus Private Services
 ------------------------------
 
-Thanks to the ``_defaults`` section in ``services.yaml``, every service defined in
-this file is ``public: false`` by default.
+Starting with Symfony 4, services are private by default.
 
 What does this mean? When a service **is** public, you can access it directly
-from the container object, which is accessible from any controller that extends
-``Controller``::
+from the container object, which can also be injected thanks to autowiring.
+This is mostly useful when you want to fetch services lazily::
 
-    use App\Service\MessageGenerator;
+    namespace App\Service;
 
-    // ...
-    public function new()
+    class MessageGenerator
     {
-        // there IS a public "logger" service in the container
-        $logger = $this->container->get('logger');
+        private $container;
 
-        // this will NOT work: MessageGenerator is a private service
-        $generator = $this->container->get(MessageGenerator::class);
-    }
+        public function __construct(ContainerInterface $container)
+        {
+            $this->container = $container;
+        }
+
+        public function generate(string $message, string $template = null, array $context = []): string
+        {
+            if ($template && $this->container->has('twig')) {
+                // there IS a public "twig" service in the container
+                $twig = $this->container->get('twig');
+
+                return $twig->render($template, $context + ['message' => $message]);
+            }
+
+            // if no template is passed, the Twig service will not be loaded
+
+            // ...
+        }
 
 As a best practice, you should only create *private* services, which will happen
 automatically. And also, you should *not* use the ``$container->get()`` method to
@@ -849,7 +861,7 @@ But, if you *do* need to make a service public, override the ``public`` setting:
             # ... same code as before
 
             # explicitly configure the service
-            App\Service\MessageGenerator:
+            App\SomePublicService:
                 public: true
 
     .. code-block:: xml
@@ -865,7 +877,7 @@ But, if you *do* need to make a service public, override the ``public`` setting:
                 <!-- ... same code as before -->
 
                 <!-- Explicitly configure the service -->
-                <service id="App\Service\MessageGenerator" public="true"></service>
+                <service id="App\SomePublicService" public="true"></service>
             </services>
         </container>
 
@@ -874,13 +886,13 @@ But, if you *do* need to make a service public, override the ``public`` setting:
         // config/services.php
         namespace Symfony\Component\DependencyInjection\Loader\Configurator;
 
-        use App\Service\MessageGenerator;
+        use App\SomePublicService;
 
         return function(ContainerConfigurator $configurator) {
             // ... same as code before
 
             // explicitly configure the service
-            $services->set(MessageGenerator::class)
+            $services->set(SomePublicService::class)
                 ->public()
             ;
         };