first draft cookbook before/after listeners

Author: ricardclau

cookbook/event_dispatcher/before_after_filters.rst

@@ -0,0 +1,135 @@
+.. index::
+   single: Event Dispatcher
+
+How to setup before and after filters
+=====================================
+
+It is quite common in web applications development to need some logic to be executed just before
+or just after our controller actions acting as filters or hooks.
+
+In Symfony1, this was achieved with the preExecute and postExecute methods, most major frameworks have similar
+methods but there is no such thing in Symfony2. Good news is that there is a much better way to interfere our
+Request -> Response process with the EventListener component.
+
+Token validation example
+========================
+
+Imagine that we need to develop an API where some controllers are public but some others are restricted
+to one or some clients. For this private features, we provide a token to our clients to identify themselves.
+
+So, before executing our controller action, we need to check if the action is restricted or not.
+And if it is restricted, we need to validate the provided token.
+
+.. note::
+
+Please note that for simplicity in the recipe, tokens will be defined in config
+and neither database setup nor authentication provider via Security component will be used
+
+Creating a before filter with a controller.request event
+========================================================
+
+Basic Setup
+-----------
+
+We can add basic tokens configuration using config.yml and parameters key
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # app/config/config.yml
+        parameters:
+            tokens:
+                client1: pass1
+                client2: pass2
+
+
+Tag controllers to be checked
+-----------------------------
+
+A kernel.controller listener gets executed at every request, so we need some way to identify
+if the controller that matches the request needs a token validation.
+
+A clean and easy way is to create an empty interface and make the controllers implement it
+
+.. code-block:: php
+
+    namespace Acme\DemoBundle\Controller;
+
+    interface TokenAuthenticatedController
+    {
+    // Nothing here
+    }
+
+    class FooController implements TokenAuthenticatedController
+    {
+    // Your actions that need authentication
+    }
+
+Creating an Event Listener
+--------------------------
+
+.. code-block:: php
+
+    namespace Acme\DemoBundle\EventListener;
+
+    use Acme\DemoBundle\Controller\TokenAuthenticatedController;
+    use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
+
+    class BeforeListener
+    {
+        private $tokens;
+        public function __contruct($tokens)
+        {
+            $this->tokens = $tokens;
+        }
+
+        public function onKernelController(FilterControllerEvent $event)
+        {
+            $controller = $event->getController();
+
+            /**
+             * $controller passed can be either a class or a Closure. This is not usual in Symfony2 but it may happen.
+             * If it is a class, it comes in array format
+             */
+            if (!is_array($controller)) return;
+
+            if($controller[0] instanceof TokenAuthenticatedController) {
+                $token = $event->getRequest()->get('token');
+                if (!in_array($token, $this->tokens)) {
+                    throw new AccessDeniedHttpException('This action needs a valid token!');
+                }
+            }
+        }
+    }
+
+Tagging the EventListener
+-------------------------
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # app/config/config.yml (or inside or your services.yml)
+        services:
+            demo.tokens.action_listener:
+              class: Acme\DemoBundle\EventListener\BeforeListener
+              arguments: [ %tokens% ]
+              tags:
+                    - { name: kernel.event_listener, event: kernel.controller, method: onKernelController }
+
+    .. code-block:: xml
+
+        <service id="demo.tokens.action_listener" class="Acme\DemoBundle\EventListener\BeforeListener">
+            <argument>%tokens%</argument>
+            <tag name="kernel.event_listener" event="kernel.controller" method="onKernelController" />
+        </service>
+
+    .. code-block:: php
+
+        use Symfony\Component\DependencyInjection\Definition;
+
+        $listener = new Definition('Acme\DemoBundle\EventListener\BeforeListener', array('%tokens%'));
+        $listener->addTag('kernel.event_listener', array('event' => 'kernel.controller', 'method' => 'onKernelController'));
+        $container->setDefinition('demo.tokens.action_listener', $listener);
+