[cookbook] initial recipe for accepting a JSON request body

doc/cookbook.rst

+Cookbook
+========
+
+The cookbook section contains recipes for solving specific problems.
+
+Recipes
+-------
+
+* :doc:`Accepting a JSON request body <recipes/json_request_body>`
+  A common need when building a restful API is the ability to accept a JSON
+  encoded entity from the request body.

doc/cookbook/json_request_body.rst

+Accepting a JSON request body
+=============================
+
+A common need when building a restful API is the ability to accept a JSON
+encoded entity from the request body.
+
+An example for such an API could be a blog post creation.
+
+Example API
+-----------
+
+In this example we will create an API for creating a blog post. The following
+is a spec of how we want it to work.
+
+Request
+~~~~~~~
+
+In the request we send the data for the blog post as a JSON object. We also
+indicate that using the ``Content-Type`` header.
+
+::
+
+    POST /blog/posts
+    Accept: application/json
+    Content-Type: application/json
+    Content-Length: 57
+    {"title":"Hello World!","body":"This is my first post!"}
+
+Response
+~~~~~~~~
+
+The server responds with a 201 status code, telling us that the post was
+created. It tells us the ``Content-Type`` of the response, which is also
+JSON.
+
+::
+
+    HTTP/1.1 201 Ceated
+    Content-Type: application/json
+    Content-Length: 65
+    Connection: close
+    {"id":"1","title":"Hello World!","body":"This is my first post!"}
+
+Parsing the request body
+------------------------
+
+The request body should only be parsed as JSON if the ``Content-Type`` header
+begins with ``application/json``. Since we want to do this for every request,
+the easiest solution is to use a before filter.
+
+We simply use ``json_decode`` to parse the content of the request and then
+replace the request data on the ``$request`` object.
+
+::
+
+    use Symfony\Component\HttpFoundation\Request;
+    use Symfony\Component\HttpFoundation\ParameterBag;
+
+    $app->before(function (Request $request) {
+        if (0 === strpos($request->headers->get('Content-Type'), 'application/json') {
+            $data = json_decode($request->getContent(), true);
+            $request->request = new ParameterBag(is_array($data) ? $data : array());
+        }
+    });
+
+Controller implementation
+-------------------------
+
+Our controller will create a new blog post from the data provided and will
+return the post object, including its ``id``, as JSON.
+
+::
+
+    use Symfony\Component\HttpFoundation\Request;
+    use Symfony\Component\HttpFoundation\Response;
+
+    $app->match('/blog/posts', function (Request $request) {
+        $post = array(
+            'title' => $request->get('title'),
+            'body'  => $request->get('body'),
+        );
+
+        $post['id'] = createPost($post);
+
+        $json = json_encode($post);
+        return new Response($json, 201);
+    });
+
+Manual testing
+--------------
+
+In order to manually test our API, we can use the ``curl`` command line
+utility, which allows sending HTTP requests.
+
+::
+
+    $ curl http://blog.lo/blog/posts -d '{"title":"Hello World!","body":"This is my first post!"}' -H 'Content-Type: application/json'
+    {"id":"1","title":"Hello World!","body":"This is my first post!"}
+    $