Updated the documentation to show how to set cookies for not yet existing responses

docs/patterns/deferredcallbacks.rst

@@ -0,0 +1,73 @@
+.. _deferred-callbacks:
+
+Deferred Request Callbacks
+==========================
+
+One of the design principles of Flask is that response objects are created
+and passed down a chain of potential callbacks that can modify them or
+replace them.  When the request handling starts, there is no response
+object yet.  It is created as necessary either by a view function or by
+some other component in the system.
+
+But what happens if you want to modify the response at a point where the
+response does not exist yet?  A common example for that would be a
+before-request function that wants to set a cookie on the response object.
+
+One way is to avoid the situation.  Very often that is possible.  For
+instance you can try to move that logic into an after-request callback
+instead.  Sometimes however moving that code there is just not a very
+pleasant experience or makes code look very awkward.
+
+As an alternative possibility you can attach a bunch of callback functions
+to the :data:`~flask.g` object and call then at the end of the request.
+This way you can defer code execution from anywhere in the application.
+
+
+The Decorator
+-------------
+
+The following decorator is the key.  It registers a function on a list on
+the :data:`~flask.g` object::
+
+    from flask import g
+
+    def after_this_request(f):
+        if not hasattr(g, 'after_request_callbacks'):
+            g.after_request_callbacks = []
+        g.after_request_callbacks.append(f)
+        return f
+
+
+Calling the Deferred
+--------------------
+
+Now you can use the `after_this_request` decorator to mark a function to
+be called at the end of the request.  But we still need to call them.  For
+this the following function needs to be registered as
+:meth:`~flask.Flask.after_request` callback::
+
+    @app.after_request
+    def call_after_request_callbacks(response):
+        for callback in getattr(g, 'after_request_callbacks', ()):
+            response = callback(response)
+        return response
+
+
+A Practical Example
+-------------------
+
+Now we can easily at any point in time register a function to be called at
+the end of this particular request.  For example you can remember the
+current language of the user in a cookie in the before-request function::
+
+    from flask import request
+
+    @app.before_request
+    def detect_user_language():
+        language = request.cookies.get('user_lang')
+        if language is None:
+            language = guess_language_from_request()
+            @after_this_request
+            def remember_language(response):
+                response.set_cookie('user_lang', language)
+        g.language = language

docs/patterns/index.rst

@@ -36,3 +36,4 @@ Snippet Archives <http://flask.pocoo.org/snippets/>`_.
    mongokit
    favicon
    streaming
+   cookies

docs/quickstart.rst

@@ -621,6 +621,10 @@ just return strings from the view functions Flask will convert them into
 response objects for you.  If you explicitly want to do that you can use
 the :meth:`~flask.make_response` function and then modify it.
 
+Sometimes you might want to set a cookie at a point where the response
+object does not exist yet.  This is possible by utilizing the
+:ref:`deferred-callbacks` pattern.
+
 For this also see :ref:`about-responses`.
 
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