.. _hooks_chapter:

Using ZCML Hooks
================

ZCML "hooks" can be used to influence the behavior of the
:mod:`repoze.bfg` framework in various ways.  This is an advanced
topic; not many people will want or need to do any of these things.

Changing the request factory
----------------------------

You may change the class used as the "request factory" from within the
:mod:`repoze.bfg` ``Router`` class (the ``Router`` class turns the
WSGI environment into a "request" object which is used ubiquitously
throughout :mod:`repoze.bfg`).  The default "request factory" is the
class ``webob.Request``.  You may change it by placing the following
ZCML in your ``configure.zcml`` file.

.. code-block:: xml
   :linenos:

   <utility provides="repoze.bfg.interfaces.IRequestFactory"
            component="helloworld.factories.request_factory"/>

Replace ``helloworld.factories.request_factory`` with the Python
dotted name to the request factory you want to use.  Here's some
sample code that implements a minimal request factory:

.. code-block:: python

   from webob import Request
   from repoze.bfg.interfaces import IRequest

   class MyRequest(Request):
       implements(IRequest)

   def request_factory():
       return MyRequest

.. warning:: If you register an ``IRequestFactory`` utility in this
   way, you *must* be sure that the factory returns an object that
   implements *at least* the ``repoze.bfg.interfaces.IRequest``
   interface.  Otherwise all application view lookups will fail (they
   will all return a 404 response code).  Likewise, if you want to be
   able to use method-related interfaces such as ``IGETRequest``,
   ``IPOSTRequest``, etc. in your view declarations, the callable
   returned by the factory must also do the same introspection of the
   environ that the default request factory does and decorate the
   returned object to implement one of these interfaces based on the
   ``HTTP_METHOD`` present in the environ.  Note that the above
   example does not do this, so lookups for method-related interfaces
   will fail.

Changing the response factory
-----------------------------

You may change the class used as the "response factory" from within
the :mod:`repoze.bfg` ``chameleon_zpt``, ``chameleon_genshi``,
``chameleon_text`` (the ``render_template_to_response`` function used
within each) and other various places where a Response object is
constructed by :mod:`repoze.bfg`.  The default "response factory" is
the class ``webob.Response``.  You may change it by placing the
following ZCML in your ``configure.zcml`` file.

.. code-block:: xml
   :linenos:

   <utility provides="repoze.bfg.interfaces.IResponseFactory"
            component="helloworld.factories.response_factory"/>

Replace ``helloworld.factories.response_factory`` with the Python
dotted name to the response factory you want to use.  Here's some
sample code that implements a minimal response factory:

.. code-block:: python

   from webob import Response

   class MyResponse(Response):
      pass

   def response_factory():
       return MyResponse

Unlike a request factory, a response factory does not need to return
an object that implements any particular interface; it simply needs
have a ``status`` attribute, a ``headerlist`` attribute, and and
``app_iter`` attribute.

Changing the NotFound application
---------------------------------

When :mod:`repoze.bfg` can't map a URL to code, it creates and invokes
a NotFound WSGI application. The application it invokes can be
customized by placing something like the following ZCML in your
``configure.zcml`` file.

.. code-block:: xml
   :linenos:

   <utility provides="repoze.bfg.interfaces.INotFoundAppFactory"
            component="helloworld.factories.notfound_app_factory"/>

Replace ``helloworld.factories.notfound_app_factory`` with the Python
dotted name to the request factory you want to use.  Here's some
sample code that implements a minimal NotFound application factory:

.. code-block:: python

   from webob.exc import HTTPNotFound

   class MyNotFound(HTTPNotFound):
       pass

   def notfound_app_factory():
       return MyNotFound

.. note:: When a NotFound application factory is invoked, it is passed
   the WSGI environ and the WSGI ``start_response`` handler by
   :mod:`repoze.bfg`.  Within the WSGI environ will be a key named
   ``message`` that has a value explaining why the not found error was
   raised.  This error will be different when the ``debug_notfound``
   environment setting is true than it is when it is false.

Changing the Unauthorized application
-------------------------------------

When :mod:`repoze.bfg` can't authorize execution of a view based on
the security policy in use, it creates and invokes an Unauthorized
WSGI application. The application it invokes can be customized by
placing something like the following ZCML in your ``configure.zcml``
file.

.. code-block:: xml
   :linenos:

   <utility provides="repoze.bfg.interfaces.IUnauthorizedAppFactory"
            component="helloworld.factories.unauthorized_app_factory"/>

Replace ``helloworld.factories.unauthorized_app_factory`` with the
Python dotted name to the request factory you want to use.  Here's
some sample code that implements a minimal Unauthorized application
factory:

.. code-block:: python

   from webob.exc import HTTPUnauthorized

   class MyUnauthorized(HTTPUnauthorized):
       pass

   def notfound_app_factory():
       return MyUnauthorized

.. note:: When an Unauthorized application factory is invoked, it is
   passed the WSGI environ and the WSGI ``start_response`` handler by
   :mod:`repoze.bfg`.  Within the WSGI environ will be a key named
   ``message`` that has a value explaining why the action was not
   authorized.  This error will be different when the
   ``debug_authorization`` environment setting is true than it is when
   it is false.

.. note:: You can influence the status code of Unauthorized responses
   by using an alterate unauthorized application factory.  For
   example, you may return an unauthorized application with a ``403
   Forbidden`` status code, rather than use the default unauthorized
   application factory, which sends a response with a ``401
   Unauthorized`` status code.