Captcha Plugin Architecture
===========================

This document contains design notes for the plone.app.discussion Captcha plugin
architecture.


Introduction
------------

When a Captcha plugin (e.g. plone.formwidget.captcha or
plone.formwidget.recaptcha) is installed, plone.app.discussion extends the
comment form with a Captcha field/widget and a Captcha validator.

The form extender and validator are only registered if there is a plugin
installed that claims to provide the "plone.app.discussion-captcha" feature in
its configure.zcml file::

    <configure
        xmlns:meta="http://namespaces.zope.org/meta"
        xmlns:zcml="http://namespaces.zope.org/zcml">

        <!-- Declare that plone.formwidget.captcha provides a Captcha field that
             can be used by plone.app.discussion to add a Captcha field to comment
             forms. -->
        <meta:provides feature="plone.app.discussion-captcha" />

    </configure>

.. note::

  Currently plone.formwidget.captcha and plone.formwidget.recaptcha claim to
  provide such a feature. If you want to write your own Captcha plugin, it has
  to provide this feature as well.

.. seealso::

  * https://svn.plone.org/svn/plone/plone.formwidget.captcha/trunk/plone/formwidget/captcha/meta.zcml
  * https://svn.plone.org/svn/plone/plone.formwidget.recaptcha/trunk/plone/formwidget/recaptcha/meta.zcml


CaptchaExtender
---------------

The CaptchaExtender class extends the comment form with a Captcha field and
widget. The CaptchaExtender currently uses either the CaptchaFieldWidget from
plone.formwidget.captcha or the ReCaptchaFieldWidget from
plone.formwidget.recaptcha. If you want to write your own Captcha solution, you
have to override the update() method of the CaptchaExtender or write your own
CaptchaExtender class.

.. literalinclude:: ../../plone/app/discussion/browser/captcha.py
   :language: python
   :pyobject: CaptchaExtender


CaptchaValidator
----------------

The CaptchaValidator class provides custom versions of the
plone.formwidget.captcha and the plone.formwidget.recaptcha validators. It does
this, because we want to be able to have more than one Captcha solution
installed in one Plone instance. We also want to be able to easily switch
between different Captcha implementations inside a single Plone instance.

Therefore we have to check which Captcha solution is enabled in the discussion
control panel and use only the selected Captcha validator. It is not enough to
check if a Captcha plugin is just installed, because there could be more than
one.

We do two checks. First we check for a suitable Captcha solution (check for the
plone.app.discussion-captcha feature, see notes above). Second, we check which
Captcha solution is enabled in the discussion control panel and apply the
corresponding field validator.

The plone.app.discussion captcha validator always checks for a view with the
name of the captcha plugin. For instance, if plone.formwidget.captcha is enabled
it checks for a "captcha" view.

.. literalinclude:: ../../plone/app/discussion/browser/validator.py
   :language: python
   :pyobject: CaptchaValidator


.. Writing your own Captcha plugin
   -------------------------------

   If you want to write your own Captcha plugin, you have to add at least the
   following things:

   1) Add your Captcha plugin to the captcha_vocabulary (vocabularies.py)

   2) Override the update() method of the CapchaExtender class (browser/captcha.py)
      to show your own Captcha widget.

   3) Override the validate() method of the CaptchaValidator class
      (browser/validator.py) to use your own Captcha validator.