[Framework-Team] plone.theme - one more package for Plone 3?

[Martin Aspeli]

Hi guys,

David, Florian and I were discussing the skinning story in Plone 3. 
David was a bit frustrated trying to customize things like the portal 
logo in a clean way. We made some headway, but discovered that it wasn't 
very easy to make use of Zope 3 browser layers in Plone.

For those not familiar with them, browser layers are markers which are 
applied to the request during traversal. You can register views, 
viewlets etc for a particular layer marker, either to override a view 
for a particular skin/theme, or to make sure a particular viewlet is 
only shown in a particular skin/theme.

For example:

  <browser:viewlet
     name="my.viewlet"
     for="*"
     layer=".interfaces.IMyTheme"
     manager="plone.app.layout.viewlets.interfaces.IAboveContent"
     template="myviewlet.pt"
     permission="zope2.View"
     />

This will only show up if the IMyTheme skin/browser layer is applied.

The most obvious way for this to would be for IMyTheme to correspond to 
a particular theme/skin as selected in portal_skins.

That's where plone.theme comes in. It provides an event handler 
listening to IBeforeTraverseEvent, and will attempt to match a browser 
layer marker to the current skin selection.

To use it, let's say you have "My Theme" in portal_skins. You could then 
have a marker interface like:

     from zope.publisher.interfaces.browser import IDefaultBrowserLayer

     class IMyTheme(IDefaultBrowserLayer):
         """Marker interface used in the tests
         """

and in configure.zcml:

     <interface
         interface=".interfaces.IMyTheme"
         type="zope.publisher.interfaces.browser.IBrowserSkinType"
         name="My Theme"
         />

The trick here is that the name of the IBrowserSkinType utility matches 
the name of the skin selection/theme in portal_skins.

The code is here:

   http://dev.plone.org/plone/browser/plone.theme/trunk/plone/theme

It's pretty simple, and there are (doc)tests. It should be pretty low risk.

Now, nothing in Plone core uses this (though NuPlone could use it, 
perhaps). I can release this to the Cheese Shop and things like 
DIYPloneStyle can depend on it. However, it may be useful to integrators 
and customisers if this is part of the Plone 3 distribution, and its 
ZCML was loaded from Products/CMFPlone to avoid slugs or needing to 
include it explicitly.

I can manage the release and add it to the lib bundle and ploneout, if 
no-one objects to it going in at this (admittedly late) stage.

Martin