[Zope3-dev] RFC: zcml:condition

[Jim Fulton]

We've identified a real need for conditional ZCML
processing.

Consider an optional feature, like onlinehelp or zsync.
These system define directives or components that individual
packages will want to provide if these systems are available.

For example, a package might want to register on-line-help
documents if the on-line help system is available:

<configure
     xmlns="http://namespaces.zope.org/zope"
     xmlns:help="http://namespaces.zope.org/help"
     zcml:condition="have onlinehelp"
     >

   <help:register
       id="zope.app.authentication"
       parent="dev"
       title="Authentication"
       doc_path="README.txt"
       class="zope.app.onlinehelp.onlinehelptopic.RESTOnlineHelpTopic"
       />

   <help:register
       id="principalfolder"
       parent="dev/zope.app.authentication"
       title="Principal Folder"
       doc_path="principalfolder.txt"
       class="zope.app.onlinehelp.onlinehelptopic.RESTOnlineHelpTopic"
       />

   <help:register
       id="groupfolder"
       parent="dev/zope.app.authentication"
       title="Group Folder"
       doc_path="groupfolder.txt"
       class="zope.app.onlinehelp.onlinehelptopic.RESTOnlineHelpTopic"
       />

</configure>

Our previous attempt at something like this:

   http://svn.zope.org/?rev=28766&view=rev

allowed tales expressions as condition values.  Martijn objected
that this was too powerful and we removed the feature.

I'd like to propose a much simpler language for condition
values:

   verb arguments

The arguments depend on the verb.

Initially, we'd implement a single verb, "have", that
takes a feature name.

We'd also provide a meta configuration directive:

   <meta:provide feature="featurename" />

(as in <meta:provide feature="onlinehelp" />)

A condition with a "have" command will be true if there was
a previous provides directive for the named feature.

Thoughts?

Marius volunteered to write this this weekend, so I'd
like to reach consensus quickly. :)


Jim

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