What Plone programmers should know about ZCML.
- conflict resolution (e.g. two plug-ins cannot overlap);
- extensible syntax based on namespaces.
Downsides of ZCML are:
- it is cumbersome to write by hand;
- lack of end-user documentation.
Plone uses ZCML to:
- register components with various places in the system, both core and add-ons.
Everything you can do in ZCML can also be done in Python code.
- ZCML reference (does not include Plone specific directives)
Each Plone component (core, add-on) has a base
in the package root. This
file can include additional nested configuration files
- ZCML is always interpreted during Plone start-up.
- Your unit test may need to manually include ZCML.
- Funny exception error messages occur if Plone is started in the production mode and ZCML was not properly read for all the packages
When Plone is started all ZCML files are read.
New way: Python egg
setup.pyfile contains a autoinclude hint and is picked up automatically when all the packages are scanned.
Old way: ZCML reference must be manually added to the
zcml = sectionin
If ZCML contains errors Plone does not start up in the foreground
Besides layer overrides, ZCML provides more hardcore ways to override things in buildout. These overrides can also override utilities etc. and overrides take effect during ZCML parsing, not when site is run.
overrides.zcmlfile in your egg to the same folder as
Syntax is 100% same as in
- Restart Plone.
Before Plone 3.3, ZCML directives could not be
automatically picked up from eggs. To make Plone pick up
the directions in
overrides.zcml, you'd have to add this line in
zcml = ... myegg-overrides
Since Plone 3.3, the
plugin can do this (https://plone.org/products/plone/roadmap/247/).
If you ever find yourself needing to use a template from another package, you can do so with using the configure tag which will then run the block of ZCML in the context of that package.
Here's an example of overriding the
'folder_contents'. It is defined in package
<browser:page for="Products.CMFCore.interfaces._content.IFolderish" class=".folder.FolderContentsView" name="folder_contents" template="templates/folder_contents.pt" permission="cmf.ListFolderContents" />
In your own package
my.package, you want to override the class, but keep the template.
Assuming you created a class
directory of your package, add this
<configure xmlns="http://namespaces.zope.org/zope" xmlns:browser="http://namespaces.zope.org/browser" i18n_domain="my.package"> <!-- override folder_contents --> <configure package="plone.app.content.browser"> <browser:page for="Products.CMFCore.interfaces._content.IFolderish" class="my.package.browser.foldercontents.MyFolderContentsView" name="folder_contents" template="folder_contents.pt" layer="my.package.interfaces.IMyPackageLayer" permission="cmf.ListFolderContents" /> </configure> </configure>
Basically, you re-define the BrowserView in the context of its original package, so that the relative path to the template stays valid. But using the full path in dotted notation, you can let it point to your own class.
You can conditionally run ZCML if a certain package or feature is installed.
First, include the namespace at the top of the ZCML file:
<configure xmlns="http://namespaces.zope.org/zope" xmlns:zcml="http://namespaces.zope.org/zcml" i18n_domain="my.package"> ....
conditionally run for package:
<include zcml:condition="installed some.package" package=".package" /> <include zcml:condition="not-installed some.package" package=".otherpackage" />
conditionally run for feature:
<include zcml:condition="have plone-4" package=".package" /> <include zcml:condition="not-have plone-4" package=".otherpackage" />