Working with new Sling developers, I find that one of the concepts
they have trouble with is in the area of Adapters and that it is hard
to find out what objects are adaptable to which classes. Although it
is possible to document this (see for example
http://dev.day.com/docs/en/cq/current/developing/sling-adapters.html),
because new Adapters can be dynamically added (or removed) through
OSGi services, any documentation is potentially incomplete.
What I would like to do is create a new Web Console plugin which
displays the list of available adaptation pairs. I'm still thinking
about the UI, but you could imagine that a first version would be a
select list on the left listing the adaptable classes and a list on
the right of the possible adapters for the selected adaptable.
For implementations of AdapterFactory, the necessary metadata already
exists in the service registry, but for cases where a class implements
its own adapters, this metadata needs to be added.
In order to do this, I propose the following:
1) The creation of such a Web Console plugin (obviously).
2) The creation of new OSGi Services for classes which implement their
own adaptations. For example:
@Component
@Service(value=Object.class)
@Properties({
@Property(name="adaptables",
value="org.apache.sling.api.resource.Resource"),
@Property(name="adapters", value={
"javax.jcr.Node",
"javax.jcr.Item",
"java.io.InputStream",
"java.net.URL",
"java.util.Map",
"org.apache.sling.api.resource.ValueMap"
})
})
public class JcrNodeResourceAdapterMeta { }
3) The establishment of a new service property, named
adapter.condition, which optionally indicates the conditions under
which an adaptation can be made. For example, in the case above:
@Property(name="adapter.condition", value="If the resource is a
JcrNodeResource")
The Web Console plugin would need to note that potentially not all
adapters are listed, but I believe that if we add the metadata for all
the adapters in Sling, that will set a good precedent for downstream
projects to follow. If we want, we could also add a note which says
"If there's an adapter not listed here, please file an issue with the
adapter provider." :)
Adding this metadata is not a trivial amount of effort, but it also
something I feel will pay dividends down the road.
WDYT?
Justin
