Re: Issues creating an RDFInferDataSource

I apologize for the cross-post, but I'm not sure if this is an XPCOM or 
XPFE problem anymore.  Please feel free to burn me in effigy if I'm wrong.

Axel Hecht wrote:
> Michael A Nachbaur schrieb:
> 
>> I'm creating an nsIRDFInferDataSource object as per the recommendation 
>> of Axel Hecht (see http://xrl.us/ei89 for the discussion) over on the 
>> XPFE mailing list.  This was an awesome recommendation because my 
>> application is much cleaner as a result.  However, I'm having some 
>> problems implementing the nsIRDFDataSource interface that I was hoping 
>> to get some help on.
>>
>> I'm using my datasource to drive XUL templates (not in threads this 
>> time), and while it works to a point, I have some strange issues I'm 
>> unable to debug.
>>
>> Basically, I create one instance of my datasource for each other RDF 
>> datasource I inspect.  This I then add to the composite datasource 
>> bound to a XUL template, and then rebuild the tree.  I can see from my 
>> debugging code that the XUL template requests all the appropriate 
>> statements from my datasource.  However, when I add a second 
>> datasource to the composite, the XUL template only queries up to a 
>> point the required statements.
>>
>> XUL code: http://xrl.us/ei9d
>> JavaScript for the interface: http://xrl.us/ei9e
>> JavaScript for the RDF datasource: http://xrl.us/ei9f
>> Log of the debugging output: http://xrl.us/ei9g
>> Screenshot showing the problem: http://xrl.us/ei9h
>>
>> The problem is really exhibited near the very end of the log output.  
>> In the test, I load the first file (file:///home/nachbaur/foaf.rdf).  
>> This loads successfully.  Then I load the second file (in this 
>> instance, file:///home/nachbaur/projects/scampclient/install.rdf).  
>> The XUL template processes the necessary statements, but when it gets 
>> to the point where it calls GetTarget on the "Watch", "StatementCount" 
>> and "DataSourceType" predicates, it queries against the first 
>> datasource in the composite (the foaf.rdf object) and not the second.
>>
>> Does anyone have any ideas as to why this might be?  I suppose I'm 
>> implementing my GetTarget method incorrectly, but I don't know what is 
>> wrong.  I apologize in advance for the verbosity of the above 
>> URLs...if anyone would like clarification on anything, please let me 
>> know.
>>
> 
> This stuff is designed to have one infer datasource working on the 
> composite datasource, triggered by a infer="rdfinspector".

I haven't seen this in any documentation anywhere.  Looking at 
nsXULTemplateBuilder.cpp, it seems that the infer attribute specifies 
the contract ID that the template builder then uses to QI the datasource.

> Now, what I think you're doing is giving each of your DSs in the 
> composite ds its own infer DS, and of course, it will ask the first for 
> a target from the template builder and bail there somewhere.

Yes, this is whats happening.  In pseudo-code:

var ds1 = new RDFInspectorDataSource(url1);
var ds2 = new RDFInspectorDataSource(url2);
tree.database.AddDataSource(ds1);
tree.database.AddDataSource(ds2);

However, since in the above pseudo-code, the GetTarget method is not 
returning any nodes for the requested arc, the composite datasource 
should AFAICT query the second datasource.

At least, it works for the "URI" arc, which is how the second item in 
the tree shows up at all.  However, the other two arcs are not.  The 
interesting thing here is that they are defined in the XUL template as 
bindings, whereas the URI is defined using a <triple/> element.  I had 
considered changing my XUL template to use all <triple/>s, but I wanted 
to solve the problem in my datasource rather than the symptom.

> Not really 
> sure how to solve this, but this is still an xpfe question to me, and 
> not a xpcom one.

Well, looking at the requirements of my project, I actually don't 
strictly need to implement this as an nsIRDFInferDataSource, but rather 
I probably should just implement a standard nsIRDFDataSource object.  In 
this case, the inference-specific XUL template code wouldn't really 
apply (at least, this is the way my code is turning out in any case).

Do you think because I'm implementing the nsIRDFInferDataSource 
interface, the XUL templates are behaving differently with my datasources?

> Try a little more digging, I'm out for food.

Same here.  I really appreciate your help.  I would like to do my best 
to document what I find here as I work on this project, since there's a 
lot about some of the XPCOM interfaces that aren't yet.  Therefore 
things like http://dev.mozref.com/reference/interfaces/nsIThread are in 
the works (though my stylesheets for XPCOM interfaces is very new)

-- 
Michael A. Nachbaur <[email protected]>
http://nachbaur.com/pgpkey.asc