[Developers] Calling Freebase from GWT?

[Alison Lee]

There are several issues to address in doing this.

First, Google Web Toolkit (GWT) supports building AJAX application in  
Java.  It provides a nice wrapper around XMLHttpRequest.  However,  
cross-site issues prevent that approach to work for normal Freebase  
application where the GWT application host and the Freebase query  
server run at different sites.  Section
4.5.2 of Developing Metaweb-Enabled Web Applications (http:// 
www.freebase.com/view/documentation/) describes several workarounds  
to the cross-site issues.  GWT's developer site provides information  
and an example of how to dynamically fetch JSON feeds from other web  
domains (http://code.google.com/support/bin/answer.py? 
answer=55954&topic=10394). Note: the approach works but the some of  
the description and the code snippets are buggy.  In summary, an  
approach is to:

a) use a dynamically created script tag with a mqlread URL and a  
callback parameter in the GWT application,
b) use the GWT approach to define a JSON callback into Java from  
external Javascript.

Second, aside from the errors in the above-mentioned GWT example,  
there are issues to be aware of.  For example, because a GWT is  
loaded in a zero-size IFrame inside the main document, this prevents  
"x instanceof Array" from working with the data returned by JSON.   
For details, see the discussion in GWT Google Groups (http:// 
groups.google.com/group/Google-Web-Toolkit/browse_thread/thread/ 
b750195091da28cb/1ba006b4577365a8? 
lnk=gst&q=instanceof&rnum=10#1ba006b4577365a8). Therefore, processing  
the JavaScriptObject provided by the JSON callback requires writing  
some JSNI code and using a workaround to test for arrays.

Without further ado, here's an application example that displays the  
MQL query result in a GWT tree widget.
   http://www.webcollab.com/freebase/gwtmqlread/

Alison

P.S.: GWT is available from http://code.google.com/webtoolkit/

> -----Original Message-----
> From: developers-bounces at freebase.com
> [mailto:developers-bounces at freebase.com] On Behalf Of William Pietri
> Sent: Tuesday, March 20, 2007 3:16 PM
> To: developers at freebase.com
> Subject: [Developers] Calling Freebase from GWT?
>
> Has anybody managed to call the Freebase API from a GWT component?  
> I'm a
> Javascript tyro, and I'm struggling to get it properly bound in  
> with JSNI.
> My current hurdle, which I think I'll get past shortly, is getting  
> the call
> to happen at all. But the one that scares me is getting callbacks  
> to work
> properly on result arrival.
>
> Thanks,
>
> William
>
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