var Observer = {
   onBeginLoad: function(aSink)
   {
     alert("Beginning to load the RDF/XML...");
   },
   onInterrupt: function(aSink) {},
   onResume: function(aSink) {},
   onEndLoad: function(aSink)
   {
     doneLoading( ); // A function that does something with the datasource
   },
  onError: function(aSink, aStatus, aErrorMsg)
  {
    alert("Error: " + aErrorMsg);
  }
};