This interface is implemented by an object that wants
 to observe an event corresponding to a topic.

 @status FROZEN
[scriptable, uuid(db242e01-e4d9-11d2-9dde-000064657374)]
interface nsIObserver : nsISupports

Methods

 Observe will be called when there is a notification for the
 topic |aTopic|.  This assumes that the object implementing
 this interface has been registered with an observer service
 such as the nsIObserverService. 

 If you expect multiple topics/subjects, the impl is 
 responsible for filtering.

 You should not modify, add, remove, or enumerate 
 notifications in the implemention of observe. 

 @param aSubject : Notification specific interface pointer.
 @param aTopic   : The notification topic or subject.
 @param aData    : Notification specific wide string.
                    subject event.
void observe(in nsISupports aSubject, in string aTopic, in wstring aData)