How to observe document setup changes

Question

Hi everybody,
I'd like to observe any change on document setup (anything you can do in menu "File" -> "Document setup..."), such as orientation, document size, etc.
The first and only way I can see is, even I know this is not the good one, is intercepting the command kSetPageSetupPrefsCmdBoss (using
ICommandInterceptor).
But I'd prefer to use the clean way, I mean Observer. But I can't find what is the subject to observe :
Here's the signature of the method in ISubject
    void AttachObserver(IObserver* observer,
                                      const PMIID& interestedIn,
                                       const PMIID& asObserver = IID_IOBSERVER);
I know observer mechanism. I just can't find what I should observe to get what I am interested in.
And in a general manner, what is the best way to find the subject I am interested in (not only for document) ?

Answer 1

I finally found the solution:
Implement an IObserver:
IObserver documentObserver = InterfacePtr<IObserver>(::CreateObject2<IObserver>(kMyDocumentObserverBoss)) ;
Get the workspace UIDRef, and then the subject to observe:
UIDRef documentWorkspaceRef = document->GetDocWorkSpace();
InterfacePtr<ISubject> documentWorkspaceSubject(documentWorkspaceRef, UseDefaultIID());
Attach the observer when needed :
documentWorkspaceSubject->AttachObserver(documentObserver, IID_IPAGEPREFERENCES, IID_IOBSERVER);
The value I was looking for was initially IID_IPAGEPREFERENCES. I found it with spy in the debug version of InDesign.