What's a good way to bind from a shared utility window and the frontmost document window?

Question

I have an application which allows for multiple NSDocuments to be open. In this application is a single utility window that contains some functionality that I want to apply to the frontmost document.

I am trying to use bindings here, so the trick is how to cleanly bind the user interface of the utility window to the frontmost document. The goal is that then switching the frontmost document window will update the view in the utility window; controls that are bound to properties of the frontmost document's model would be updated appropriately when state changes in the document's model, etc.

For sending actions from such a window, it's easy to just use first responder; the document object can intercept actions via the responder chain. But I want more than this, and of course you can't bind to the first responder.

A few ideas that I have:

• put an object controller in my nib for the shared window. When a document window changes frontmost status, change the content of that binding. A disadvantage of this is that if I were to have another kind of utility window, I'd have to remember to hook up the bindings from the document window to that utility window too!
• Make an accessor in the application delegate that gets the frontmost document window by traversing the window list. My utility window would just bind through the application delegate's method. A disadvantage here is that it's not KVO compliant
• Have a getter and setter in the application delegate to determine (and perhaps set to be KVO-compliant? would that make sense?) the frontmost document. Perhaps use window notifications set an ivar to the appropriate value when a window loses main status. Update: I'm using this for now, and it actually seems pretty clean. I set the value from the windowDidBecomeMain notification of my doc window and clear it (if it's the current value) in windowWillClose. Unless there is any major objection, this is probably the approach I'll use.
• One idea was to bind to mainWindow.windowController.document ... this comes close, except that when my shared window becomes main, then this binding goes away. So really I need to find the frontmost document window's controller (and of the right class).

None of these seem quite right. Is there a better way to do this that I'm missing?

Answer 1

I’ve always bound through Shared Application, mainWindow.document, which works fine. if you have windows w/o documents, you may want to add a mainYourKindOfWindow key that is implemented by watching mainWindow and updating the value based on some filter criteria.

Answer 2

Leopard's TextEdit does this for its inspector. Check it out in file:///Developer/Examples/AppKit/TextEdit.