Question

NSObjectController and its subclasses return a proxy object for the selection. I’d like to understand why they chose to design it this way. The documentation doesn’t seem to shed much light on why this is.

Answer 1

Apple’s Cocoa Bindings Programming Topics has this to say:

You can establish bindings to a controller’s selection method or the
selectedObjects method. However, you should avoid binding through the
selectedObjects array, for example selectedObjects.name. Instead, you
should use selection.name. Similarly, you should avoid observing keys
through the array returned by selectedObjects. The proxy returned by
selection is more efficient at managing changes in key-value observing
as the selection changes.

I think you’ll find that your answer lies in that final sentence.

For what it’s worth, [objectController valueForKeyPath:@"selection.self"] returns the selected object itself.