> For example, in ObjC an int is an object, but it is not observable. However, an ObjC object with an int property is observable using Key-Value Observing

I haven't written Objective-C in a decade or so, but isn't this a pretty big mischaracterization of the language? NSInteger is a typedef to a C type IIRC, while there's NSNumber for the cases you want an object and/or are deserializing -- and which has observable propeties?