DI is a property of a runtime execution enviroment requiring dynamic binding. I'm very new to Obj-C and Cocoa so I may speak out of turn. Unless I'm missing something, I don't see how one could implement DI except by interpreting Obj C rather than compiling it, or by modifying the runtime environment.
I suspect that the DI like behaviour of IB is because there is a domain specific runtime environment associated with apps that are built with it.
I'm happy to be corrected though.
Categories appear to be an implementation of mixin's, allowing dynamic dispatch of methods to a delegate. Rather cool and similar to Java's interface concept, thought the details differ and from the following, I can't see if constants can be defined in a category, though member fields cannot.
objective-c categories