Obj-C has changed a lot since I first used it. For instance, reference counting was performed manually via retain, release, and auto-release. There was a brief year where the Boehm garbage collector could be used, but then that was deprecated. Automatic Reference Counting (ARC) came a bit later and properties were added. For a few years you could mix manual reference counting and ARC. Early on you would override dealloc, but later that was a no no. And on it goes...

If you write Apple software, you are generally rewriting you apps every year. The API's are deprecated very often, and the languages are in constant flux as well.