Using Categories in Objective C

Question

I have a file called ViewMessages.m which is becoming quite verbose. I'd like to refactor and put part of my methods into a separate file. It seems that Categories are the right way to go, so I've created a Category on ViewMessages called DataEngineViewMessages

It created 2 files, DataEngineViewMessages.h and DataEngineViewMessages.m

My Question: Do I need to #import "DataEngineViewMessage.h" in my original ViewMessage.h or .m in order to access my new methods?

Answer 1

Not into the .h file. The category header should import ViewMessages.h, and if required the category header should be imported into ViewMessages.m. This prevents any kind of circularity.

Usually you want to differentiate between category methods that should be public (defined in a header perhaps like ViewMessages+DataEngine.h) and category methods that are for internal use (defined in a header perhaps like ViewMessages+Private.h). The former shouldn't be included by ViewMessages and the latter should (again, in the .m file).

Answer 2

Any code that calls the methods will raise a compiler warning if the compiler can't see the method definition. They may also raise an error if the compiler can't figure out exactly what to do about the return type and you're expecting it to be an atomic C type.

In terms of keeping the external interface untouched, you can probably just add a #import for your category methods at the bottom of the main class's header file.