Which is the best way to define private methods for a class in Objective-C?

Question

Like:

@interface ClassXXName(private)
- (void)  xxxfunctions
@end

or user category methods?

Answer 1

@interface Foo() creates a class extension (I stand corrected, props to bbum) on interface Foo which is like additional methods added to the interface. Some people also use @interafce Foo(Private) (category) instead of a class extension with (). It's more like "injecting" new methods into a class from outside the class.

Placing this in the .m file just keeps other things from "seeing it" in the .h file, but that's it. Basically people normally use categories or class extensions in .m files to specify private interfaces, but they are also used for things like UIKit uses categories to add row and section public methods to NSIndexPath. (This can be confusing.)

You don't really need to define private methods this way, but if you have a method called bar that calls method foo before foo is defined in the source file you'll get a compiler warning something like "object self may not respond to foo". You can get rid of that by defining foo before you define bar or any other foo-calling code. It's the same with plain C and functions.

Like Ole says this doesn't stop anyone from calling the private methods, it just declares your intention that they be private and causes the compiler to generate the "may not respond to" warnings even if they import the .h file.