How to create private methods and ivars in Objective-C in iOS >= 4.0?

Question

I've heard that lately it's possible to create private methods by declaring the interface once again in the .m file. But what exactly is the syntax like?

If it matters: Under ARC.

• Must it appear right before @implementation?
• Does it look exactly like the @interface declaration in the .h file?
• Must it duplicate the information about inheritance and protocol compliance?
• Since when is this available? What's the oldest iOS and Objc-C runtime that supports it?

Answer 1

Header file:

//YourClass.h:
@interface YourClass {
    @private //optional
    //private scope ivars
    @protected //default, optional
    //protected scope ivars
    @public //optional
    //public scope ivars
    @package //optional
    //package scope ivars
}

    //public methods

@end

Implementation file:

//YourClass.m:
#import "YourClass.h"

//you could also import this class extension (that's what it's called) from an
//external header file which can be helpful for making pseudo-protected methods/ivars
//Don't forget the additional import statement then, though.
@interface YourClass () <PrivateProtocol> //protocol tag optional, of cource

    @private //optional
    //private scope hidden ivars
    @protected //default, optional
    //protected scope hidden ivars
    @public //optional
    //public scope hidden ivars
    @package //optional
    //package scope hidden ivars

@end

@implementation YourClass

//your class' method implementations

@end

Docs on ivar scopes.

For more information on what's new in the above snippet and what it's all about (compatibility, etc) see the WWDC 2011 Session #322

Xcode 4.2 brought the hidden private ivars. Class extensions have been around for quite a while.