What do brackets do in an interface declaration

Question

I stumbled upon https://github.com/AlanQuatermain/aqtoolkit/blob/master/Extensions/NSObject%2BProperties.h

The interface is defined as:

@interface NSObject (AQProperties)

What does the (AQProperties) part do?

Answer 1

They are categories - a great way to extend existing classes without subclassing them.

To sum up, your NSObject (AQProperties) defines an NSObject with some extra methods that pertains to AQProperties.

Unlike a subclass, you can only add methods to a category, not an extra data member.

Answer 2

It's called a "category". You can extend existing classes, even if you do not have the source code for them, by adding your methods to a named category:

@interface NSString (MyFancyStuff)
- (NSString*) stringByCopyingStringAndReplacingDollarCharsWithEuros;
- (NSUInteger) lengthIgnoringWhitespace;
@end

You can then implement these new methods as usual

@implementation NSString (MyFancyStuff)
- (NSString*) stringByCopyingStringAndReplacingDollarCharsWithEuros
{
    ...
}

- (NSUInteger) lengthIgnoringWhitespace 
{
    ...
}
@end

These new methods then behave like all other methods of NSString (provided, you did include the header file, where you declared the category):

NSString* text = [NSString stringWithFormat: @"Hello, %@", param];
NSUInteger length = [text lengthIgnoringWhitespace];

...

Apart from being a tool to extend classes, whose code you don't have, categories can also be useful to structure the interfaces of your own classes. For example:

FancyStuff.h

@interface FancyStuff 
// Public methods and properties are declared here
@end

FancyStuff+Protected.h

#import "FancyStuff.h"

@interface FancyStuff (Protected)
// Methods, which may be useful for classes, which derive from 
// FancyStuff, but should not be considered part of the public API
@end

FancyStuff.m

#import "FancyStuff+Protected.h"

@implementation FancyStuff
...
@end

@implementation FancyStuff (Protected)
...
@end