Question

DataController.h

@class Play;

@interface DataController : NSObject 

- (unsigned)countOfList;
- (Play *)objectInListAtIndex:(unsigned)theIndex;

@end

DataController.m

#import "DataController.h"
#import "Play.h"


@interface DataController ()
@property (nonatomic, copy, readwrite) NSMutableArray *list;
- (void)createDemoData;
@end


@implementation DataController

@synthesize list;


- (id)init {
    if (self = [super init]) {
        [self createDemoData];
    }
    return self;
}

Why do you think that @interface is defined twice? And also whats the meaning of ()? Shouldn’t there be a class name maybe the super class between the parentheses?

Answer 1

In general, the syntax @interface ClassName (CategoryName) is for declaring a category. Categories are a way to add methods to a class. You can do this even with classes for which you don’t have the source code. See more here.

@interface ClassName () (with nothing in the parentheses) is essentially a special case of a category and is called a class extension. The primary difference between a class extension and a category is that methods declared in a class extension must be defined/implemented in the main @implementation block for the class, or you’ll get a compiler warning. Methods in a regular category can be defined in an external @implementation block.

The most common use for class extensions (as in this case) is for declaring private methods. Objective-C doesn’t have support for true private methods, so an easy way to accomplish the same basic end result is to declare private methods in a class extension at the top of the .m file. Since these methods aren’t defined in the .h file, other classes won’t see them, and you’ll get a compiler warning if you try to use them outside the class they belong to.

You can also redeclare a readonly @property as readwrite in a class extension. That way, code external to the class implementation can only read a property’s value, but inside the class’s implementation, you can write to it too. This is the only case where it’s allowable to redeclare an @property.

(Note that class extensions were a new feature in Objective-C 2.0 and aren’t available on Mac OS X 10.4 and earlier.)