Extract category

// Cat.h
@interface Cat : NSObject
- (void)purr;
- (void)meow;
@end
// Cat.m
@implementation Cat {
}
// This method will be extracted
// to a category
- (void)purr {
    NSLog(@"Purr, purr, purr...");
}
- (void)meow {
    NSLog(@"Meow!");
}
@end

// New category
// Cat+Purr.h
@interface Cat (Purr)
- (void)purr;
@end
// Cat+Purr.m
@implementation Cat (Purr)
- (void)purr {
    NSLog(@"Purr, purr, purr...");
}
@end

// Cat.h
@interface Cat : NSObject
- (void)meow;
@end
// Cat.m
@implementation Cat {
}
- (void)meow {
    NSLog(@"Meow!");
}
@end