Objective-C Category and new iVar

Question

I try to extend the functionality of SimpleAudioEngine of cocos2d with the ability to play several sound effect one after another as some kind of chain. I tried to do this w

Answer 1

You can not add iVars but can add property variables. Something like below:

In your .h:

#import <objc/runtime.h>

@interface Chair (Liking)

     @property (nonatomic, assign)BOOL liked;
 @end

In your .m:

#import "Chair+ChairCat.h"

@implementation Chair (Liking)

 -(BOOL)liked{

    return [ objc_getAssociatedObject( self, "_aliked" ) boolValue ] ;    
 }

-(void)setLiked:(BOOL)b{    
    objc_setAssociatedObject(self, "_aliked",  [ NSNumber numberWithBool:b ],OBJC_ASSOCIATION_RETAIN_NONATOMIC ) ;

 }

@end

Then somewhere in some other class say myViewController.m

#import "Chair+ChairCat.h"
- (void)viewDidLoad {
    /////
    Chair *chair = [[Chair alloc] init];
    [chair setLiked:NO];
}

Answer 2

You are correct that you can't add instance variables (or synthesized @properties) to a category. You can workaround this limitation using the Objective-C runtime's support for Associative References

Something like this:

In your .h:

@interface SimpleAudioEngine (SoundChainHelper)
    @property (nonatomic, retain) NSMutableArray *soundsInChain;
@end

In your .m:

#import <objc/runtime.h>
static char soundsInChainKey;

@implementation SimpleAudioEngine (SoundChainHelper)

- (NSMutableArray *)soundsInChain
{
   return objc_getAssociatedObject(self, &soundsInChainKey);
}

- (void)setSoundsInChain:(NSMutableArray *)array
{
    objc_setAssociatedObject(self, &soundsInChainKey, array, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

(The standard disclaimer applies. I typed this in the browser, and didn't test it, but I have used this technique before.)

The documentation I linked to has a lot more information about how associative references work.