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FLOCow+isMilkable.h

#import "FLOCow.h"

@interface FLOCow (isMilkable)

- (BOOL) milkable;
- (void) setMilkable:(BOOL)canMilk;

@end
FLOCow+isMilkable

FLOCow+isMilkable.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "FLOCow+isMilkable.h"

@implementation FLOCow (isMilkable)

static char const _milkableKey;

- (BOOL) milkable{
    return [objc_getAssociatedObject(self, &_milkableKey) boolValue];
}

- (void) setMilkable:(BOOL)canMilk{
    objc_setAssociatedObject (self, &_milkableKey, [NSNumber numberWithBool:canMilk], OBJC_ASSOCIATION_RETAIN);
}

@end

FLOCow.h

#import <Foundation/Foundation.h>

@interface FLOCow : NSObject

- (NSString *) moo;

@end

FLOCow.m

#import "FLOCow.h"

@implementation FLOCow

- (NSString *) moo{
    return @"Moo!";
}

- (NSString *) description{
    return [NSString stringWithFormat:@"I am a %@",[self className]];
}

@end

FLOCowisMilkable.h

#import "FLOCow.h"

@interface FLOCow (isMilkable)

- (BOOL) milkable;
- (void) setMilkable:(BOOL)canMilk;

@end

FLOCowisMilkable.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "FLOCow+isMilkable.h"

@implementation FLOCow (isMilkable)

static char const _milkableKey;

- (BOOL) milkable{
    return [objc_getAssociatedObject(self, &_milkableKey) boolValue];
}

- (void) setMilkable:(BOOL)canMilk{
    objc_setAssociatedObject (self, &_milkableKey, [NSNumber numberWithBool:canMilk], OBJC_ASSOCIATION_RETAIN);
}

@end

main.m

#import <Foundation/Foundation.h>
#import "FLOCow.h"
#import "FLOCow+isMilkable.h"

int main(int argc, const char * argv[])
{
    @autoreleasepool {
                
        FLOCow * betsy = [[FLOCow alloc] init];
        FLOCow * stan = [[FLOCow alloc] init];
        
        NSLog(@"%@", betsy);
        NSLog(@"%@", [betsy moo]);
        
        [betsy setMilkable:YES];
        [stan setMilkable:NO];
        
        NSLog(@"Is stan milkable? %@", ([stan milkable] ? @"Yes, it's milkable." : @"No, that's silly."));
        NSLog(@"Is betsy milkable? %@", ([betsy milkable] ? @"Yes, it's milkable." : @"No, that's silly."));

    }
    return 0;
}