Objective-C Associated Objects, Subscripting Edition

main.m

#import <Foundation/Foundation.h>
#import "NSObject+OP6AssociatedObjects.h"

int main(int argc, char *argv[]) {
    
    NSObject *testObject = [NSObject new];
    testObject.associatedObjects[@"pi"] = @(M_PI);
    NSLog(@"%@", [testObject.associatedObjects[@"pi"] stringValue]); // Logs 3.141592653589793
    return EXIT_SUCCESS;
    
}

NSObject+OP6AssociatedObjects.h

//
//  NSObject+OP6AssociatedObjects.h
//
//  Created by Carter Allen on 8/8/12.
//  Copyright (c) 2012 Opt-6 Products, LLC. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject (OP6AssociatedObjects)
@property (readonly) NSMutableDictionary *associatedObjects;
@end

NSObject+OP6AssociatedObjects.m

//
//  NSObject+OP6AssociatedObjects.m
//
//  Created by Carter Allen on 8/8/12.
//  Copyright (c) 2012 Opt-6 Products, LLC. All rights reserved.
//

#import "NSObject+OP6AssociatedObjects.h"
#import <objc/runtime.h>

const char *OP6AssociatedObjectsDictionaryKey = "OP6AssociatedObjectsDictionaryKey";

@implementation NSObject (OP6AssociatedObjects)

- (NSMutableDictionary *)associatedObjects {
    NSMutableDictionary *associatedObjects = objc_getAssociatedObject(self, OP6AssociatedObjectsDictionaryKey);
    if (!associatedObjects) {
        associatedObjects = [NSMutableDictionary new];
        objc_setAssociatedObject(self, OP6AssociatedObjectsDictionaryKey, associatedObjects, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return associatedObjects;
}

@end

NSObjectOP6AssociatedObjects.h

//
//  NSObject+OP6AssociatedObjects.h
//
//  Created by Carter Allen on 8/8/12.
//  Copyright (c) 2012 Opt-6 Products, LLC. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject (OP6AssociatedObjects)
@property (readonly) NSMutableDictionary *associatedObjects;
@end

NSObjectOP6AssociatedObjects.m

//
//  NSObject+OP6AssociatedObjects.m
//
//  Created by Carter Allen on 8/8/12.
//  Copyright (c) 2012 Opt-6 Products, LLC. All rights reserved.
//

#import "NSObject+OP6AssociatedObjects.h"
#import <objc/runtime.h>

const char *OP6AssociatedObjectsDictionaryKey = "OP6AssociatedObjectsDictionaryKey";

@implementation NSObject (OP6AssociatedObjects)

- (NSMutableDictionary *)associatedObjects {
    NSMutableDictionary *associatedObjects = objc_getAssociatedObject(self, OP6AssociatedObjectsDictionaryKey);
    if (!associatedObjects) {
        associatedObjects = [NSMutableDictionary new];
        objc_setAssociatedObject(self, OP6AssociatedObjectsDictionaryKey, associatedObjects, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return associatedObjects;
}

@end