Objective-C Associated Objects

Introduction

First introduced in iOS 3.1 as part of the Objective-C runtime, associated objects provide a way to add instance variables to an existing class object (w\o subclassing.

This means you'll be able to attach any object to any other object without subclassing.

Syntax

  • void objc_setAssociatedObject(id object, void *key, id value, objc_AssociationPolicy policy)

  • id objc_getAssociatedObject(id object, void *key)

  • void objc_removeAssociatedObjects(id object)

Parameters

ParamDetails
objectThe existing object you want to modify
keyThis can basically be any pointer that has a constant memory address, but a nice practice is to use here a computed property (getter)
valueThe object you want to add
policyThe memory policy for this new value i.e. should it be retained / assigned, copied etc.. just like any other property you'd declare

Remarks

More details here:

NSHipster

@kostiakoval

kingscocoa

Basic Associated Object Example

Assume we need to add an NSString object to SomeClass (we cant subclass).

In this example we not only create an associated object but also wrap it in a computed property in a category for extra neatness

#import <objc/runtime.h>

@interface SomeClass (MyCategory)
// This is the property wrapping the associated object. below we implement the setter and getter which actually utilize the object association
@property (nonatomic, retain) NSString *associated;
@end

@implementation SomeClass (MyCategory)

- (void)setAssociated:(NSString *)object {
    objc_setAssociatedObject(self, @selector(associated), object,
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSString *)associated {
    return objc_getAssociatedObject(self, @selector(associated));
}

Now it would be as easy as this to use the property

SomeClass *instance = [SomeClass alloc] init];
instance.associated = @"this property is an associated object under the hood";