Condition of using objc_setAssociated and objc_getAssociated

Question

I read the apple developer document of Customizing Existing Classes and Objective-C Runtime Reference.

I am wondering whether the objc_getAssociatedObject and objc_setAssociatedObject methods must be used with CATEGORY or not.

Is it mean category is used for customing method and objc_getAssociatedObject and objc_setAssociatedObject is used for customing (ususally add) ivar?

Is that make senses to use methods above to add another instance variable individually?

If yes, what's the condition to add instance variable ?

Thanks.

Latest edit:

ViewController.m

[XXView showView:[UIColor greenColor]];

XXView.m

+ (void)showView: (UIColor *)bgcolor {
   XXViewController *vc = [[XXViewController alloc] init];
  vc.backgroundColor = [self BackgroundColor];
}

+ (void)setBackgroundColor:(UIColor *)BackgroundColor {
   objc_setAssociatedObject(self, @selector(backgroundColor), BackgroundColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

+ (UIColor *)BackgroundColor {
   if (!objc_getAssociatedObject(self, _cmd)) {
     [self setBackgroundColor:[UIColor redColor]];
   }
   return objc_getAssociatedObject(self, _cmd);
}

Answer 1

The associated object functions can be used in any code; the code just needs to have a reference to the object to which you which to associate another object, a reference to the object you wish to associated, and access to the unique key value.

So yes, you can use them in category methods and this is a way to achieve something similar to an instance variable created and maintained by category methods.

For the unique key value use the address of a static variable – just declared such a variable, any type will do as you are only going to use its address, in the same file as you define the category methods. An address is used as every address in a program is unique.

For the policy argument you probably want OBJC_ASSOCIATION_RETAIN, which means the associated object reference will be retained and released along with the associating object - this mimics the default behaviour of instance variables under ARC.

Not exactly sure what you are asking with "condition to add instance variable", the only conditions are the key must be unique and you can only associate object references - no primitive values, but that is easily addressed if needed by associating an object which contains a primitive valued property. If you wish to associate multiple objects ("add multiple instance variables") then associating a single object with multiple properties is worth consideration.

HTH