Swizzling a single instance, not a class

Question

I have a category on NSObject which supposed to so some stuff. When I call it on an object, I would like to override its dealloc method to do some cleanups.

I wanted to

Answer 1

Method selection is based on the class of an object instance, so method swizzling affects all instances of the same class - as you discovered.

But you can change the class of an instance, but you must be careful! Here is the outline, assume you have a class:

@instance MyPlainObject : NSObject

- (void) doSomething;

@end

Now if for just some of the instances of MyPlainObject you'd like to alter the behaviour of doSomething you first define a subclass:

@instance MyFancyObject: MyPlainObject

- (void) doSomething;

@end

Now you can clearly make instances of MyFancyObject, but what we need to do is take a pre-existing instance of MyPlainObject and make it into a MyFancyObject so we get the new behaviour. For that we can swizzle the class, add the following to MyFancyObject:

static Class myPlainObjectClass;
static Class myFancyObjectClass;

+ (void)initialize
{
   myPlainObjectClass = objc_getClass("MyPlainObject");
   myFancyObjectClass = objc_getClass("MyFancyObject");
}

+ (void)changeKind:(MyPlainObject *)control fancy:(BOOL)fancy
{
   object_setClass(control, fancy ? myFancyObjectClass : myPlainObjectClass);
}

Now for any original instance of MyPlainClass you can switch to behave as a MyFancyClass, and vice-versa:

MyPlainClass *mpc = [MyPlainClass new];

...

// masquerade as MyFancyClass
[MyFancyClass changeKind:mpc fancy:YES]

... // mpc behaves as a MyFancyClass

// revert to true nature
[MyFancyClass changeKind:mpc: fancy:NO];

(Some) of the caveats:

You can only do this if the subclass overrides or adds methods, and adds static (class) variables.

You also need a sub-class for ever class you wish to change the behaviour of, you can't have a single class which can change the behaviour of many different classes.