Objective C -- is there a keypath that will cause an object to return itself?

Question

Given an object foo of class Foo, I want to do the following:

NSString *key = @"some key";
id myObj = [foo valueForKey: key];

and have myObj equal to foo.

Is there a way to do this without defining a category on Foo?

score 11 · Accepted Answer · answered Feb 04 '10 at 15:36

11

The following works for me, returning foo:

id myObj = [foo valueForKey: @"self"];

answered Feb 04 '10 at 15:36

Costique

Agreed, it is pretty darn cool, and it makes sense that you could ask for it this way. Is there a scenario where this is your only option, though? Perhaps you can only send a key path query to an object for which you don't have a pointer to its owning object? Sounds like an odd situation... – Quinn Taylor Feb 05 '10 at 06:34
Yes, I admit I can't think of any real-world situation where I would really need this. But the coolness of @"self" is in its existence. I guess if you come across the rare case where you have to heavily rely on KVC, you will owe the engineer who did it a truckload of beer. – Costique Feb 05 '10 at 07:07
I didn't know that this is possible, but if I think about it, it is not so surprising. – swegi Feb 05 '10 at 08:01
1

@Quinn Taylor: it's good for sort descriptors that need a keypath, and you're sorting the actual object in the array, rather than a property of the object. i.e. an array of `NSNumber` – ohhorob Jul 08 '10 at 21:08

score 3 · Answer 2 · answered Feb 04 '10 at 15:42

3

id myObj = foo;

answered Feb 04 '10 at 15:42

swegi

That was a perfectly valid question about KVC, by the way :) – Costique Feb 04 '10 at 15:44
Occam's Razor strikes again. I tend to agree with this answer — if you can get a pointer to message an object, just use that pointer. I understand this is probably just an academic exercise, but why spend cycles on key-value observing when the answer is obvious? – Quinn Taylor Feb 05 '10 at 06:32

2 Answers2