Comments on: Fun with KVC

By: Kirk Kerekes

Kirk Kerekes — Wed, 05 Aug 2009 20:27:34 +0000

I would dispute the contention that these techniques are “abuse”, indeed I feel that they are now central to efficient, maintainable coding in Cocoa, and their virtuousness is implied by:

1. The very nature of object properties/accessors.
2. ObjC’s “Duck Typing”
3. The existence of the KVC array and set “operators”
4. The “@” prefix KVC override on the NS collection class clusters

BTW: makeObjectsPerformSelector:withObject: where the second param is a mutable collection is a fine way to get the return values of a method call. It can be much faster (but much less obvious) than more conventional strategies for binning/indexing.

Guy English’s extensions to the concept are intriguing, but swizzling the behavior of KVC seems like a pretty hazardous notion for production code.

By: Aaron Burghardt

Aaron Burghardt — Wed, 01 Jul 2009 00:49:15 +0000

Right, I didn’t mean to imply it was equivalent, just sharing a nice complement that is useful for similar situations and something I would actually use. Your point that the post was an indulgence wasn’t lost. I use KVC on arrays regularly, but you had some neat tricks. Overall, a great read!

By: mr_noodle

mr_noodle — Tue, 30 Jun 2009 23:56:44 +0000

Guy: I was going to post your alloc/init example as well but couldn’t recall where it would be used.

Paul/Aaron: I updated my post to clarify that this was much more of a random tangent. I also posted a more kosher way of doing a map function on an array.

Aaron: -makeObjectsPerformSelector here won’t work for doing a map-like function where you need to collect the return values of the method call. It is the natural and correct replacement for abuse of -setValue:forKey: though.

By: Aaron Burghardt

Aaron Burghardt — Tue, 30 Jun 2009 23:11:36 +0000

Not KVC, but another great option to avoid manually iterating over a collection is NSArray’s and NSSet’s makeObjectsPerformSelector: and makeObjectsPerformSelector:withObject:. So, you could:

[classes makeObjectsPerformSelector:@selector(@"autorelease")];

[people makeObjectsPerformSelector:@selector(@"setCompany:") withObject:@"Apple, Inc."];

[people makeObjectsPerformSelector:@selector(@"setDepartment:") withObject:dept];

The first example isn’t as short as your KVC form, but feels less like it’s abusing the frameworks. It’s also probably more efficient.

By: Paul

Paul — Tue, 30 Jun 2009 22:16:22 +0000

What you’re pointing out is that KVC (in this case) is actually a form of ‘map’ (from functional idioms).

Now I’m not sure I’d actually want to use it as such, but I’ve certainly missed map and it’s cohorts at times. I think I’d rather have a version that was intended for the purpose though, and didn’t have the possibility of working differently at some point in the future.

By: Guy

Guy — Tue, 30 Jun 2009 20:09:31 +0000

NSArray *classes = [NSArray arrayWithObjects: [NSObject class], [NSObject class], [NSObject class], nil];

NSArray *instances = [classes valueForKeyPath: @"alloc.init.autorelease"];

I think Mike Ash showed me that first. He actually had a reason (well … close enough) to want to do it but I forget what it was. And, yeah, my KVC stuff is more for the sake of interest than hard core use. Or, at least, don’t swizzle valueForKeyPath: to do it but add another method that’ll parse the queries. (and I really should fix up my blog …)

- Guy