> On Nov 10, 2015, at 8:32 AM, Alex Zavatone <z...@mac.com> wrote:
>
> It's been about 4 or 5 years since I made this mistake but I've just seen a
> massive swath of code where every access of a dictionary object is using
> valueForKey instead of objectForKey.
>
> I've got a few examples of why this is a "really bad idea"™, and certainly
> might explain why lots of that code is wrapped in try/catch blocks.
>
> Am I correct that using valueForKey will raise an exception if the key is
> missing?
>
> Looking for reasons why I can explain "yeah, I know it works, but here's why
> it's a terrible idea to use to access a dictionary's objects".
-valueForKey: interprets keys whose name starts with '@' specially. Examples:
[dict valueForKey:@"@allKeys"] returns an array of all keys in the dictionary.
valueForKey: throws an exception if the key name starts with '@' and the name
isn't recognized by KVC as special.
% cat test.m
#import <Foundation/Foundation.h>
int main(int argc, char *argv[]) {
NSDictionary *dict = @{ @"ordinary" : @"ordinary value",
@"@allKeys" : @"@allKeys value",
@"something" : @"something value",
@"@something" : @"@something value" };
// -objectForKey does the usual thing
NSLog(@"-- objectForKey --");
NSLog(@"%@", [dict objectForKey:@"ordinary"]);
NSLog(@"%@", [dict objectForKey:@"@allKeys"]);
NSLog(@"%@", [dict objectForKey:@"@something"]);
// -valueForKey does not
NSLog(@"-- valueForKey --");
NSLog(@"%@", [dict valueForKey:@"ordinary"]);
NSLog(@"%@", [dict valueForKey:@"@allKeys"]);
NSLog(@"%@", [dict valueForKey:@"@something"]);
}
% clang test.m -framework Foundation && ./a.out
2015-11-10 10:00:55.295 a.out[38293:4747164] -- objectForKey --
2015-11-10 10:00:55.296 a.out[38293:4747164] ordinary value
2015-11-10 10:00:55.296 a.out[38293:4747164] @allKeys value
2015-11-10 10:00:55.296 a.out[38293:4747164] @something value
2015-11-10 10:00:55.296 a.out[38293:4747164] -- valueForKey --
2015-11-10 10:00:55.296 a.out[38293:4747164] ordinary value
2015-11-10 10:00:55.297 a.out[38293:4747164] (
something,
ordinary,
"@allKeys",
"@something"
)
2015-11-10 10:00:55.298 a.out[38293:4747164] *** Terminating app due to
uncaught exception 'NSUnknownKeyException', reason: '[<__NSDictionaryI
0x7fc0b850ead0> valueForUndefinedKey:]: this class is not key value
coding-compliant for the key something.'
--
Greg Parker gpar...@apple.com Runtime Wrangler
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