> Le 10 nov. 2015 à 19:52, Alex Zavatone <z...@mac.com> a écrit :
>
>
> On Nov 10, 2015, at 12:35 PM, Greg Weston 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"™, ....
>>
>> I would like to see these examples, because I can't think of any reason why
>> it should be a bad idea. Per the documentation, it's a wrapper around
>> objectForKey: with one special case.
>
> I'm looking at a large chunk of code that uses all dictionary object access
> using valueForKey. All of it is wrapped within @try/@catch blocks, implying
> that the original authors ran into lots of exceptions being raised and they
> didn't know why.
>
> My main inclination for using objectForKey over valueForKey to access the
> object for a key within a dictionary is that objectForKey is actually a
> dictionary method while valueForKey is a method declared in the
> NSKeyValueCoding.h file.
>
> I did find this example on SO yesterday, and this important issue with
> regards to missing keys.
>
> http://stackoverflow.com/questions/1062183/difference-between-objectforkey-and-valueforkey
>
>
> Here's a great reason to use objectForKey: wherever possible instead of
> valueForKey: - valueForKey: with an unknown key will throw
> NSUnknownKeyException saying "this class is not key value coding-compliant
> for the key ".
>
> But according to Charles and Mike comment and my testing, it doesn't - in
> some cases.
>
> Take this code on iOS 9:
> NSDictionary *myStuff = [NSDictionary dictionaryWithObjectsAndKeys:@"a",
> @"A", nil];
>
> po myStuff
> {
> A = a;
> }
>
> po [myStuff valueForKey:@"a"];
> nil
>
> // Just as Charles and Mike mentioned
>
> po [myStuff objectForKey:@"a"];
>
> nil
> // Just as we expect
>
> But if we remove the @ (obviously unintended), valueForKey causes an
> EXC_BAD_ACCES while objectForKey reports "error: string literal must be
> prefixed by '@'".
>
> And David does state that whether an exception is thrown or not is
> implementation dependent.
>
> From what Greg showed and in my testing, valueForKey will crater if you use
> an @ in your key name, something I haven't seen until I tested it.
>
> If we're not using @ within our key names, I haven't seen much here that
> indicates why it's dangerous, nor why the code I'm looking at that uses
> valueForKey on dictionaries needs to be wrapped in @try/@catch statements.
>
>
> For what we're doing on a daily basis, if we're not using @ within our keys,
> I still can't see anything concrete besides "objectForKey is an NSDictionary
> method" while "valueForKey is a KVO method".
>
> Hmm.
>
> Thanks to all for their input on this.
>
> - Alex Zavatone
>
If you don’t want to have to ask yourself if it should be valueForKey or
objectForKey, just use the new bracket syntax to access dictionary.
NSDictionary *myStuff = @{ @"A" : @"a" };
myStuff[@"a"]
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