Well, if you need a nil keypath you probably shouldn't be using sort
descriptors. The primary value of sort descriptors are (a) the KVC caching and
(b) an OO representation of a sort criteria.
The standard -sortedArrayUsingComparator: is for blocks like the one you wrote
that are completely free form.
- Ben
On Mar 24, 2010, at 3:55 AM, Jochen Moeller wrote:
> Hello Ben,
>
> thanks very much for your answer. Your idea with the nil keypath works!
>
> So changing the line in question to:
>
> NSSortDescriptor *descr3 = [ NSSortDescriptor sortDescriptorWithKey:nil
> ascending:YES
> comparator:^NSComparisonResult(id obj1, id obj2) {
>
> is sorting the array without claims.
>
> The syntax of that method is a bit confusing. When using a selector the key
> is required, and when using a comparator it is not.
>
> Thanks again,
> Jochen Moeller
>
>
> Am 24.03.2010 um 11:12 schrieb Ben Trumbull:
>
>>> while experimenting with sorting methods I got a strange error message with
>>> -sortDescriptorWithKey:ascending:comparator:.
>>>
>>> In the following listing (as simple as possible) an Array can be sorted with
>>> -sortedArrayUsingDescriptors: with a selector, and
>>> -sortedArrayUsingComparator:
>>>
>>> But -sortedArrayUsingDescriptors: with a comparator results in
>>> Error: -[NSCFNumber rx]: unrecognized selector.
>>>
>>> So the method expects the key "rx" not in my class as it should but in the
>>> NSNumber class.
>>> A bug? Or is something wrong in my approach?
>>
>> The arguments to the Block with the NSSortDescriptor are the results of
>> calling valueForKeyPath, not the original objects in the array.
>> NSSortDescriptor has already pulled the values for the keypaths out and
>> caches them. So your using the key @"rx" with that block is like:
>>
>> [[obj1 valueForKeyPath:@"rx"] rx]
>>
>> If you don't want NSSortDescriptors KVC functionality, you can just use
>> -sortedArrayUsingComparator:. You can also try a nil keypath, although I'm
>> not certain that works.
>>
>> - Ben
>>
>>> // Sort array with -sortDescriptorWithKey:ascending:comparator:
>>> // Error: -[NSCFNumber rx]: unrecognized selector
>>> NSSortDescriptor *descr3 = [ NSSortDescriptor sortDescriptorWithKey:@"rx"
>>> ascending:YES
>>> comparator:^NSComparisonResult(id obj1, id obj2) {
>>> if ([obj1 rx] < [obj2 rx]) { return NSOrderedAscending; }
>>> else if ([obj1 rx] > [obj2 rx]) { return NSOrderedDescending; }
>>> return NSOrderedSame;
>>> }];
>>
>>
>
- Ben
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