Sorting an array of NSURL objects by creation date - iphone

Question

My caches images to a directory (/Library/Caches/ImageCache/). When the directory exceeds a certain size I would like to delete the oldest file in the directory. To accomplish this task I use NSFileManager to retrieve the directory contents. I then try to sort this array by date and delete the oldest object.
My problem is that my program crashes when I try to sort the array by the key NSURLCreationDateKey.
NSFileManager *defaultFM = [NSFileManager defaultManager];
NSArray *keys = [NSArray arrayWithObjects:NSURLNameKey, NSURLCreationDateKey, nil];
NSURL *cacheDirectory = [self photoCacheDirectory]; // method that returns cache URL
NSArray *cacheContents = [defaultFM contentsOfDirectoryAtURL:cacheDirectory
includingPropertiesForKeys:keys
options:NSDirectoryEnumerationSkipsSubdirectoryDescendants
error:nil];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:NSURLCreationDateKey ascending:YES];
NSArray *sortedArray = [cacheContents sortedArrayUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
The program crashes on the last line. With error:
* Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ valueForUndefinedKey:]: this class is not key value coding-compliant for the key NSURLCreationDateKey.'

Answer 1

EDIT : Better answer
If that doesn't work, you will have to write your own comparator block and get the dates to compare manually :(
[cacheContents sortUsingComparator:^ (NSURL *a, NSURL *b) {
// get the two dates
id da = [[a resourceValuesForKeys:[NSArray arrayWithObject:NSURLCreationDateKey] error:nil] objectForKey:NSURLCreationDateKey];
id db = [[b resourceValuesForKeys:[NSArray arrayWithObject:NSURLCreationDateKey] error:nil] objectForKey:NSURLCreationDateKey];
// compare them
return [da compare:db];
}];
(Same disclaimer still applies but I'm not even sure that will compile ;) - you get the idea though!)
Here is my first answer (included here for posterity but mostly it just shows how important it is to read the question properly :)
It's because you're getting an array of NSURL objects; these don't have a NSURLCreationDateKey property.
Try this (disclaimer - not 100% it will work)
NSString *key = [NSString stringWithFormat:#"fileAttributes.%#", NSURLCreationDateKey];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:key ascending:YES];
Your key to sort by is a property of the fileAttributes dictionary which is, in turn, a property of your enumerator.

Answer 2

I'm not positive about this, but I think that the contentsOfDirectoryAtURL: method returns a list of URLs which are not KVC objects. To get the property you want from the NSURL, you need to call:
getResourceValue:forKey:error:
Since this is not KVC compatible, you won't be able to use a sort descriptor, and instead need to use a routine like [cacheContents sortedArrayUsingComparator:...]