gistfile1.m

// NSString から NSSortDescriptor の NSArrayを生成
//
// @"sortDescriptorsBynameDESCANDcharacterDOTname"
// を受けて、name(DESC) と character.name の二つからなるそれを返す。

NSString *sortDesctiptorsBy = @"sortDescriptorsBy";

- (id)sortDescriptorsFromString:(NSString *)keysString
{
	NSArray *keys;
	NSString *aKey;
	NSMutableArray *result = [NSMutableArray array];
	
	keysString = [keysString stringByReplacingOccurrencesOfString:@"DOT"
				     	     withString:@"."];
	keys = [keysString componentsSeparatedByString:@"AND"];
	for(aKey in keys) {
		BOOL ascending = YES;
		if([aKey hasSuffix:@"DESC"]) {
			ascending = NO;
			aKey = [aKey substringToIndex:[aKey length] - 4/* length of DESC */];
		}
		NSSortDescriptor *s = [[[NSSortDescriptor alloc] initWithKey:aKey
							ascending:ascending] autorelease];
		[result addObject:s];
	}
	
	return result;
}

- (id)valueForKey:(NSString *)key
{
	if([key hasPrefix:sortDesctiptorsBy]) {
		NSUInteger prefixLength;
		NSUInteger length;
		NSString *sortKeys;
		
		length = [key length];
		prefixLength = [sortDesctiptorsBy length];
		if(prefixLength == length) goto end;
		sortKeys = [key substringFromIndex:prefixLength];
		
		return [self sortDescriptorsFromString:sortKeys];
	}
	
end:	return [super valueForKey:key];
}