gistfile1.m

+ (NSValueTransformer *)URLArrayTransformer {
    return [MTLValueTransformer reversibleTransformerWithForwardBlock:^NSArray *(NSArray *URLStrings) {
        NSMutableArray *result = [NSMutableArray arrayWithCapacity:[URLStrings count]];
        for (NSString *URLString in URLStrings) {
            [result addObject:[NSURL URLWithString:URLString]];
        }
        return result;
    } reverseBlock:^NSArray *(NSArray *URLs) {
        NSMutableArray *result = [NSMutableArray arrayWithCapacity:[URLs count]];
        for (NSURL *URL in URLs) {
            [result addObject:URL.absoluteString];
        }
        return result;
    }];
}