iOS JSONとNSDictionaryの相互転送

//   JSON
+(NSString *)convertJSONWithDic:(NSDictionary *)dic {
    NSError *err;
    NSData *data = [NSJSONSerialization dataWithJSONObject:dic options:NSJSONWritingPrettyPrinted error:&err];
    if (err) {
        return @"   JSON  ";
    }
    return [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
}

//JSON   
+(NSDictionary *)convertDicWithJSON:(NSString *)jsonStr {
    if (jsonStr.length == 0) {
        return nil;
    }
    NSError *err;
    NSData *jsondata = [jsonStr dataUsingEncoding:NSUTF8StringEncoding];
    NSDictionary *dic = [NSJSONSerialization JSONObjectWithData:jsondata options:NSJSONReadingMutableContainers error:&err];
    if (err) {
        return nil;
    }
    
    return dic;
}