将对象转换为json

- (NSString *)toJSONData:(id)theData{
    
    NSError *error = nil;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:theData
                                                       options:NSJSONWritingPrettyPrinted
                                                         error:&error];
    
    if ([jsonData length] > 0 && error == nil){
        NSString *jsonString = [[NSString alloc] initWithData:jsonData
                                                     encoding:NSUTF8StringEncoding];
        return jsonString;
    }else{
        return nil;
    }
   
}