Objective-Cで非同期通信

.hに

NSURLConnection *connection;
NSMutableData *listData;

viewDidLoadとかに

// パラメータを作成
NSString *data = [[NSString alloc] init];
data = [NSString stringWithFormat:@"user_id=%@", uuid_];

NSURL *url = [NSURL URLWithString:[NSString stringWithCString:URL encoding:NSUTF8StringEncoding]];
NSMutableURLRequest *urlRequest = [[NSMutableURLRequest alloc]initWithURL:url];
[urlRequest setHTTPMethod:@"POST"];
[urlRequest setHTTPBody:[data dataUsingEncoding:NSUTF8StringEncoding]];

// 取得データを格納
listData = [[NSMutableData alloc] initWithCapacity:0];

if (connection) [connection cancel];
connection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self startImmediately:NO];
[connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
[connection start];

別にメソッドを作成

// レスポンスを受け取った時点で呼び出される
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    [listData setLength:0];
}

// データを受け取る度に呼び出される
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data2 {
   [listData appendData:data2];
}

// 非同期通信ダウンロード完了
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSLog(@"%@", listData);
}

解析するために、NSDataをNSStringに変換

NSString *json_string = [[NSString alloc] initWithData:listData encoding:NSUTF8StringEncoding];