objective-c - NSURLConnection を sendSynchronousRequest から sendAsynchronousRequest に変更しますか?

Question

sendSynchronousRequest を使用した NSURLConnection のコードは正常に動作しますが、非同期要求に変更するにはどうすればよいですか? 私はたくさん試しましたが、何もうまくいきませんでした。

リクエストが空の場合、 [[]] で空の配列を取得します。アラート メッセージをキャッチするにはどうすればよいですか?

助けてください ...

     [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;

    NSString *urlString = @"http://www.xyz.at/sample.php";

    NSURL *url = [NSURL URLWithString:urlString];

    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
    [request setURL:url];
    [request setHTTPMethod:@"POST"];

    NSMutableData *body = [NSMutableData data];

    NSString *postWerte = [NSString stringWithFormat:@"id=%@", self.textfeld.text];

    [body appendData:[postWerte dataUsingEncoding:NSUTF8StringEncoding]];

    [request setHTTPBody:body];

    NSError *error = nil;
    NSData *returnData = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:&error];
    NSLog(@"Error: %@", error.description);

    NSString *returnString = [[NSString alloc] initWithData:returnData encoding:NSUTF8StringEncoding];

    const char *convert = [returnString UTF8String];
    NSString *responseString = [NSString stringWithUTF8String:convert];
    NSMutableArray *meinErgebnis = [responseString JSONValue];

    NSString *cycle = @"";

    NSString *kopfdaten = [NSString stringWithFormat:@"Sendungsart: %@\r\nGewicht: %@ kg\r\n\r\n", [[meinErgebnis objectAtIndex:0] objectForKey:@"ParcelTypeDescription"], [[meinErgebnis objectAtIndex:0] objectForKey:@"Weight"]];

    cycle = [cycle stringByAppendingString:kopfdaten];

    for(int i = 1; i < meinErgebnis.count; i++)
        {

        NSString *myValue = [NSString stringWithFormat:@"%@     PLZ: %@\r\nStatus: %@\r\n\r\n",
                [[meinErgebnis objectAtIndex:i] objectForKey:@"EventTimestamp"],
                [[meinErgebnis objectAtIndex:i] objectForKey:@"EventPostalCode"],
                [[meinErgebnis objectAtIndex:i] objectForKey:@"ParcelEventReasonDescription"]];

        cycle = [cycle stringByAppendingString:myValue];

        }
        self.ergebnis.text = [NSString stringWithFormat:@"%@", cycle];

    [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
    [self.textfeld resignFirstResponder];