Objective c Post XML数据时出现ASIFormDataRequest错误

//[请求addRequestHeader:@“内容类型”值：@“应用程序/json”]//对JSON的测试很好 [请求附件postdata:theData]； [请求正文]； [请求设置请求方法：@“POST”]

我没有从下面的代码中得到任何信息，这也不会产生错误

[request setDelegate:self];
[request startAsynchronous];
[request setTimeOutSeconds:20];// 测试连接的时间，默认10s
[postUrl release];

当我使用JSON时，我得到了结果

ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:postUrl];
[request setResponseEncoding:NSUTF8StringEncoding];

NSString *msgLength = [NSString stringWithFormat:@"%d", [xmlString length]];
[request addRequestHeader:@"Content-Length" value:msgLength];

NSString *contentType = [NSString stringWithFormat:@"application/xml"];
[request addRequestHeader:@"Content-Type" value:contentType];

[request setDelegate:self];
[request startAsynchronous];
[request setTimeOutSeconds:20];// 测试连接的时间，默认10s
[postUrl release];

(void)requestFinished:(ASIHTTPRequest *)request{
    NSData *theData = [request responseData];
    NSString *ret = [request responseString];
    NSLog(@"ret--------------%@",ret);
}