iOS nsurlconnection 回调方法无效的分析！

 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:urlStr] cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:CONNECTIONT_TIMEOUT];
        NSString *encodedStr = [body stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        NSData *sendData = [encodedStr dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:YES];
        NSString *dataLengthStr =[NSString stringWithFormat:@"%lu",(unsigned long)[sendData length]];
        [request setValue:dataLengthStr forHTTPHeaderField:@"Content-Length"];
        [request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
        [request setHTTPMethod:@"POST"];
        [request setHTTPBody:sendData];
        self.connection = [NSURLConnection connectionWithRequest:request delegate:self];
    });

这是一个在主线程中执行的代码，执行结果有问题，无法收到网络回掉。

分析待续。。。

需要研究为什么connection不能阻止线程结束！什么东西能阻止？？