在内容100%完成之前,从HttpResponseMessage读取标头

Question

1. 在整个响应流回来之前,如何访问响应标头？
2. 如何在流到达时读取流？
3. HttpClient是接收http响应的粒度控制的最佳选择吗？

这是一个可以说明我的问题的剪辑:

using (var response = await _httpClient.SendAsync(request,
  HttpCompletionOption.ResponseHeadersRead))
{
   var streamTask = response.Content.ReadAsStreamAsync();
   //how do I check if headers portion has completed? 
   //Does HttpCompletionOption.ResponseHeadersRead guarantee that?
   //pseudocode
   while (!(all headers have been received)) 
     //maybe await a Delay here to let Headers get fully populated
   access_headers_without_causing_entire_response_to_be_received

   //how do I access the response, without causing an await until contents downloaded?
   //pseudocode
   while (stremTask.Resul.?) //i.e. while something is still streaming
     //? what goes here? a chunk-read into a buffer? or line-by-line since it's http?
   ...

编辑为我澄清另一个灰色区域:
我发掘的任何引用都有某种阻塞语句,这将导致等待内容到达.引用我通常读取streamTask.Result或内容上的访问方法或属性,我不知道足以排除哪些引用是正常的,因为streamTask正在进行而哪些将导致等待直到任务完成.

Answer 1

根据我自己的测试,在你开始阅读内容流之前,内容不会被转移,而你是正确的,呼叫Task.Result是一个阻塞呼叫,但它的本质是,它是一个同步点.但是,它不会阻止预缓存整个内容,它只会阻止内容开始来自服务器.

所以无限的流不会阻塞无限的时间.因此,尝试异步获取流可能被视为过度,特别是如果您的头处理操作相对较短.但是,如果您愿意,您可以随时处理标题,同时在另一个任务上处理内容流.这样的事情会实现这一点.

static void Main(string[] args)
{
    var url = "http://somesite.com/bigdownloadfile.zip";
    var client = new HttpClient();
    var request = new HttpRequestMessage(HttpMethod.Get, url);

    var getTask = client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);
    Task contentDownloadTask = null;

    var continuation = getTask.ContinueWith((t) =>
    {
        contentDownloadTask = Task.Run(() =>
        {
            var resultStream = t.Result.Content.ReadAsStreamAsync().Result;
            resultStream.CopyTo(File.Create("output.dat"));
        });

        Console.WriteLine("Got {0} headers", t.Result.Headers.Count());
        Console.WriteLine("Blocking after fetching headers, press any key to continue...");
        Console.ReadKey(true);
    });

    continuation.Wait();
    contentDownloadTask.Wait();
    Console.WriteLine("Finished downloading {0} bytes", new FileInfo("output.dat").Length);

    Console.WriteLine("Finished, press any key to exit");
    Console.ReadKey(true);
}

请注意,无需检查标题部分是否完整,您已使用该HttpCompletionOption.ResponseHeadersRead选项明确指定.该SendAsync任务将不会继续下去,直到头已经被取出.

Answer 2

使用await / async关键字的结果更具可读性：

var url = "http://somesite.com/bigdownloadfile.zip";

using (var httpClient = new HttpClient())
using (var httpRequest = new HttpRequestMessage(HttpMethod.Get, url ))
using(HttpResponseMessage response = await httpClient.SendAsync(httpRequest, HttpCompletionOption.ResponseHeadersRead))
using (Stream stream = await response.Content.ReadAsStreamAsync())
{
    //Access to the Stream object as it comes, buffer it or do whatever you need
}