Question

如何在整个响应流回来之前访问响应标头？
如何在流式传输时读取流？
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，会导致等待内容到达。~~引用我通常读取streamTask.Result或Content上的方法或属性，我不知道我知道足以排除哪些这样的引用是正常的，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，在您开始阅读内容流之前不会传输内容，而且您调用Task.Result是阻塞调用是正确的，但它本质上是一个同步点。但是，它不会阻止预先缓冲整个内容，它只会阻止内容启动来自服务器。

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。这样的事情可以实现这一点。

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HttpCompletionOption.ResponseHeadersRead选项明确指定。在检索到标题之前，SendAsync任务不会继续。

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
}

在内容100％完成之前，从HttpResponseMessage读取标头

2 个答案: