下载时 HttpClient 不写入流

Question

目前我正在实施一种使用 HttpClient 报告进度的方法，因为我们与 .NET4 WPF 和 Windows 通用应用程序共享代码，所以我们使用来自 NuGet 的 Microsoft HTTP 客户端库。想法是将目标文件流包装在 CountingInputStream 中并在那里报告进度：

 public override void Write(byte[] buffer, int offset, int count)
    {
        _stream.Write(buffer, offset, count);

        _bytesRead += count;
        _progress.Report(_bytesRead);

        if (_cancellationToken.IsCancellationRequested)
        {
            _cancellationToken.ThrowIfCancellationRequested();
        }
    }

然后我发送我的请求：HttpResponseMessage httpResponseMessage = AsyncHelpers.RunSync(() => _httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken));

之后我打开文件流，然后复制内容流。响应正确 headers： Content-Length: 213334 Content-Type: application/octet-stream; charset=UTF-8 Content-Disposition: attachment; filename="Bondi Beach.jpg"; filename*=UTF-8''Bondi%20Beach.jpg

using(Stream fileStream = new CountingInputStream(storage.Open(downloadRequest.TargetPath, FileMode.Create), downloadRequest.Progress, cancellationToken )) {                                               
                await HttpHeaderResponseMessage.Content.CopyToAsync(fileStream);
       }

问题是 StreamContent 仅在下载完成后才开始写入文件流。当它开始编写进度报告时工作正常。

我已经尝试过不同的方法，例如：

ReadAsStreamAsync 然后将响应流复制到文件流
ReadAsStreamAsync手动读取到缓冲区然后写入文件流
_httpClient = new System.Net.Http.HttpClient(){MaxResponseContentBufferSize = 4096};限制BufferSize

关于如何在 ContentStream 仍在下载时强制写入文件流有什么想法吗？

更新：按照 Luaans 的建议，我尝试重写 WriteAsync 并实现 StreamContent Extensions 方法：

//CountingInputStream
public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
    {
        _bytesRead += count;
        _progress.Report(_bytesRead);

        if (_cancellationToken.IsCancellationRequested)
        {
            _cancellationToken.ThrowIfCancellationRequested();
        }
        return _stream.WriteAsync(buffer, offset, count, cancellationToken);
    }
//static Extensions Class
public static async Task CopyToAs(this StreamContent source, Stream targetStream)
    {
        int read;
        byte[] buffer = new byte[4096];
        using(Stream responseStream = await source.ReadAsStreamAsync()) {
            while ((read = await responseStream.ReadAsync(buffer,0,buffer.Length))>0) {
                await targetStream.WriteAsync(buffer, 0, read);
            }
        }
    }

它仍然等到下载完成，直到它第一次调用 ReadAsync。有什么提示我做错了什么吗？

Answer 1

事实上，ReadAsStreamAsync 是 async，这让人很怀疑。你为什么要异步等待获取流？您应该异步读取它，但您应该立即准备好流本身。

阅读文档使这一点显而易见：

This operation will not block. The returned task object will complete after the whole response (including content) is read.

但是，在读取 headers 之后，您可以使用重载来获得它 return。这仍然意味着您需要等待服务器处理请求，然后才能开始取得进展，但对于下载本身，您很幸运。

示例代码：

var response = 
 await 
 (
   new HttpClient()
   .GetAsync("http://www.microsoft.com/", HttpCompletionOption.ResponseHeadersRead)
 );

var stream = await response.Content.ReadAsStreamAsync();
var buffer = new byte[2048];    

while (await stream.ReadAsync(buffer, 0, buffer.Length) > 0)
{
  // Report progress and write to a different stream
}

编辑：

听起来你应该使用 Windows.Web.Http.HttpClient 而不是 System.Net.Http.HttpClient:

async Task DownloadWithProgress()
{
 var awaitable = httpClient.GetAsync(yourUrl)

 awaitable.Progress = (res, progress) =>
 {
   // Report progress
 }

 await awaitable;   
}

下载时 HttpClient 不写入流

HttpClient not writing to stream while downloading

c#

http

stream

win-universal-app

dotnet-httpclient