目前,我正在实现一种使用 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 NuGet 中的 Microsoft HTTP 客户端库。这个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();
}
}
然后我发送请求: HttpResponseMessage httpResponseMessage = AsyncHelpers.RunSync(() => _httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, CancelToken));
之后,我打开文件流,然后复制内容流。响应具有正确的 header :
内容长度:213334
内容类型:应用程序/八位字节流;字符集=UTF-8
内容处置:附件; filename="邦迪海滩.jpg";文件名*=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,效果就很好。
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
手动读取缓冲区然后写入文件流_httpClient = new System.Net.Http.HttpClient(){MaxResponseContentBufferSize = 4096};
限制BufferSize
有什么想法可以强制 ContentStream
在下载时写入文件流吗?
更新:
根据 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);
}
}
}
它仍然会等待下载完成,直到第一次调用ReadAsync
。有什么提示我做错了什么吗?
最佳答案
事实上,ReadAsStreamAsync
是异步
,这使得这一点相当可疑。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。
阅读文档可以让这一点变得显而易见:
This operation will not block. The returned task object will complete after the whole response (including content) is read.
但是,您可以使用重载使其在读取 header 后返回。这仍然意味着您需要等待服务器处理请求,然后才能开始获取进度,但对于下载本身来说,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:
听起来您应该使用 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;
}
关于c# - HttpClient 在下载时不写入流,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/29015127/