我正在用 C# 创建一个下载器。我正在使用 WebClient 类。要通过单击按钮暂停下载，我可以考虑使用 Thread。因此，当我创建线程并将其与我的文件下载一起附加时，如下所示
WebClient web = new WebLCient();
Thread dwnd_thread = new Thread(Program.web.DownloadFileAsync(new Uri(Program.src), Program.dest));
它给了我以下错误：“'System.Threading.Thread.Thread(System.Threading.ThreadStart)'的最佳重载方法匹配有一些无效参数”和“参数'1'：无法从'void'转换为' System.Threading.ThreadStart'”。
然后我想如果我暂停我的系统主线程，那么它可能会阻止我在下面的代码行中使用的整个进程
System.Threading.Thread.Sleep(100);
但它什么也没做。有人可以告诉我什么是暂停/下载的更好方法以及如何使用线程来暂停我的下载过程。
由于没有暂停/恢复下载请求的标准方法，您必须实现自己的机制。下面是一段代码，其中包含一个示例，说明这种机制的外观。该类FileDownload采用 3 个参数：
source- 文件的url，下载。
destination- 保存文件的位置。
chunkSize- 在检查是暂停还是继续下载之前要读取多少字节。
public class FileDownload
{
private volatile bool _allowedToRun;
private string _source;
private string _destination;
private int _chunkSize;
private Lazy<int> _contentLength;
public int BytesWritten { get; private set; }
public int ContentLength { get { return _contentLength.Value; } }
public bool Done { get { return ContentLength == BytesWritten; } }
public FileDownload(string source, string destination, int chunkSize)
{
_allowedToRun = true;
_source = source;
_destination = destination;
_chunkSize = chunkSize;
_contentLength = new Lazy<int>(() => Convert.ToInt32(GetContentLength()));
BytesWritten = 0;
}
private long GetContentLength()
{
var request = (HttpWebRequest)WebRequest.Create(_source);
request.Method = "HEAD";
using (var response = request.GetResponse())
return response.ContentLength;
}
private async Task Start(int range)
{
if (!_allowedToRun)
throw new InvalidOperationException();
var request = (HttpWebRequest)WebRequest.Create(_source);
request.Method = "GET";
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)";
request.AddRange(range);
using (var response = await request.GetResponseAsync())
{
using (var responseStream = response.GetResponseStream())
{
using (var fs = new FileStream(_destination, FileMode.Append, FileAccess.Write, FileShare.ReadWrite))
{
while (_allowedToRun)
{
var buffer = new byte[_chunkSize];
var bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length);
if (bytesRead == 0) break;
await fs.WriteAsync(buffer, 0, bytesRead);
BytesWritten += bytesRead;
}
await fs.FlushAsync();
}
}
}
}
public Task Start()
{
_allowedToRun = true;
return Start(BytesWritten);
}
public void Pause()
{
_allowedToRun = false;
}
}
用法：
static void Main(string[] args)
{
var fw = new FileDownload("http://download.microsoft.com/download/E/E/2/EE2D29A1-2D5C-463C-B7F1-40E4170F5E2C/KinectSDK-v1.0-Setup.exe", @"D:\KinetSDK.exe", 5120);
// Display progress...
Task.Factory.StartNew(() =>
{
while (!fw.Done)
{
Console.SetCursorPosition(0, Console.CursorTop);
Console.Write(string.Format("ContentLength: {0} | BytesWritten: {1}", fw.ContentLength, fw.BytesWritten));
}
});
// Start the download...
fw.Start();
// Simulate pause...
Thread.Sleep(500);
fw.Pause();
Thread.Sleep(2000);
// Start the download from where we left, and when done print to console.
fw.Start().ContinueWith(t => Console.WriteLine("Done"));
Console.ReadKey();
}
我采用了@ebb 的解决方案（效果很好！）并对其进行了一些调整。现在：
IProgresspublic class FileDownload
{
private volatile bool _allowedToRun;
private Stream _sourceStream;
private string _sourceUrl;
private string _destination;
private bool _disposeOnCompletion;
private int _chunkSize;
private IProgress<double> _progress;
private Lazy<long> _contentLength;
public long BytesWritten { get; private set; }
public long ContentLength { get { return _contentLength.Value; } }
public bool Done { get { return ContentLength == BytesWritten; } }
public FileDownload(Stream source, string destination, bool disposeOnCompletion = true, int chunkSizeInBytes = 10000 /*Default to 0.01 mb*/, IProgress<double> progress = null)
{
_allowedToRun = true;
_sourceStream = source;
_destination = destination;
_disposeOnCompletion = disposeOnCompletion;
_chunkSize = chunkSizeInBytes;
_contentLength = new Lazy<int>(() => Convert.ToInt32(GetContentLength()));
_progress = progress;
BytesWritten = 0;
}
public FileDownload(string source, string destination, int chunkSizeInBytes = 10000 /*Default to 0.01 mb*/, IProgress<double> progress = null)
{
_allowedToRun = true;
_sourceUrl = source;
_destination = destination;
_chunkSize = chunkSizeInBytes;
_contentLength = new Lazy<int>(() => Convert.ToInt32(GetContentLength()));
_progress = progress;
BytesWritten = 0;
}
private long GetContentLength()
{
if (_sourceStream != null)
return _sourceStream.Length;
else
{
var request = (HttpWebRequest)WebRequest.Create(_sourceUrl);
request.Method = "HEAD";
using (var response = request.GetResponse())
return response.ContentLength;
}
}
private async Task Start(int range)
{
if (!_allowedToRun)
throw new InvalidOperationException();
if (_sourceStream != null)
{
using (var fs = new FileStream(_destination, FileMode.Append, FileAccess.Write, FileShare.ReadWrite))
{
while (_allowedToRun)
{
var buffer = new byte[_chunkSize];
var bytesRead = await _sourceStream.ReadAsync(buffer, 0, buffer.Length).ConfigureAwait(false);
if (bytesRead == 0) break;
await fs.WriteAsync(buffer, 0, bytesRead);
BytesWritten += bytesRead;
_progress?.Report((double)BytesWritten / ContentLength);
}
await fs.FlushAsync();
}
//Control whether the stream should be disposed here or outside of this class
if (BytesWritten == ContentLength && _disposeOnCompletion)
_sourceStream?.Dispose();
}
else
{
var request = (HttpWebRequest)WebRequest.Create(_sourceUrl);
request.Method = "GET";
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)";
request.AddRange(range);
using (var response = await request.GetResponseAsync())
{
using (var responseStream = response.GetResponseStream())
{
using (var fs = new FileStream(_destination, FileMode.Append, FileAccess.Write, FileShare.ReadWrite))
{
while (_allowedToRun)
{
var buffer = new byte[_chunkSize];
var bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length).ConfigureAwait(false);
if (bytesRead == 0) break;
await fs.WriteAsync(buffer, 0, bytesRead);
BytesWritten += bytesRead;
_progress?.Report((double)BytesWritten / ContentLength);
}
await fs.FlushAsync();
}
}
}
}
}
public Task Start()
{
_allowedToRun = true;
return Start(BytesWritten);
}
public void Pause()
{
_allowedToRun = false;
}
}
不幸的是，WebClient 无法暂停下载。因此，您必须在后台线程上使用 WebRequest 并暂停获取带有标志的响应流。这是示例代码。但是请确保您不能无限制地暂停，因为如果一段时间没有任何传输，TCP 连接将被关闭。所以如果恢复下载失败，你必须重新开始下载。
public class DownloadJob {
public delegate void DownloadCompletedDelegate(Stream stream);
//completion download event
public event DownloadCompletedDelegate OnDownloadCompleted;
//sync object
private object _lock = new object();
//pause flag
private bool _bPause = false;
//Pause download
public void Pause() {
lock (_lock) {
_bPause = true;
}
}
//Resume download
public void Resume() {
lock (_lock) {
_bPause = false;
}
}
//Begin download with URI
public void BeginDowload(Uri uri) {
//Create Background thread
Thread downLoadThread = new Thread(
delegate() {
WebRequest pWebReq = WebRequest.Create(uri);
WebResponse pWebRes = pWebReq.GetResponse();
using (MemoryStream pResultStream = new MemoryStream())
using (Stream pWebStream = pWebRes.GetResponseStream()) {
byte[] buffer = new byte[256];
int readCount = 1;
while (readCount > 0) {
//Read download stream
readCount = pWebStream.Read(buffer, 0, buffer.Length);
//Write to result MemoryStream
pResultStream.Write(buffer, 0, readCount);
//Waiting 100msec while _bPause is true
while (true) {
lock (_lock) {
if (_bPause == true) {
Thread.Sleep(100);
}
else {
break;
}
}
}
pResultStream.Flush();
}
//Fire Completion event
if (OnDownloadCompleted != null) {
OnDownloadCompleted(pResultStream);
}
}
}
);
//Start background thread job
downLoadThread.Start();
}
}
我已经对其进行了调整，以处理大于 2GB 的文件并处理损坏的下载，因此无论出于何种原因，下载都已停止，您需要在下一次下载尝试时重试，而不是从头开始，它将继续从头开始被阻止了。来自@derekantrican、@ebb 和我自己的所有上述解决方案只有在服务器支持 Range 请求时才有效。您可能需要先测试服务器是否接受范围请求，然后才能继续执行此解决方案。 https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests
public class FileDownload
{
private volatile bool _allowedToRun;
private readonly string _sourceUrl;
private readonly string _destination;
private readonly int _chunkSize;
private readonly IProgress<double> _progress;
private readonly Lazy<long> _contentLength;
public long BytesWritten { get; private set; }
public long ContentLength => _contentLength.Value;
public bool Done => ContentLength == BytesWritten;
public FileDownload(string source, string destination, int chunkSizeInBytes = 10000 /*Default to 0.01 mb*/, IProgress<double> progress = null)
{
if(string.IsNullOrEmpty(source))
throw new ArgumentNullException("source is empty");
if (string.IsNullOrEmpty(destination))
throw new ArgumentNullException("destination is empty");
_allowedToRun = true;
_sourceUrl = source;
_destination = destination;
_chunkSize = chunkSizeInBytes;
_contentLength = new Lazy<long>(GetContentLength);
_progress = progress;
if (!File.Exists(destination))
BytesWritten = 0;
else
{
try
{
BytesWritten = new FileInfo(destination).Length;
}
catch
{
BytesWritten = 0;
}
}
}
private long GetContentLength()
{
var request = (HttpWebRequest)WebRequest.Create(_sourceUrl);
request.Method = "HEAD";
using (var response = request.GetResponse())
return response.ContentLength;
}
private async Task Start(long range)
{
if (!_allowedToRun)
throw new InvalidOperationException();
if(Done)
//file has been found in folder destination and is already fully downloaded
return;
var request = (HttpWebRequest)WebRequest.Create(_sourceUrl);
request.Method = "GET";
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)";
request.AddRange(range);
using (var response = await request.GetResponseAsync())
{
using (var responseStream = response.GetResponseStream())
{
using (var fs = new FileStream(_destination, FileMode.Append, FileAccess.Write, FileShare.ReadWrite))
{
while (_allowedToRun)
{
var buffer = new byte[_chunkSize];
var bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length).ConfigureAwait(false);
if (bytesRead == 0) break;
await fs.WriteAsync(buffer, 0, bytesRead);
BytesWritten += bytesRead;
_progress?.Report((double)BytesWritten / ContentLength);
}
await fs.FlushAsync();
}
}
}
}
public Task Start()
{
_allowedToRun = true;
return Start(BytesWritten);
}
public void Pause()
{
_allowedToRun = false;
}
}