using System;
using System.IO;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
 
/// <summary>
/// 異步文件下載器核心類
/// </summary>
public class AsyncDownloader : IDisposable
{
    private HttpClient _client;
    private CancellationTokenSource _cts;
    private long _totalBytes;
    private long _receivedBytes;
    private bool _isResuming;
 
    /// <summary>
    /// 下載進度變更事件
    /// </summary>
    public event EventHandler<DownloadProgressArgs> ProgressChanged;
 
    public AsyncDownloader()
    {
        _client = new HttpClient
        {
            Timeout = TimeSpan.FromMinutes(30) // 長連接超時設置
        };
    }
 
    /// <summary>
    /// 啟動異步下載任務
    /// </summary>
    /// <param name="url">文件URL</param>
    /// <param name="savePath">保存路徑</param>
    /// <param name="resumeDownload">是否啟用斷點續(xù)傳</param>
    public async Task StartDownloadAsync(string url, string savePath, bool resumeDownload = false)
    {
        _cts = new CancellationTokenSource();
        _isResuming = resumeDownload;
        
        try
        {
            using (var response = await _client.GetAsync(
                url, 
                resumeDownload ? GetResumeHeader(savePath) : HttpCompletionOption.ResponseHeadersRead,
                _cts.Token))
            {
                await ProcessResponse(response, savePath);
            }
        }
        catch (OperationCanceledException)
        {
            // 處理用戶取消邏輯
        }
    }
 
    /// <summary>
    /// 處理HTTP響應流
    /// </summary>
    private async Task ProcessResponse(HttpResponseMessage response, string savePath)
    {
        _totalBytes = response.Content.Headers.ContentLength ?? 0;
        _receivedBytes = GetExistingFileSize(savePath);
 
        using (var stream = await response.Content.ReadAsStreamAsync())
        using (var fileStream = new FileStream(
            savePath,
            _isResuming ? FileMode.Append : FileMode.Create,
            FileAccess.Write))
        {
            var buffer = new byte[8192 * 4]; // 32KB緩沖區(qū)
            int bytesRead;
            
            while ((bytesRead = await stream.ReadAsync(buffer, 0, buffer.Length, _cts.Token)) > 0)
            {
                await fileStream.WriteAsync(buffer, 0, bytesRead, _cts.Token);
                _receivedBytes += bytesRead;
                ReportProgress();
            }
        }
    }
 
    /// <summary>
    /// 觸發(fā)進度更新事件
    /// </summary>
    private void ReportProgress()
    {
        ProgressChanged?.Invoke(this, new DownloadProgressArgs
        {
            TotalBytes = _totalBytes,
            ReceivedBytes = _receivedBytes,
            ProgressPercentage = _totalBytes > 0 ? 
                (double)_receivedBytes / _totalBytes * 100 : 0
        });
    }
 
    /// <summary>
    /// 獲取續(xù)傳請求頭
    /// </summary>
    private HttpRequestMessage GetResumeHeader(string path)
    {
        var fileInfo = new FileInfo(path);
        return new HttpRequestMessage
        {
            Headers = { Range = new System.Net.Http.Headers.RangeHeaderValue(fileInfo.Length, null) }
        };
    }
 
    // 其他輔助方法省略...
}
 
/// <summary>
/// 下載進度事件參數(shù)
/// </summary>
public class DownloadProgressArgs : EventArgs
{
    public long TotalBytes { get; set; }
    public long ReceivedBytes { get; set; }
    public double ProgressPercentage { get; set; }
}

ProgressPercentage = receivedBytes / totalBytes * 100

// 初始化下載器
var downloader = new AsyncDownloader();
downloader.ProgressChanged += (s, e) =>
{
    Dispatcher.Invoke(() => 
    {
        progressBar.Value = e.ProgressPercentage;
        speedText.Text = $"{CalculateSpeed(e)} MB/s";
    });
};
 
// 啟動下載任務
await downloader.StartDownloadAsync(
    "https://example.com/largefile.zip",
    @"D:\Downloads\largefile.zip",
    resumeDownload: true);
 
// 取消下載
cancelButton.Click += (s, e) => downloader.Cancel();

var elapsed = DateTime.Now - _lastUpdate;
var speed = bytesDelta / elapsed.TotalSeconds;

int retryCount = 0;
while(retryCount < 3)
{
    try { ... }
    catch { await Task.Delay(1000 * Math.Pow(2, retryCount)); }
}

HttpClientHandler.EnableMultipleHttp2Connections = true;