实现下载列表中版本的功能

Tools/DownloadService.cs

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+using System;
+using System.IO;
+using System.Net.Http;
+using System.Threading.Tasks;
+
+namespace MCSJ.Tools
+{
+    public class DownloadService
+    {
+        private readonly VersionManager _versionManager;
+        private readonly HttpClient _httpClient;
+
+        public DownloadService(VersionManager versionManager)
+        {
+            _versionManager = versionManager;
+            _httpClient = new HttpClient();
+        }
+
+        public async Task DownloadVersion(string version)
+        {
+            var url = _versionManager.GetDownloadUrl(version);
+            if (url == null)
+            {
+                Console.WriteLine($"版本 {version} 不存在");
+                return;
+            }
+
+            try
+            {
+                Console.WriteLine($"开始下载 {version}...");
+                
+                var response = await _httpClient.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
+                response.EnsureSuccessStatusCode();
+
+                var totalBytes = response.Content.Headers.ContentLength ?? -1;
+                var downloadedBytes = 0L;
+                var buffer = new byte[8192];
+                var isMoreToRead = true;
+
+                using (var stream = await response.Content.ReadAsStreamAsync())
+                using (var fileStream = new FileStream($"{version}.jar", FileMode.Create, FileAccess.Write))
+                {
+                    while (isMoreToRead)
+                    {
+                        var read = await stream.ReadAsync(buffer, 0, buffer.Length);
+                        if (read == 0)
+                        {
+                            isMoreToRead = false;
+                        }
+                        else
+                        {
+                            await fileStream.WriteAsync(buffer, 0, read);
+
+                            downloadedBytes += read;
+                            if (totalBytes > 0)
+                            {
+                                var progress = (double)downloadedBytes / totalBytes * 100;
+                                Console.Write($"\r下载进度: {progress:F2}%");
+                            }
+                        }
+                    }
+                }
+
+                Console.WriteLine($"\n{version} 下载完成!");
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine($"下载失败: {ex.Message}");
+            }
+        }
+    }
+}