feat: introduce a multi-task downloader

Net/NDownload/IDlConnection.cs

@@ -0,0 +1,29 @@
+using System.Threading.Tasks;
+
+namespace PCL.Core.Net.NDownload;
+
+/// <summary>
+/// 下载连接，负责与服务器进行通信。
+/// </summary>
+public interface IDlConnection
+{
+    /// <summary>
+    /// 开始连接，发起与服务器的通信。
+    /// </summary>
+    /// <param name="beginOffset">起始偏移，为 <c>0</c> 表示不使用分块</param>
+    /// <returns>连接信息</returns>
+    public Task<NDlConnectionInfo> StartAsync(long beginOffset);
+
+    /// <summary>
+    /// 停止连接，同时停止服务器通信并释放资源。
+    /// </summary>
+    /// <returns></returns>
+    public Task StopAsync();
+
+    /// <summary>
+    /// 读取指定长度的数据，若无法继续读取则返回空数组。
+    /// </summary>
+    /// <param name="length">读取长度</param>
+    /// <returns>字节数组形式的数据</returns>
+    public Task<byte[]> ReadAsync(int length);
+}

Net/NDownload/IDlResourceMapping.cs

@@ -0,0 +1,10 @@
+namespace PCL.Core.Net.NDownload;
+
+/// <summary>
+/// 资源 ID 映射。
+/// </summary>
+/// <typeparam name="TMappingValue">映射目标类型</typeparam>
+public interface IDlResourceMapping<out TMappingValue>
+{
+    public TMappingValue? Parse(string resId);
+}

Net/NDownload/IDlWriter.cs

@@ -0,0 +1,31 @@
+using System.IO;
+using System.Threading.Tasks;
+
+namespace PCL.Core.Net.NDownload;
+
+/// <summary>
+/// 下载写入器。
+/// </summary>
+public interface IDlWriter
+{
+    /// <summary>
+    /// 是否支持并行写入，即是否支持多次调用 <see cref="CreateStreamAsync"/>。
+    /// </summary>
+    public bool IsSupportParallel { get; }
+
+    /// <summary>
+    /// 创建写入流。
+    /// </summary>
+    /// <returns>写入流</returns>
+    public Task<Stream> CreateStreamAsync();
+
+    /// <summary>
+    /// 停止写入并释放资源。
+    /// </summary>
+    public Task StopAsync();
+
+    /// <summary>
+    /// 完成写入，用于执行某些并行操作的收尾工作 (例如合并文件)。
+    /// </summary>
+    public Task FinishAsync();
+}

Net/NDownload/NDlConnectionInfo.cs

@@ -0,0 +1,15 @@
+namespace PCL.Core.Net.NDownload;
+
+/// <summary>
+/// 下载连接信息。
+/// </summary>
+/// <param name="Length">内容长度，单位为字节</param>
+/// <param name="BeginOffset">起始偏移</param>
+/// <param name="EndOffset">结束偏移</param>
+/// <param name="IsSupportSegment">是否支持分块</param>
+public record NDlConnectionInfo(
+    long Length,
+    long BeginOffset,
+    long EndOffset,
+    bool IsSupportSegment
+);

Net/NDownload/NDlFactory.cs

@@ -0,0 +1,65 @@
+namespace PCL.Core.Net.NDownload;
+
+/// <summary>
+/// 无泛型的下载器构建工厂。
+/// </summary>
+public abstract class NDlFactory
+{
+    /// <summary>
+    /// 新建连接。
+    /// </summary>
+    /// <param name="resId">资源 ID</param>
+    /// <returns>下载连接</returns>
+    public abstract IDlConnection? CreateConnection(string resId);
+
+    /// <summary>
+    /// 新建写入器。
+    /// </summary>
+    /// <param name="resId">资源 ID</param>
+    /// <returns>下载写入器</returns>
+    public abstract IDlWriter? CreateWriter(string resId);
+}
+
+/// <summary>
+/// 下载器构建工厂。
+/// </summary>
+/// <typeparam name="TSourceArgument">下载源参数类型</typeparam>
+/// <typeparam name="TTargetArgument">写入目标参数类型</typeparam>
+public abstract class NDlFactory<TSourceArgument, TTargetArgument> : NDlFactory
+{
+    /// <summary>
+    /// 下载源映射。
+    /// </summary>
+    protected abstract IDlResourceMapping<TSourceArgument> SourceMapping { get; }
+
+    /// <summary>
+    /// 写入目标映射。
+    /// </summary>
+    protected abstract IDlResourceMapping<TTargetArgument> TargetMapping { get; }
+
+    /// <summary>
+    /// 新建连接。
+    /// </summary>
+    /// <param name="source">下载源参数</param>
+    /// <returns>下载连接</returns>
+    protected abstract IDlConnection CreateConnection(TSourceArgument source);
+
+    public override IDlConnection? CreateConnection(string resId)
+    {
+        var source = SourceMapping.Parse(resId);
+        return (source == null) ? null : CreateConnection(source);
+    }
+
+    /// <summary>
+    /// 新建写入器。
+    /// </summary>
+    /// <param name="target">写入目标</param>
+    /// <returns>下载写入器</returns>
+    protected abstract IDlWriter CreateWriter(TTargetArgument target);
+
+    public override IDlWriter? CreateWriter(string resId)
+    {
+        var target = TargetMapping.Parse(resId);
+        return (target == null) ? null : CreateWriter(target);
+    }
+}

Net/NDownload/NDlScheduler.cs

@@ -0,0 +1,5 @@
+namespace PCL.Core.Net.NDownload;
+
+public class NDlScheduler
+{
+}

Net/NDownload/NDlSourceManager.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+
+namespace PCL.Core.Net.NDownload;
+
+public class NDlSourceManager<TSourceArgument>(IList<IDlResourceMapping<TSourceArgument>> sources)
+    : IDlResourceMapping<TSourceArgument>
+{
+    public IList<IDlResourceMapping<TSourceArgument>> Sources { get; } = sources;
+
+    public TSourceArgument? Parse(string resId)
+    {
+        throw new System.NotImplementedException();
+    }
+}

Net/NDownload/NDlSourceReport.cs

@@ -0,0 +1,13 @@
+namespace PCL.Core.Net.NDownload;
+
+/// <summary>
+/// 下载源质量报告
+/// </summary>
+/// <param name="MaxSegmentCount">支持最大分块数量</param>
+/// <param name="RetryCount">重试计数</param>
+/// <param name="AverageSpeed">总体平均速度</param>
+public record NDlSourceReport(
+    int MaxSegmentCount = 1,
+    int RetryCount = 0,
+    long AverageSpeed = -1
+);

Net/NDownload/NDlTask.cs

@@ -0,0 +1,5 @@
+namespace PCL.Core.Net.NDownload;
+
+public class NDlTask
+{
+}

Net/NDownload/NDlTaskSegment.cs

@@ -0,0 +1,5 @@
+namespace PCL.Core.Net.NDownload;
+
+public class NDlTaskSegment
+{
+}