使用DownloadToFileParallelAsync下载大文件如何追踪下载进度？

针对DownloadToFileParallelAsync方法的进度追踪需求，以下两种方案均可以保留原有下载性能，实现稳定的进度统计：

方案1：使用库内置TransferContext进度回调 #

该方案无额外开发成本，完全复用原有下载逻辑，性能无损耗：

1. 先调用Blob的属性查询接口获取待下载文件的总字节大小
2. 初始化SingleTransferContext对象，绑定IProgress<TransferStatus>回调
3. 调用DownloadToFileParallelAsync时传入创建的TransferContext实例

代码示例：

// 获取目标文件总大小
await cloudBlob.FetchAttributesAsync();
long totalSize = cloudBlob.Properties.Length;

var transferContext = new SingleTransferContext
{
    Progress = new Progress<TransferStatus>(status =>
    {
        double progress = (double)status.BytesTransferred / totalSize * 100;
        // 自定义进度处理逻辑
        Console.WriteLine($"下载进度：{progress:F2}%");
    })
};

// 执行下载，传入上下文参数
await blob.DownloadToFileParallelAsync(localPath, FileMode.Create, default, null, transferContext, default);

方案2：自定义进度统计包装Stream #

如果需要更细粒度的进度控制，可以通过拦截写入流的方式统计已写入字节，性能损耗可以忽略：

1. 实现自定义ProgressStream类，包装原生FileStream，在Write方法中累计已写入字节数并触发回调
2. 调用DownloadToFileParallelAsync的Stream重载版本，传入包装后的ProgressStream对象

自定义Stream实现代码：

public class ProgressStream : Stream
{
    private readonly Stream _baseStream;
    private readonly Action<long> _onWrite;
    private long _writtenBytes;

    public ProgressStream(Stream baseStream, Action<long> onWriteCallback)
    {
        _baseStream = baseStream ?? throw new ArgumentNullException(nameof(baseStream));
        _onWrite = onWriteCallback ?? throw new ArgumentNullException(nameof(onWriteCallback));
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        _baseStream.Write(buffer, offset, count);
        _writtenBytes += count;
        _onWrite(_writtenBytes);
    }

    // 其余Stream成员直接转发到基础流实现即可
    public override bool CanRead => _baseStream.CanRead;
    public override bool CanSeek => _baseStream.CanSeek;
    public override bool CanWrite => _baseStream.CanWrite;
    public override long Length => _baseStream.Length;
    public override long Position { get => _baseStream.Position; set => _baseStream.Position = value; }
    public override void Flush() => _baseStream.Flush();
    public override int Read(byte[] buffer, int offset, int count) => _baseStream.Read(buffer, offset, count);
    public override long Seek(long offset, SeekOrigin origin) => _baseStream.Seek(offset, origin);
    public override void SetLength(long value) => _baseStream.SetLength(value);
}

使用示例：

await cloudBlob.FetchAttributesAsync();
long totalSize = cloudBlob.Properties.Length;

await using var baseFs = new FileStream(localPath, FileMode.Create, FileAccess.Write, FileShare.None);
await using var progressFs = new ProgressStream(baseFs, writtenBytes =>
{
    double progress = (double)writtenBytes / totalSize * 100;
    Console.WriteLine($"下载进度：{progress:F2}%");
});

await blob.DownloadToFileParallelAsync(progressFs, default, null, default, default);

内容的提问来源于stack exchange，提问作者Jon