如何在 C# 中实现 IRandomAccessStream？

Question

我想在 C# 中实现IRandomAccessStream 的实例（它将返回实时生成的数据）。该流实际上不需要是可写或可搜索的，但我想在ReadAsync 方法中返回我自己的数据（实际上是IInputStream 的一部分）。

public IAsyncOperationWithProgress<IBuffer, uint> ReadAsync(IBuffer buffer, uint count, InputStreamOptions options)
{
    throw new NotImplementedException("To be done");
}

我的两个主要问题是：

1. 如何返回实现IAsyncOperationWithProgress 的东西？框架中是否有任何东西可以帮助解决这个问题？
2. 如何将数据写入缓冲区？ IBuffer 只有 Length 和 Capacity 属性（具体的 Buffer 类也不再提供）。

Answer 1

How to Convert byte Array to IRandomAccessStream

我找到了这篇博客文章，希望IRandomAccessStream 的实现可以成为您的起点。

class MemoryRandomAccessStream : IRandomAccessStream
{
    private Stream m_InternalStream;

    public MemoryRandomAccessStream(Stream stream)
    {
        this.m_InternalStream = stream;
    }

    public MemoryRandomAccessStream(byte[] bytes)
    {
        this.m_InternalStream = new MemoryStream(bytes);
    }

    public IInputStream GetInputStreamAt(ulong position)
    {
        this.m_InternalStream.Seek((long)position, SeekOrigin.Begin);

        return this.m_InternalStream.AsInputStream();
    }

    public IOutputStream GetOutputStreamAt(ulong position)
    {
        this.m_InternalStream.Seek((long)position, SeekOrigin.Begin);

        return this.m_InternalStream.AsOutputStream();
    }

    public ulong Size
    {
        get { return (ulong)this.m_InternalStream.Length; }
        set { this.m_InternalStream.SetLength((long)value); }
    }

    public bool CanRead
    {
        get { return true; }
    }

    public bool CanWrite
    {
        get { return true; }
    }

    public IRandomAccessStream CloneStream()
    {
        throw new NotSupportedException();
    }

    public ulong Position
    {
        get { return (ulong)this.m_InternalStream.Position; }
    }

    public void Seek(ulong position)
    {
        this.m_InternalStream.Seek((long)position, 0);
    }

    public void Dispose()
    {
        this.m_InternalStream.Dispose();
    }

    public Windows.Foundation.IAsyncOperationWithProgress<IBuffer, uint> ReadAsync(IBuffer buffer, uint count, InputStreamOptions options)
    {
        var inputStream = this.GetInputStreamAt(0);
        return inputStream.ReadAsync(buffer, count, options);
    }

    public Windows.Foundation.IAsyncOperation<bool> FlushAsync()
    {
        var outputStream = this.GetOutputStreamAt(0);
        return outputStream.FlushAsync();
    }

    public Windows.Foundation.IAsyncOperationWithProgress<uint, uint> WriteAsync(IBuffer buffer)
    {
        var outputStream = this.GetOutputStreamAt(0);
        return outputStream.WriteAsync(buffer);
     }
}

Answer 2

1. 使用AsyncInfo.Run(Func<CancellationToken, IProgress<uint>, Task<IBuffer>>) 方法从委托创建 IAsyncOperationWithProgress 实例。

   public IAsyncOperationWithProgress<IBuffer, uint> ReadAsync(IBuffer buffer, uint count, InputStreamOptions options)
   {    
       if (buffer == null) throw new ArgumentNullException("buffer");
   
       Func<CancellationToken, IProgress<uint>, Task<IBuffer>> taskProvider =
       (token, progress) => ReadBytesAsync(buffer, count, token, progress, options);
   
       return AsyncInfo.Run(taskProvider);
   }
   
   private async Task<IBuffer> ReadBytesAsync(IBuffer buffer, uint count, CancellationToken token, IProgress<uint> progress, InputStreamOptions options)
   {
   ... Fill the buffer here. Report the progress.
       return buffer;
   }
   

2. 通常您不需要直接访问缓冲区数据。但如果您需要在 c# 中执行此操作，您可以使用 System.Runtime.InteropServices.WindowsRuntime.WindowsRuntimeBufferExtensions 类将数据复制到缓冲区/从缓冲区复制数据。