如何将部分预缓存的 MemoryStream 与 FileStream 结合起来？

Question

对于时间紧迫的媒体演示应用程序，重要的是媒体文件在用户选择它时立即显示。这些文件驻留在一个真正庞大的目录结构中，由数千个媒体文件组成。

显然，将媒体文件缓存在 MemoryStream 中是可行的方法；然而，由于文件的数量庞大，完全缓存每个文件是不可行的。相反，我的想法是预先缓存每个文件的某个缓冲区，一旦文件出现，就从该缓存中播放，直到文件的其余部分从硬盘加载。

我没有看到的是如何“连接”MemoryStream 和 FileStream 以提供无缝播放体验。我在数据流方面还不是很强，我看到了几个问题：

如何跟踪 MemoryStream 中的当前阅读位置并将其提供给 FileStream 而不会 MemoryStream 阅读更多？
如何从一种流切换到另一种流而不让两种流彼此部分重叠或造成“播放中断”？
如果使用流队列（如 How do I concatenate two System.Io.Stream instances into one? 中所建议），我如何指定第二个流必须在第一个流完成后立即准备好进行读取访问？在这里，特别是，我看不出 MemoryStream 有什么帮助，因为 FileStream 作为队列中的第二个，只有在实际使用后才会开始访问硬盘。
同时打开数百个（如果不是数千个）流真的是一种可行的方法吗？

请注意，我不需要写入权限——读取权限足以解决手头的问题。此外，这个问题类似于 Composite Stream Wrapper providing partial MemoryStream and full original Stream，但解决方案提供了 Windows Phone 8 的错误修复，但不适用于我的情况。

我非常想拓宽我对此相当有限的理解，因此非常感谢任何帮助。

Answer 1

我会建议类似以下解决方案：

从FileStream

CachableFileStream

实现一个非常简单的 Cache，它使用您喜欢的数据结构（例如 Queue）
允许Preload将数据写入内部缓存
允许Reload将数据写入内部缓存
以某种方式修改原始 Read 行为，使用您的缓存

为了让您了解我的想法，我会建议一些实现方式，如下所示：

用法可能是这样的：

CachableFileStream cachedStream = new CachableFileStream(...)
{
    PreloadSize = 8192,
    ReloadSize = 4096,
};

// Force preloading data into the cache
cachedStream.Preload();

...
cachedStream.Read(buffer, 0, buffer.Length);
...

警告：下面的代码既没有经过正确测试也不理想 - 这只是给你一个想法！

CachableFileStream class:

using System;
using System.IO;
using System.Threading.Tasks;

/// <summary>
/// Represents a filestream with cache.
/// </summary>
public class CachableFileStream : FileStream
{
    private Cache<byte> cache;
    private int preloadSize;
    private int reloadSize;

    /// <summary>
    /// Gets or sets the amount of bytes to be preloaded.
    /// </summary>
    public int PreloadSize
    {
        get
        {
            return this.preloadSize;
        }

        set
        {
            if (value <= 0)
                throw new ArgumentOutOfRangeException(nameof(value), "The specified preload size must not be smaller than or equal to zero.");

            this.preloadSize = value;
        }
    }

    /// <summary>
    /// Gets or sets the amount of bytes to be reloaded.
    /// </summary>
    public int ReloadSize
    {
        get
        {
            return this.reloadSize;
        }

        set
        {
            if (value <= 0)
                throw new ArgumentOutOfRangeException(nameof(value), "The specified reload size must not be smaller than or equal to zero.");

            this.reloadSize = value;
        }
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="CachableFileStream"/> class with the specified path and creation mode.
    /// </summary>
    /// <param name="path">A relative or absolute path for the file that the current CachableFileStream object will encapsulate</param>
    /// <param name="mode">A constant that determines how to open or create the file.</param>
    /// <exception cref="System.ArgumentException">
    /// Path is an empty string (""), contains only white space, or contains one or more invalid characters.
    /// -or- path refers to a non-file device, such as "con:", "com1:", "lpt1:", etc. in an NTFS environment.
    /// </exception>
    /// <exception cref="System.NotSupportedException">
    /// Path refers to a non-file device, such as "con:", "com1:", "lpt1:", etc. in a non-NTFS environment.
    /// </exception>
    /// <exception cref="System.ArgumentNullException">
    /// Path is null.
    /// </exception>
    /// <exception cref="System.Security.SecurityException">
    /// The caller does not have the required permission.
    /// </exception>
    /// <exception cref="System.IO.FileNotFoundException">
    /// The file cannot be found, such as when mode is FileMode.Truncate or FileMode.Open, and the file specified by path does not exist.
    /// The file must already exist in these modes.
    /// </exception>
    /// <exception cref="System.IO.IOException">
    /// An I/O error, such as specifying FileMode.CreateNew when the file specified by path already exists, occurred.-or-The stream has been closed.
    /// </exception>
    /// <exception cref="System.IO.DirectoryNotFoundException">
    /// The specified path is invalid, such as being on an unmapped drive.
    /// </exception>
    /// <exception cref="System.IO.PathTooLongException">
    /// The specified path, file name, or both exceed the system-defined maximum length.
    /// For example, on Windows-based platforms, paths must be less than 248 characters, and file names must be less than 260 characters.
    /// </exception>
    /// <exception cref="System.ArgumentOutOfRangeException">
    /// Mode contains an invalid value
    /// </exception>
    public CachableFileStream(string path, FileMode mode) : base(path, mode)
    {
        this.cache = new Cache<byte>();
        this.cache.CacheIsRunningLow += CacheIsRunningLow;
    }

    /// <summary>
    /// Reads a block of bytes from the stream and writes the data in a given buffer.
    /// </summary>
    /// <param name="array">
    /// When this method returns, contains the specified byte array with the values between
    /// offset and (offset + count - 1) replaced by the bytes read from the current source.
    /// </param>
    /// <param name="offset">The byte offset in array at which the read bytes will be placed.</param>
    /// <param name="count">The maximum number of bytes to read.</param>
    /// <returns>
    /// The total number of bytes read into the buffer. This might be less than the number
    /// of bytes requested if that number of bytes are not currently available, or zero
    /// if the end of the stream is reached.
    /// </returns>
    /// <exception cref="System.ArgumentNullException">
    /// Array is null.
    /// </exception>
    /// <exception cref="System.ArgumentOutOfRangeException">
    /// Offset or count is negative.
    /// </exception>
    /// <exception cref="System.NotSupportedException">
    /// The stream does not support reading.
    /// </exception>
    /// <exception cref="System.IO.IOException">
    /// An I/O error occurred.
    /// </exception>
    /// <exception cref="System.ArgumentException">
    /// Offset and count describe an invalid range in array.
    /// </exception>
    /// <exception cref="System.ObjectDisposedException">
    /// Methods were called after the stream was closed.
    /// </exception>
    public override int Read(byte[] array, int offset, int count)
    {
        int readBytesFromCache;

        for (readBytesFromCache = 0; readBytesFromCache < count; readBytesFromCache++)
        {
            if (this.cache.Size == 0)
                break;

            array[offset + readBytesFromCache] = this.cache.Read();
        }

        if (readBytesFromCache < count)
            readBytesFromCache += base.Read(array, offset + readBytesFromCache, count - readBytesFromCache);

        return readBytesFromCache;
    }

    /// <summary>
    /// Preload data into the cache.
    /// </summary>
    public void Preload()
    {
        this.LoadBytesFromStreamIntoCache(this.PreloadSize);
    }

    /// <summary>
    /// Reload data into the cache.
    /// </summary>
    public void Reload()
    {
        this.LoadBytesFromStreamIntoCache(this.ReloadSize);
    }

    /// <summary>
    /// Loads bytes from the stream into the cache.
    /// </summary>
    /// <param name="count">The number of bytes to read.</param>
    private void LoadBytesFromStreamIntoCache(int count)
    {
        byte[] buffer = new byte[count];
        int readBytes = base.Read(buffer, 0, buffer.Length);

        this.cache.AddRange(buffer, 0, readBytes);
    }

    /// <summary>
    /// Represents the event handler for the CacheIsRunningLow event.
    /// </summary>
    /// <param name="sender">The sender of the event.</param>
    /// <param name="e">Event arguments.</param>
    private void CacheIsRunningLow(object sender, EventArgs e)
    {
        this.cache.WarnIfRunningLow = false;

        new Task(() =>
        {
            Reload();
            this.cache.WarnIfRunningLow = true;
        }).Start();
    }
}

Cache class:

using System;
using System.Collections.Concurrent;

/// <summary>
/// Represents a generic cache.
/// </summary>
/// <typeparam name="T">Defines the type of the items in the cache.</typeparam>
public class Cache<T>
{
    private ConcurrentQueue<T> queue;

    /// <summary>
    /// Is executed when the number of items within the cache run below the
    /// specified warning limit and WarnIfRunningLow is set.
    /// </summary>
    public event EventHandler CacheIsRunningLow;

    /// <summary>
    /// Gets or sets a value indicating whether the CacheIsRunningLow event shall be fired or not.
    /// </summary>
    public bool WarnIfRunningLow
    {
        get;
        set;
    }

    /// <summary>
    /// Gets or sets a value that represents the lower warning limit.
    /// </summary>
    public int LowerWarningLimit
    {
        get;
        set;
    }

    /// <summary>
    /// Gets the number of items currently stored in the cache.
    /// </summary>
    public int Size
    {
        get;
        private set;
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="Cache{T}"/> class.
    /// </summary>
    public Cache()
    {
        this.queue = new ConcurrentQueue<T>();
        this.Size = 0;
        this.LowerWarningLimit = 1024;
        this.WarnIfRunningLow = true;
    }

    /// <summary>
    /// Adds an item into the cache.
    /// </summary>
    /// <param name="item">The item to be added to the cache.</param>
    public void Add(T item)
    {
        this.queue.Enqueue(item);
        this.Size++;
    }

    /// <summary>
    /// Adds the items of the specified array to the end of the cache.
    /// </summary>
    /// <param name="items">The items to be added.</param>
    public void AddRange(T[] items)
    {
        this.AddRange(items, 0, items.Length);
    }

    /// <summary>
    /// Adds the specified count of items of the specified array starting
    /// from offset to the end of the cache.
    /// </summary>
    /// <param name="items">The array that contains the items.</param>
    /// <param name="offset">The offset that shall be used.</param>
    /// <param name="count">The number of items that shall be added.</param>
    public void AddRange(T[] items, int offset, int count)
    {
        for (int i = offset; i < count; i++)
            this.Add(items[i]);
    }

    /// <summary>
    /// Reads one item from the cache.
    /// </summary>
    /// <returns>The item that has been read from the cache.</returns>
    /// <exception cref="System.InvalidOperationException">
    /// The cache is empty.
    /// </exception>
    public T Read()
    {
        T item;

        if (!this.queue.TryDequeue(out item))
            throw new InvalidOperationException("The cache is empty.");

        this.Size--;

        if (this.WarnIfRunningLow &&
            this.Size < this.LowerWarningLimit)
        {
            this.CacheIsRunningLow?.Invoke(this, EventArgs.Empty);
        }

        return item;
    }

    /// <summary>
    /// Peeks the next item from cache.
    /// </summary>
    /// <returns>The item that has been read from the cache (without deletion).</returns>
    /// <exception cref="System.InvalidOperationException">
    /// The cache is empty.
    /// </exception>
    public T Peek()
    {
        T item;

        if (!this.queue.TryPeek(out item))
            throw new InvalidOperationException("The cache is empty.");

        return item;
    }
}

希望对您有所帮助，玩得开心 ;-)

如何将部分预缓存的 MemoryStream 与 FileStream 结合起来？

How to combine partially pre-cached MemoryStream with FileStream?

c#

caching

memorystream

stream

filestream