如何將部分預緩存的 MemoryStream 與 FileStream 結合起來？

Question

對於時間緊迫的媒體呈現應用程序，重要的是在用戶選擇媒體文件時立即呈現媒體文件。 這些文件駐留在一個真正龐大的目錄結構中，由數千個媒體文件組成。

顯然，在MemoryStream緩存媒體文件是要走的路； 然而，由於文件數量龐大，完全緩存每個文件是不可行的。 相反，我的想法是預先緩存每個文件的某個緩沖區，一旦文件出現，就從該緩存播放，直到文件的其余部分從硬盤加載。

我沒有看到的是如何“連接” MemoryStream和FileStream以提供無縫播放體驗。 我在數據流方面不是很強大（還），我看到了幾個問題：

• 如何跟蹤MemoryStream中的當前讀取位置並將其提供給FileStream而MemoryStream不會讀取更多內容？
• 如何在不讓兩個流彼此部分重疊或創建“播放中斷”的情況下從一個流切換到另一個流？
• 如果使用流隊列（如如何將兩個 System.Io.Stream 實例連接成一個？中所建議的那樣？ ），我如何指定第二個流必須在第一個流完成后立即准備好進行讀取訪問？ 在這里，特別是，我根本看不到MemoryStream什么幫助，因為FileStream作為隊列中的第二個，只有在實際使用后才開始訪問硬盤。
• 一次擁有數百個甚至數千個開放流真的是一種可行的方法嗎？

請注意，我不需要寫訪問權限——閱讀就足以解決手頭的問題。 此外，這個問題類似於Composite Stream Wrapper 提供部分 MemoryStream 和完整的原始 Stream ，但提供的解決方案有 Windows Phone 8 的錯誤修復，不適用於我的情況。

我非常想擴大我對這一點相當有限的理解，因此非常感謝任何幫助。

Answer 1

我會建議類似以下解決方案：

• 從FileStream繼承你自己的CachableFileStream
• 實現一個非常簡單的Cache ，它使用您喜歡的數據結構（如Queue ）
• 允許將數據Preload到內部緩存中
• 允許將數據Reload到內部緩存中
• 以某種方式修改原始Read行為，即使用您的緩存

為了讓您了解我的想法，我會建議一些實現，如下所示：

用法可能是這樣的：

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

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

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

警告：下面的代碼既沒有經過正確的測試，也沒有達到理想的效果——這只是給你一個想法！

CachableFileStream類：

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類：

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;
    }
}

我希望這會有所幫助，玩得開心;-)