最佳化檔案存取

發行項
10/05/2023

建立可有效存取檔案系統的通用 Windows 平台 (UWP) 應用程式，避免因為磁碟延遲和記憶體/CPU 週期而發生效能問題。

當您想要存取大量檔案，並且想要存取一般 Name、FileType 和 Path 屬性以外的屬性值時，請建立 QueryOptions 並呼叫 SetPropertyPrefetch 來存取這些檔案。 SetPropertyPrefetch 方法可以大幅改善顯示從檔案系統所取得項目集合的應用程式效能，例如影像集合。下一組範例示範幾個存取多個檔案的方式。

第一個範例會使用 Windows.Storage.StorageFolder.GetFilesAsync 來擷取一組檔案的名稱資訊。此方法提供良好的效能，因為範例只會存取 name 屬性。

StorageFolder library = Windows.Storage.KnownFolders.PicturesLibrary;
IReadOnlyList<StorageFile> files = await library.GetFilesAsync(Windows.Storage.Search.CommonFileQuery.OrderByDate);

for (int i = 0; i < files.Count; i++)
{
    // do something with the name of each file
    string fileName = files[i].Name;
}

Dim library As StorageFolder = Windows.Storage.KnownFolders.PicturesLibrary
Dim files As IReadOnlyList(Of StorageFile) =
    Await library.GetFilesAsync(Windows.Storage.Search.CommonFileQuery.OrderByDate)

For i As Integer = 0 To files.Count - 1
    ' do something with the name of each file
    Dim fileName As String = files(i).Name
Next i

第二個範例會使用 Windows.Storage.StorageFolder.GetFilesAsync，然後擷取每個檔案的映像屬性。此方法提供效能不佳。

StorageFolder library = Windows.Storage.KnownFolders.PicturesLibrary;
IReadOnlyList<StorageFile> files = await library.GetFilesAsync(Windows.Storage.Search.CommonFileQuery.OrderByDate);
for (int i = 0; i < files.Count; i++)
{
    ImageProperties imgProps = await files[i].Properties.GetImagePropertiesAsync();

    // do something with the date the image was taken
    DateTimeOffset date = imgProps.DateTaken;
}

Dim library As StorageFolder = Windows.Storage.KnownFolders.PicturesLibrary
Dim files As IReadOnlyList(Of StorageFile) = Await library.GetFilesAsync(Windows.Storage.Search.CommonFileQuery.OrderByDate)
For i As Integer = 0 To files.Count - 1
    Dim imgProps As ImageProperties =
        Await files(i).Properties.GetImagePropertiesAsync()

    ' do something with the date the image was taken
    Dim dateTaken As DateTimeOffset = imgProps.DateTaken
Next i

第三個範例會使用 QueryOptions 來取得一組檔案的相關資訊。此方法提供比上一個範例更好的效能。

// Set QueryOptions to prefetch our specific properties
var queryOptions = new Windows.Storage.Search.QueryOptions(CommonFileQuery.OrderByDate, null);
queryOptions.SetThumbnailPrefetch(ThumbnailMode.PicturesView, 100,
        ThumbnailOptions.ReturnOnlyIfCached);
queryOptions.SetPropertyPrefetch(PropertyPrefetchOptions.ImageProperties, 
       new string[] {"System.Size"});

StorageFileQueryResult queryResults = KnownFolders.PicturesLibrary.CreateFileQueryWithOptions(queryOptions);
IReadOnlyList<StorageFile> files = await queryResults.GetFilesAsync();

foreach (var file in files)
{
    ImageProperties imageProperties = await file.Properties.GetImagePropertiesAsync();

    // Do something with the date the image was taken.
    DateTimeOffset dateTaken = imageProperties.DateTaken;

    // Performance gains increase with the number of properties that are accessed.
    IDictionary<String, object> propertyResults =
        await file.Properties.RetrievePropertiesAsync(
              new string[] {"System.Size" });

    // Get/Set extra properties here
    var systemSize = propertyResults["System.Size"];
}

' Set QueryOptions to prefetch our specific properties
Dim queryOptions = New Windows.Storage.Search.QueryOptions(CommonFileQuery.OrderByDate, Nothing)
queryOptions.SetThumbnailPrefetch(ThumbnailMode.PicturesView,
            100, Windows.Storage.FileProperties.ThumbnailOptions.ReturnOnlyIfCached)
queryOptions.SetPropertyPrefetch(PropertyPrefetchOptions.ImageProperties,
                                 New String() {"System.Size"})

Dim queryResults As StorageFileQueryResult = KnownFolders.PicturesLibrary.CreateFileQueryWithOptions(queryOptions)
Dim files As IReadOnlyList(Of StorageFile) = Await queryResults.GetFilesAsync()


For Each file In files
    Dim imageProperties As ImageProperties = Await file.Properties.GetImagePropertiesAsync()

    ' Do something with the date the image was taken.
    Dim dateTaken As DateTimeOffset = imageProperties.DateTaken

    ' Performance gains increase with the number of properties that are accessed.
    Dim propertyResults As IDictionary(Of String, Object) =
        Await file.Properties.RetrievePropertiesAsync(New String() {"System.Size"})

    ' Get/Set extra properties here
    Dim systemSize = propertyResults("System.Size")

Next file

如果您要在 Windows.Storage 物件上執行多個作業，例如 Windows.Storage.ApplicationData.Current.LocalFolder，請建立區域變數來參考該儲存體來源，如此一來，您就不會在每次存取時重新建立中繼物件。

C# 與 Visual Basic 中的串流效能

UWP 與 .NET 串流之間的緩衝

您在許多案例中可能需要將 UWP 串流 (例如 Windows.Storage.Streams.IInputStream 或 IOutputStream) 轉換成 .NET 串流 (System.IO.Stream)。例如，當您撰寫 UWP 應用程式，並想要使用現有的 .NET 程式碼搭配 UWP 檔案系統處理串流時，這會很有用。若要啟用此功能，適用於 UWP 應用程式的 .NET API 會提供延伸方法，讓您在 .NET 與 UWP 資料流類型之間進行轉換。如需詳細資訊，請參閱 WindowsRuntimeStreamExtensions。

當您將 UWP 串流轉換成 .NET 串流時，您可以有效地建立基礎 UWP 串流的配接器。在某些情況下，與在 UWP 串流上叫用方法相關聯的執行階段成本。這可能會影響應用程式的速度，特別是在您執行許多小型、頻繁讀取或寫入作業的案例。

為了加速應用程式，UWP 串流配接器包含資料緩衝區。下列程式碼範例示範使用具有預設緩衝區大小的 UWP 串流配接器進行小型連續讀取。

StorageFile file = await Windows.Storage.ApplicationData.Current
    .LocalFolder.GetFileAsync("example.txt");
Windows.Storage.Streams.IInputStream windowsRuntimeStream = 
    await file.OpenReadAsync();

byte[] destinationArray = new byte[8];

// Create an adapter with the default buffer size.
using (var managedStream = windowsRuntimeStream.AsStreamForRead())
{

    // Read 8 bytes into destinationArray.
    // A larger block is actually read from the underlying 
    // windowsRuntimeStream and buffered within the adapter.
    await managedStream.ReadAsync(destinationArray, 0, 8);

    // Read 8 more bytes into destinationArray.
    // This call may complete much faster than the first call
    // because the data is buffered and no call to the 
    // underlying windowsRuntimeStream needs to be made.
    await managedStream.ReadAsync(destinationArray, 0, 8);
}

Dim file As StorageFile = Await Windows.Storage.ApplicationData.Current -
.LocalFolder.GetFileAsync("example.txt")
Dim windowsRuntimeStream As Windows.Storage.Streams.IInputStream =
    Await file.OpenReadAsync()

Dim destinationArray() As Byte = New Byte(8) {}

' Create an adapter with the default buffer size.
Dim managedStream As Stream = windowsRuntimeStream.AsStreamForRead()
Using (managedStream)

    ' Read 8 bytes into destinationArray.
    ' A larger block is actually read from the underlying 
    ' windowsRuntimeStream and buffered within the adapter.
    Await managedStream.ReadAsync(destinationArray, 0, 8)

    ' Read 8 more bytes into destinationArray.
    ' This call may complete much faster than the first call
    ' because the data is buffered and no call to the 
    ' underlying windowsRuntimeStream needs to be made.
    Await managedStream.ReadAsync(destinationArray, 0, 8)

End Using

在將 UWP 串流轉換成 .NET 串流的大部分案例中，這都是理想的預設緩衝行為。不過，在某些情況下，您可能需要調整緩衝行為，以提高效能。

使用大型資料集

讀取或寫入較大的資料集時，您可以向 AsStreamForRead、AsStreamForWrite 和 AsStream 擴充方法提供大型緩衝區大小，以增加讀取或寫入輸送量。這會為串流配接器提供較大的內部緩衝區大小。例如，將來自大型檔案的串流傳遞至 XML 剖析器時，剖析器可以從串流進行許多循序小型讀取。大型緩衝區可以減少對基礎 UWP 串流的呼叫數目，並提升效能。

注意在設定大於約 80 KB 的緩衝區大小時，請務必謹慎，因為這可能導致記憶體回收行程堆積分散 (請參閱改善記憶體回收效能)。下列程式碼範例會建立具有 81,920 位元組緩衝區的受控串流配接器。

// Create a stream adapter with an 80 KB buffer.
Stream managedStream = nativeStream.AsStreamForRead(bufferSize: 81920);

' Create a stream adapter with an 80 KB buffer.
Dim managedStream As Stream = nativeStream.AsStreamForRead(bufferSize:=81920)

Stream.CopyTo 和 CopyToAsync 方法也會配置本機緩衝區，以在串流之間複製。如同 AsStreamForRead 擴充方法，您可以覆寫預設緩衝區大小，以取得大型串流複本的較佳效能。下列程式碼範例示範如何變更 CopyToAsync 呼叫的預設緩衝區大小。

MemoryStream destination = new MemoryStream();
// copies the buffer into memory using the default copy buffer
await managedStream.CopyToAsync(destination);

// Copy the buffer into memory using a 1 MB copy buffer.
await managedStream.CopyToAsync(destination, bufferSize: 1024 * 1024);

Dim destination As MemoryStream = New MemoryStream()
' copies the buffer into memory using the default copy buffer
Await managedStream.CopyToAsync(destination)

' Copy the buffer into memory using a 1 MB copy buffer.
Await managedStream.CopyToAsync(destination, bufferSize:=1024 * 1024)

此範例使用 1 MB 的緩衝區大小，其大於先前建議的 80 KB。使用這類大型緩衝區可以改善非常大型資料集的複製作業輸送量 (也就是數百 MB)。不過，此緩衝區會配置在大型物件堆積上，且可能會降低廢棄項目收集效能。只有在大幅改善應用程式的效能時，才應該使用大型緩衝區大小。

當您同時使用大量串流時，可能會需要減少或消除緩衝區的記憶體額外負荷。您可以指定較小的緩衝區，或將 bufferSize 參數設定為 0，以完全關閉該串流配接器的緩衝處理。如果您對受控串流執行大型讀取和寫入，仍可以在不需要緩衝處理的情況下達到良好的輸送量效能。

執行延遲敏感性作業

如果您想要低延遲讀取和寫入，且不想在基礎 UWP 串流外的大型區塊中讀取，則也可以避免緩衝處理。例如，如果您使用串流進行網路通訊，可能會需要要低延遲讀取和寫入。

在聊天應用程式中，您可以使用透過網路介面的串流來回傳送訊息。在此情況下，您需要在訊息就緒後立即傳送訊息，而不是等待緩衝區填滿。如果您在呼叫 AsStreamForRead、AsStreamForWrite 和 AsStream 擴充方法時將緩衝區大小設定為 0，則產生的配接器將不會配置緩衝區，且所有呼叫都會直接操作基礎 UWP 串流。

共用方式為