Windows Phone 8.0到Windows Phone 8.1應用程序

Question

我正在寫一個Windows Phone 8.1（WINRT）應用程序 。 我有一個@ Sergio0694給出的代碼，它在Windows Phone 8.0 / Silverlight上運行良好。 它壓縮從圖庫中選取的用戶照片，然后轉換為base64字符串。

它不適用於Windows Phone 8.1。 誰能幫我 ？

public static async Task<String> ToCompressedBase64(this StorageFile imageFile, Page localPage)
{
    //Get the stream from the StorageFile
    IRandomAccessStream imageStream = await imageFile.OpenAsync(FileAccessMode.Read);

    System.Diagnostics.Debug.WriteLine("Original size ---> " + imageStream.ToFileSize());

    //Compresses the image if it exceedes the maximum file size
    imageStream.Seek(0);
    BitmapDecoder compressDecoder = await BitmapDecoder.CreateAsync(imageStream);
    PixelDataProvider compressionData = await compressDecoder.GetPixelDataAsync();
    byte[] compressionBytes = compressionData.DetachPixelData();

    //Set target compression quality
    BitmapPropertySet propertySet = new BitmapPropertySet();
    BitmapTypedValue qualityValue = new BitmapTypedValue(0.5, PropertyType.Single);
    propertySet.Add("ImageQuality", qualityValue);

    imageStream.Seek(0);
    imageStream = new InMemoryRandomAccessStream();
    BitmapEncoder compressionEncoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, imageStream, propertySet);
    compressionEncoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Straight,
                                    compressDecoder.PixelWidth, compressDecoder.PixelHeight,
                                    compressDecoder.DpiX, compressDecoder.DpiY, compressionBytes);
    await compressionEncoder.FlushAsync();

    //Create a BitmapDecoder from the stream
    BitmapDecoder resizeDecoder = await BitmapDecoder.CreateAsync(imageStream);
#if DEBUG
    System.Diagnostics.Debug.WriteLine("Old height and width ---> " + resizeDecoder.PixelHeight + " * " + resizeDecoder.PixelWidth + "\nCompressed size ---> " + imageStream.ToFileSize());
#endif
    //Resize the image if needed
    TaskCompletionSource<bool> completionSource = new TaskCompletionSource<bool>();
    localPage.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
    {
        const int maxImageWidth = 48;
        if (resizeDecoder.PixelWidth > maxImageWidth)
        {
            //Resize the image if it exceedes the maximum width
            int newHeight = (int)(maxImageWidth * resizeDecoder.PixelHeight / resizeDecoder.PixelWidth);
            WriteableBitmap tempBitmap = new WriteableBitmap((int)resizeDecoder.PixelWidth, (int)resizeDecoder.PixelHeight);
            imageStream.Seek(0);
            await tempBitmap.SetSourceAsync(imageStream);
            WriteableBitmap resizedImage = tempBitmap.Resize(maxImageWidth, newHeight, WriteableBitmapExtensions.Interpolation.Bilinear);

            //Assign to imageStream the resized WriteableBitmap
            InMemoryRandomAccessStream resizedStream = new InMemoryRandomAccessStream();
            await resizedImage.ToStream(resizedStream, BitmapEncoder.JpegEncoderId);
            imageStream = resizedStream;
        }
        completionSource.SetResult(true);
    }).Forget();
    await completionSource.Task;           

    //Converts the final image into a Base64 String
    imageStream.Seek(0);

    BitmapDecoder decoder = await BitmapDecoder.CreateAsync(imageStream);
    PixelDataProvider pixels = await decoder.GetPixelDataAsync();
#if DEBUG
    System.Diagnostics.Debug.WriteLine("New height and width ---> " + decoder.PixelHeight + " * " + decoder.PixelWidth + "\nSize after resize ---> " + imageStream.ToFileSize());
#endif
    byte[] bytes = pixels.DetachPixelData();

    //Encode image
    InMemoryRandomAccessStream encoded = new InMemoryRandomAccessStream();
    BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, encoded);
    encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Straight, decoder.PixelWidth, decoder.PixelHeight, decoder.DpiX, decoder.DpiY, bytes);
    await encoder.FlushAsync();
    encoded.Seek(0);

    //Read bytes
    byte[] outBytes = new byte[encoded.Size];
    await encoded.AsStream().ReadAsync(outBytes, 0, outBytes.Length);

    //Create Base64
    return Convert.ToBase64String(outBytes);
}

Answer 1

當使用TaskCompletionSource並等待它時，除了使用TaskCompletionSource設置結果TaskCompletionSource.SetResult您還應該處理可能發生的任何異常，當您通過調用TaskCompletionSource.SetException等待TaskCompletionSource.Task時，這些異常不會被冒泡。

很可能當你處理一個大文件時拋出一個異常，因此你的await completionSource.Task會因為沒有被調用SetResult而掛起。

為了說明這一點，這里有一個代碼示例，該代碼執行您正在嘗試使用您的邏輯調度程序執行的操作，但也處理異常，如上所述：

public async Task<string> TestTaskCompletionSource(CoreDispatcher dispatcher)
{
    TaskCompletionSource<bool> completionSource = new TaskCompletionSource<bool>();
    await dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
    {
        try
        {
            Debug.WriteLine("Before delay");
            await Task.Delay(100);
            Debug.WriteLine("After delay and before exception");
            throw new Exception("Test");
#pragma warning disable 162
            completionSource.SetResult(true);
#pragma warning restore 162
        }
        catch (Exception e)
        {
            completionSource.SetException(e);
        }
    });
    await completionSource.Task;

    return "Test";
}