在Metro Windows 8中将ImageSource转换为WriteableBitmap

在Metro Windows 8中，将ImageSource转换为WriteableBitmap可以通过以下步骤实现：

首先，需要在项目中引入Windows.UI.Xaml.Media.Imaging命名空间。
然后，可以使用ImageSourceConverter类将ImageSource转换为WriteableBitmap。

以下是一个示例代码：

// 将ImageSource转换为WriteableBitmap
private async Task<WriteableBitmap> ConvertImageSourceToWriteableBitmap(ImageSource imageSource)
{
    WriteableBitmap writeableBitmap = null;
    if (imageSource != null)
    {
        var stream = new InMemoryRandomAccessStream();
        var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, stream);
        var pixelData = await imageSource.ToPixelDataAsync();
        encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore, (uint)imageSource.PixelWidth, (uint)imageSource.PixelHeight, 96.0, 96.0, pixelData);
        await encoder.FlushAsync();
        stream.Seek(0);
        var bitmapImage = new BitmapImage();
        bitmapImage.SetSource(stream);
        writeableBitmap = new WriteableBitmap(bitmapImage.PixelWidth, bitmapImage.PixelHeight);
        stream.Seek(0);
        await writeableBitmap.SetSourceAsync(stream);
    }
    return writeableBitmap;
}

在这个示例代码中，我们首先创建了一个InMemoryRandomAccessStream对象，然后使用BitmapEncoder将ImageSource转换为PNG格式的数据，并将其写入到InMemoryRandomAccessStream对象中。接着，我们使用BitmapImage对象将InMemoryRandomAccessStream对象中的数据读取出来，并将其设置为WriteableBitmap对象的源。最后，我们返回WriteableBitmap对象。

需要注意的是，这个示例代码中的ToPixelDataAsync()方法是扩展方法，需要自己实现。具体实现可以参考以下代码：

public static class ImageSourceExtensions
{
    public static async Task<byte[]> ToPixelDataAsync(this ImageSource imageSource)
    {
        byte[] pixelData = null;
        if (imageSource != null)
        {
            var randomAccessStream = new InMemoryRandomAccessStream();
            var bitmapEncoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, randomAccessStream);
            var softwareBitmap = await imageSource.ToSoftwareBitmapAsync();
            bitmapEncoder.SetSoftwareBitmap(softwareBitmap);
            await bitmapEncoder.FlushAsync();
            pixelData = (await randomAccessStream.ToArrayAsync()).ToArray();
        }
        return pixelData;
    }

    private static async Task<SoftwareBitmap> ToSoftwareBitmapAsync(this ImageSource imageSource)
    {
        SoftwareBitmap softwareBitmap = null;
        if (imageSource is SoftwareBitmapSource softwareBitmapSource)
        {
            softwareBitmap = softwareBitmapSource.SoftwareBitmap;
        }
        else if (imageSource is WriteableBitmap writeableBitmap)
        {
            softwareBitmap = SoftwareBitmap.CreateCopyFromBuffer(writeableBitmap.PixelBuffer, BitmapPixelFormat.Bgra8, writeableBitmap.PixelWidth, writeableBitmap.PixelHeight);
        }
        else if (imageSource is BitmapImage bitmapImage)
        {
            softwareBitmap = await bitmapImage.ToSoftwareBitmapAsync();
        }
        return softwareBitmap;
    }

    private static async Task<SoftwareBitmap> ToSoftwareBitmapAsync(this BitmapImage bitmapImage)
    {
        SoftwareBitmap softwareBitmap = null;
        if (bitmapImage != null)
        {
            var randomAccessStream = new InMemoryRandomAccessStream();
            var bitmapEncoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, randomAccessStream);
            bitmapEncoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore, (uint)bitmapImage.PixelWidth, (uint)bitmapImage.PixelHeight, 96.0, 96.0, await bitmapImage.ToPixelDataAsync());
            await bitmapEncoder.FlushAsync();
            randomAccessStream.Seek(0);
            softwareBitmap = await BitmapDecoder.CreateAsync(randomAccessStream).AsTask().ContinueWith(async (task) =>
            {
                var bitmapDecoder = task.Result;
                return await bitmapDecoder.GetSoftwareBitmapAsync();
            }).Unwrap().ConfigureAwait(false);
        }
        return softwareBitmap;
    }

    private static async Task<byte[]> ToArrayAsync(this IRandomAccessStream randomAccessStream)
    {
        byte[] data = new byte[randomAccessStream.Size];
        await randomAccessStream.ReadAsync(data.AsBuffer(), (uint)randomAccessStream.Size, InputStreamOptions.None);
        return data;
    }
}

这个扩展方法包含了ImageSource对象到SoftwareBitmap对象的转换，以及SoftwareBitmap对象到字节数组的转换。