WPF 之上的 C# async/await Task<BitmaptImage> 问题

Question

微型 WPF 应用程序以 async/await 方式使用灰度输入参数转换图像 (BitmapImage)。

我阅读了很多实现，但没能成功:/

按键方式：

private async void btnConvertImage_ClickAsync(object sender, RoutedEventArgs e)
{
    try
    {
        var cts = new CancellationTokenSource();
        BitmapImage result = await ImageProcessing.GreyscaleAsync(orginalImage, cts.Token).ConfigureAwait(false);
        imgPhotoConverted.Source = result;
    }
}

灰度任务定义：

public static async Task<BitmapImage> GreyscaleAsync(BitmapImage inputBitmapImage, CancellationToken cancellationToken)
{
    return await Task.Run(() =>
    {
        Bitmap inputBitmap = ToBitmap(inputBitmapImage);
        Bitmap outputImage = new Bitmap(inputBitmap.Width, inputBitmap.Height);
        for (int i = 0; i < inputBitmap.Width; i++)
        {
            for (int x = 0; x < inputBitmap.Height; x++)
            {
                cancellationToken.ThrowIfCancellationRequested();
                Color imageColor = inputBitmap.GetPixel(i, x);
                int grayScale = (int)((imageColor.R * 0.21) + (imageColor.G * 0.72) + (imageColor.B * 0.07));
                Color newColor = Color.FromArgb(imageColor.A, grayScale, grayScale, grayScale);
                outputImage.SetPixel(i, x, newColor);
            }
        }

        return ToBitmapImage(outputImage);
    }, cancellationToken);
}

在线：

imgPhotoConverted.Source = result;

抛出错误：

System.InvalidOperationException: 'The calling thread cannot access this object because a different thread owns it.'

Answer 1

我设法解决了：

我不必使用 BitmapSource，但我必须 Freeze() 输出 BitmaImage 并将 result 包装在适当的上下文中使用：

ThreadPool.QueueUserWorkItem(async delegate
{
    // ThreadPool
    var cts = new CancellationTokenSource();
    BitmapImage result = await ImageProcessing.GreyscaleAsync(orginalImage, cts.Token);
    result.Freeze();

    sc.Post(delegate
    {
        // original context (UI)
        imgPhotoConverted.Source = result;
        cts.Cancel();
    }, null);
}

我希望这对其他人有用。谢谢！

Answer 2

您应该在 Stephen Clearys Blog 阅读更多关于 async/await 的信息。

按照那里的建议，您将找到一个非常聪明的解决方案

private async void btnConvertImage_ClickAsync(object sender, RoutedEventArgs e)
{
    var originalImage = ( imgPhotoOriginal.Source as BitmapImage );
    BitmapImage result = await Task.Run( () => originalImage.ToBitmap().ToGrayscale().ToBitmapImage() );
    imgPhotoConverted.Source = result;
}

正在使用此扩展 class

public static class BitmapExtensions
{
    public static Bitmap ToGrayscale( this Bitmap source, CancellationToken cancellationToken = default )
    {
        Bitmap output = new Bitmap( source.Width, source.Height );
        for ( int i = 0; i < source.Width; i++ )
        {
            for ( int x = 0; x < source.Height; x++ )
            {
                cancellationToken.ThrowIfCancellationRequested();
                var imageColor = source.GetPixel( i, x );
                int grayScale = (int)( ( imageColor.R * 0.21 ) + ( imageColor.G * 0.72 ) + ( imageColor.B * 0.07 ) );
                var newColor = System.Drawing.Color.FromArgb( imageColor.A, grayScale, grayScale, grayScale );
                output.SetPixel( i, x, newColor );
            }
        }
        return output;
    }

    public static Bitmap ToBitmap( this BitmapImage source )
    {
        using ( MemoryStream outStream = new MemoryStream() )
        {
            BitmapEncoder enc = new BmpBitmapEncoder();
            enc.Frames.Add( BitmapFrame.Create( source ) );
            enc.Save( outStream );
            System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap( outStream );

            return new Bitmap( bitmap );
        }
    }

    public static BitmapImage ToBitmapImage( this Bitmap source )
    {
        using ( var memory = new MemoryStream() )
        {
            source.Save( memory, ImageFormat.Png );
            memory.Position = 0;

            var bitmapImage = new BitmapImage();
            bitmapImage.BeginInit();
            bitmapImage.StreamSource = memory;
            bitmapImage.CacheOption = BitmapCacheOption.OnLoad;
            bitmapImage.EndInit();
            bitmapImage.Freeze();

            return bitmapImage;
        }
    }
}

Answer 3

我认为有一种更简单的方法可以做到这一点：在返回之前冻结 () 位图。

那么哪个线程访问它并不重要（当然实际的 UI 元素仍然只需要从 WPF 线程访问）

我通过如下修改解决了类似问题。

Task.Run(() =>
{
...
        var bmp = ToBitMapImage(outputImage);
        bmp.Freeze();
        return bmp;
}...

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