正在线程池中创建Windows Phone 8上的图像缩略图

好吧，我想我也会在这里给出我自己的答案，因为它从一个不同的角度展示了事情。贾斯汀·安吉尔的答案是可以的，但有几个问题：

1. 当代码深入模型层并在后台线程上运行时，不可能引用Dispatcher
2. 我需要从该方法返回缩略图，然后在同一同步上下文中使用它。否则，我将不得不围绕这种创建缩略图的方法更改很多代码

考虑到这些要求，以下是我的解决方案：

private WriteableBitmap CreateThumbnail(Stream stream, int width, int height, SynchronizationContext uiThread)
        {
            // This hack comes from the problem that classes like BitmapImage, WritableBitmap, Image used here could 
            // only be created or accessed from the UI thread. And now this code called from the threadpool. To avoid
            // cross-thread access exceptions, I dispatch the code back to the UI thread, waiting for it to complete
            // using the Monitor and a lock object, and then return the value from the method. Quite hacky, but the only
            // way to make this work currently. It's quite stupid that MS didn't provide any classes to do image 
            // processing on the non-UI threads.
            WriteableBitmap result = null;
            var waitHandle = new object();
            lock (waitHandle)
            {
                uiThread.Post(_ => 
                {
                    lock (waitHandle)
                    {
                        var bi = new BitmapImage();
                        bi.SetSource(stream);
                        int w, h;
                        double ws = (double)width / bi.PixelWidth;
                        double hs = (double)height / bi.PixelHeight;
                        double scale = (ws > hs) ? ws : hs;
                        w = (int)(bi.PixelWidth * scale);
                        h = (int)(bi.PixelHeight * scale);
                        var im = new Image();
                        im.Stretch = Stretch.UniformToFill;
                        im.Source = bi;
                        result = new WriteableBitmap(width, height);
                        var tr = new CompositeTransform();
                        tr.CenterX = (ws > hs) ? 0 : (width - w) / 2;
                        tr.CenterY = (ws < hs) ? 0 : (height - h) / 2;
                        tr.ScaleX = scale;
                        tr.ScaleY = scale;
                        result.Render(im, tr);
                        result.Invalidate();
                        Monitor.Pulse(waitHandle);
                    }
                }, null);
                Monitor.Wait(waitHandle);
            }
            return result;
        }

当我还在UI线程中（在视图模型中）时，我会捕获UI线程的SynchronizationContext，并进一步传递它，然后我会使用闭包来捕获局部变量，这样它们就可以用于在UI线程上运行的回调。我还使用锁和监视器来同步这两个线程，并等待图像准备好。

如果有投票，我会接受我或贾斯汀·安吉尔的回答。：）

EDIT:在UI线程上（例如，在按钮单击处理程序中），您可以通过System.Threading.SynchronizationContext.Current获取Dispatcher的SynchronizationContext实例。像这样：

private async void CreateThumbnailButton_Clicked(object sender, EventArgs args)
    {
        SynchronizationContext uiThread = SynchronizationContext.Current;
        var result = await Task.Factory.StartNew<WriteableBitmap>(() =>
            {
                Stream img = GetOriginalImage();// get the original image through a long synchronous operation
                return CreateThumbnail(img, 163, 163, uiThread);
            });
        await SaveThumbnailAsync(result);
    }