win2d - 如何从画布控件（win2d）获取绘图会话

Question

在win8应用程序中，如何从canvascontrol（win2d）中获取绘图会话，或者，如何通过canvasControl_Draw函数在canvascontrol上绘制图像。

Answer 1

You cannot - this is a key part of policy provided by CanvasControl.

This ensures that:

• the drawingsession is created and closed at the appropriate time

• drawing isn't attempted before resources have been created

• handling device lost errors are handled

If you want to force a redraw you can use CanvasControl.Invalidate().

Alternatively, you may find that you want to render to an offscreen CanvasRenderTarget (that you can call CreateDrawingSession). Then use DrawImage in your CanvasControl_Draw to draw the render target to the control.

Answer 2

如果您的目标只是渲染到图像，您可以在不使用 CanvasControl.Draw 方法的情况下执行此操作。这是我的一个应用程序中的一些代码，它们呈现为图像并将其保存到文件中（PageRenderer 是我的渲染类）：

public async Task GenerateThumbnailAsync(IRandomAccessStream stream, int width, int height, CanvasBitmapFileFormat imageType)
{
    CanvasDevice device = CanvasDevice.GetSharedDevice();
    PageRenderer renderer = new PageRenderer(device);
    using (CanvasRenderTarget offscreen = new CanvasRenderTarget(device, width, height, 96))
    {
        using (CanvasDrawingSession ds = offscreen.CreateDrawingSession())
        {
            ds.Clear(Colors.Black);
            renderer.DrawPage(ds);
        }
        await offscreen.SaveAsync(stream, imageType);
    }
}