Pause plugins when covered by other window

src/DesktopMagicPluginAPI/SkiaAsyncPlugin.cs

@@ -8,14 +8,17 @@ namespace DesktopMagic.Api;
 
 /// <summary>
 /// Provides an abstract base class for creating asynchronous plugins using SkiaSharp rendering.
-/// Plugins override <see cref="MainAsync(SKCanvas, CancellationToken)"/> instead of <see cref="Plugin.Main()"/>.
+/// Plugins override <see cref="RenderAsync(CancellationToken)"/> and return an <see cref="SKImage"/>
+/// of their own size, just like <see cref="AsyncPlugin.MainAsync(CancellationToken)"/> returns a
+/// <see cref="Bitmap"/>. The returned image is scaled to fit the plugin window while preserving
+/// its aspect ratio.
 /// All existing <see cref="Plugin"/>, <see cref="AsyncPlugin"/>, and <see cref="SkiaPlugin"/> code is unaffected.
 /// </summary>
 public abstract class SkiaAsyncPlugin : AsyncPlugin
 {
     /// <summary>
     /// This method is sealed and returns null for async Skia plugins.
-    /// Override <see cref="MainAsync(SKCanvas, CancellationToken)"/> instead.
+    /// Override <see cref="RenderAsync(CancellationToken)"/> instead.
     /// </summary>
     public sealed override Task<Bitmap?> MainAsync(CancellationToken cancellationToken)
     {
@@ -24,8 +27,9 @@ public abstract class SkiaAsyncPlugin : AsyncPlugin
 
     /// <summary>
     /// Called every <see cref="Plugin.UpdateInterval"/> milliseconds to render the plugin asynchronously.
+    /// Return <see langword="null"/> to keep the previously rendered image.
     /// </summary>
-    /// <param name="canvas">The SkiaSharp canvas to draw on.</param>
     /// <param name="cancellationToken">Token signaled when the host requests cancellation.</param>
-    public abstract Task MainAsync(SKCanvas canvas, CancellationToken cancellationToken);
+    /// <returns>The rendered image, or <see langword="null"/> to keep the previous frame.</returns>
+    public abstract Task<SKImage?> RenderAsync(CancellationToken cancellationToken);
 }