SKAutoCanvasRestore avoid crash at Dispose when canvas was already disposed

[taublast]

Description of Change

Have catched this few times myself: might be my bad coding style, but nevertheless SKAutoCanvasRestore should not crash app if the canvas inside was already disposed for any reason whatever. So checking to avoid that inside SKAutoCanvasRestore Dispose method.

Bugs Fixed

SKAutoCanvasRestore could crash if GC-ed/disposed after the containing canvas native Handler was already released while canvas field still not NULL and Restore() would be called, then crash app accessing Handle 0.

EDIT: also added safety checks to mappers (second commit), after was running n Debug and catched landing into mapper code with a NULL handler parameter when navigating between pages with accelerated canvases. Would guess there want be any harm for adding those too to solve occasional very rare and random crashes.

• Fixes

No known issues identified, still could be related to some unknown random crashes.

No Api changes, a small code change, please see 1 file change below.

Required skia PR

None.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard
• Updated documentation

[taublast]

omg. might be the fix for the navigating away from page with skglview crash.

EDIT:

Yeah! It doesn't crash when i just comment this out! So when we merge i could uncomment :)

  //using (new SKAutoCanvasRestore(_retainedSurface.Canvas, true))
  {
      OnPaintSurface(new(_retainedSurface, _renderTarget, SurfaceOrigin, ColorType));
  }

Re-EDIT:

Ha just realized i can just use a custom-fixed SKAutoCanvasRestore meanwhile..

[mattleibow]

Thanks for the continued work!

[mattleibow]

/azp run

[azure-pipelines]

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[mattleibow]

/azp run

[azure-pipelines]

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[mattleibow]

/azp run

[azure-pipelines]

Azure Pipelines could not run because the pipeline triggers exclude this branch/path.

[Copilot]

Pull Request Overview

This PR adds safety checks to prevent null reference and disposed-object crashes in several canvas and handler classes.

• Guard Invalidate calls against null dispatchers in SKXamlCanvas
• Add null-check guards in SKGLViewHandler and SKCanvasViewHandler mapping and invalidate methods
• Update SKCanvas.Restore to verify the native handle before restoring and always clear the canvas reference

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
source/SkiaSharp.Views/SkiaSharp.Views.WinUI/SKXamlCanvas.cs	Use null-conditional calls for DispatcherQueue/Dispatcher
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKGLView/SKGLViewHandler.iOS.cs	Add null guard for handler.PlatformView in invalidate and mappers
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKGLView/SKGLViewHandler.Windows.cs	Add null guards before calling Invalidate and mapping methods
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKGLView/SKGLViewHandler.MacCatalyst.cs	Add null guards for invalidate and mapping methods
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKGLView/SKGLViewHandler.Android.cs	Add null guards for invalidate and mapping methods
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKCanvasView/SKCanvasViewHandler.Windows.cs	Add null guard around Invalidate and map methods
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKCanvasView/SKCanvasViewHandler.Tizen.cs	Add null guards for invalidate and mapping calls
source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Handlers/SKCanvasView/SKCanvasViewHandler.Apple.cs	Add null guards for invalidate and mapping calls
binding/SkiaSharp/SKCanvas.cs	Check Handle before RestoreToCount and always clear canvas

Comments suppressed due to low confidence (2)

binding/SkiaSharp/SKCanvas.cs:1082

• Consider adding a unit test to verify that calling Restore() does not throw when canvas is null or canvas.Handle is IntPtr.Zero, ensuring this safety branch is covered.

			if (canvas != null && canvas.Handle != IntPtr.Zero) {

binding/SkiaSharp/SKCanvas.cs:1079

• The XML doc for Restore() should be updated to note that it now safely handles cases where the underlying canvas has already been disposed or its handle is invalid.

		public void Restore ()

[Copilot]

[nitpick] This null-check pattern is repeated across multiple handler methods; consider extracting a shared guard or helper method to reduce duplication and improve maintainability.

Suggested change

	public static void OnInvalidateSurface(SKGLViewHandler handler, ISKGLView view, object? args)
	{
	if (handler?.PlatformView == null)
	private static bool IsHandlerValid(SKGLViewHandler? handler) =>
	handler?.PlatformView != null;
	public static void OnInvalidateSurface(SKGLViewHandler handler, ISKGLView view, object? args)
	{
	if (!IsHandlerValid(handler))