Remove SpriteVisual from visual tree in attached shadows before disposing of composition resources

[Ryken100]

Fixes #816

This PR ensures that the SpriteVisual of each attached shadow is removed from the visual tree before disposing of the SpriteVisual or any other composition resources.

PR Type

What kind of change does this PR introduce?

Bugfix

What is the current behavior?

The attached shadow logic disposes of composition resources before the SpriteVisual is removed from the visual tree.

What is the new behavior?

AttachedShadowBase.OnElementContextUninitialized() and AttachedShadowElementContext.Uninitialize() were updated to remove the SpriteVisual from the visual tree before disposing of it and linked composition resources. This should prevent XAML from trying to access the SpriteVisual or its resources after they have been disposed.

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs
• New component
  • Documentation has been added
  • Sample in sample app has been added
  • Analyzers are passing for documentation and samples
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (if applicable)
• Header has been added to all new source files
• Contains NO breaking changes

Other information

N/A

[Arlodotexe]

We should write a test to highlight the broken/fixed behavior.

[Ryken100]

We should write a test to highlight the broken/fixed behavior.

Unfortunately I'm unable to repro the issue, so I'm not sure how to recreate broken/fixed states in a test.

For info, in our internal bug report someone from composition informed us that XAML is trying to access a hand-in visual (a visual added using ElementCompositionPreview.SetElementChildVisual) that has already been disposed, and showed the issue originated from a visual tree that uses an AttachedCardShadow's SpriteVisual as its hand-in visual.

The only spot I saw where we could be creating this issue is that we dispose of the SpriteVisual and other composition resources before removing the visual from the tree using ElementCompositionPreview.SetElementChildVisual(_, null).

[Arlodotexe]

Thanks for the added details @Ryken100! I'll see if I can repro in a test using the info you've provided, we don't want to end up using prod to debug these things the long way around.

[zubinqayam]

Ok

[zubinqayam]

Ok