Add custom alpha mask support to DropShadowPanel

[arcadiogarcia]

Fixes

#3693

PR Type

What kind of change does this PR introduce?

Feature

What is the current behavior?

DropShadowPanel does not allow user controls to specify a custom alpha mask

What is the new behavior?

User controls can choose to implement IAlphaMaskProvider and their alpha mask will be picked up by the DropShadowPanel

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

[ghost]

Thanks arcadiogarcia for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[michael-hawker]

Thanks @arcadiogarcia! We're working on some new Drop Shadow stuff too (outside DropShadowPanel), hopefully will have a PR for that open soon. So we should coordinate and see if this neat interface trick can work there too. Should know more by next week I think?

I'll convert this to a draft for now until we sync on coordination.

[michael-hawker]

Can we move this to the Microsoft.Toolkit.Uwp.UI library? (Helpers folder I think would be fine.)

If no namespace is provided where does it end up, the root of the assembly's one? (It should just live in the root UI namespace.)

[michael-hawker]

Think there's no namespace the comment can't be resolved properly?

D:\a\1\s\Microsoft.Toolkit.Uwp.UI.Controls.Core\DropShadowPanel\IAlphaMaskProvider.cs(8,108): error CS1574: XML comment has cref attribute 'DropShadowPanel' that could not be resolved [D:\a\1\s\Microsoft.Toolkit.Uwp.UI.Controls.Core\Microsoft.Toolkit.Uwp.UI.Controls.Core.csproj]

Once moved though we won't be able to reference the control directly anyway, just in text.

[michael-hawker]

@arcadiogarcia mind making the quick file move and addressing the comments and we can get this in? Thanks!

[michael-hawker]

The physical file needs to move within the Microsoft.Toolkit.Uwp.UI assembly as well.

[arcadiogarcia]

Ah right, I'll move it there! Any suggestion on where to place it? None of the existing folders seems relevant, is it ok if I create one called DropShadowPanel (following the same pattern as AdvancedCollectionView)?

[michael-hawker]

I think just putting in the Helpers folder for now would be alright. It'll be in the root namespace still, so folder structure doesn't really matter, we can always move it later if we want without breaking anyone. Thanks!

[ghost]

This PR has been marked as "needs attention 👋" and awaiting a response from the team.