feat: ItemTypeAttribute

[workgroupengineering]

Part of AvaloniaUI/AvaloniaVS#386

What does the pull request do?

Allow AvaloniaVS Intellisense to identify the item type of ItemsControl, ListBox, etc..

What is the current behavior?

AvaloniaVS Intellisense show all class derived from AvaloniaObject

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

ItemTypeAttribute indicates AvaloniaVS Intellisense what the base type of the items in the ListBoxItem example control is for ListBox.

ItemTypeAttribute derives from DesignerAttribute, to allow in the future in a PR to eliminate all the attributes derived from DesignerAttribute during the packaging phase and keep them only in the reference assemblies.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0040370-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

ItemsControl can have any types as items. ContentControl is just a default for virtualized containers.

[workgroupengineering]

ItemType does not add any constraints, it is only used to allow Intellisense to propose the types.

I use this because it is default container that can be used to display an item.

Avalonia/src/Avalonia.Controls/ItemsControl.cs

Lines 338 to 344 in 8ea685e

	/// <summary>
	/// Creates or a container that can be used to display an item.
	/// </summary>
	protected internal virtual Control CreateContainerForItemOverride(object? item, int index, object? recycleKey)
	{
	return new ContentPresenter();
	}

[maxkatz6]

In general, I am not sure about this PR.
Default controls could be guessed from the naming (ListBox -> ListBoxItem) or hardcoded. When this PR assumes that every custom control will apply this attribute to be efficient. Which is unlikely to be followed.

[workgroupengineering]

ItemTypeAttribute is marked Inherited true. So if you create the public class MyFashionListBox:ListBox, it will inherit the ItemTypeAttribute from the ListBox. Additionally we could create an analyzer/fixer or create a source generator that parses CreateContainerForItemOverride and generates ItemTypeAttribute.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048046-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048883-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0050224-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]