DataGrid: expose row index

[rabbitism]

What does the pull request do?

Make DataGridRow.Index public get-able.

What is the current behavior?

It's hard to show row number.

What is the updated/expected behavior with this PR?

User can use below binding to show row index.

<DataGridTextColumn Header=" " Binding="{Binding $parent[DataGridRow].Index}" Width="60" x:DataType="local:Country" />

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

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

I think no.

Obsoletions / Deprecations

GetIndex method is obsoleted, should be replaced by Index property

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048779-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[rabbitism]

@cla-avalonia agree

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048787-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[stevemonaco]

What are the performance implications if you insert or remove the first row of a 100k row DataGrid? Does this cause 100k PropertyChanged events for Index? Or only for the rows that are presently materialized via virtualization?

[maxkatz6]

It should affect only rows that are currently materialized.