Removed all nullability overrides from TimePickerPresenter and DatePickerPresenter

[TomEdwardsEnscape]

Some mis-use of nullability checking was noted in #19210, but the problem goes much deeper. This PR moves template part fields into a struct, which is used by a nullable field on the presenter. This two-layer approach allows us to differentiate between optional and required template parts at compile time, while still allowing the template to be applied after the constructor has executed.

The only remaining uses of the ! character in the two affected files are boolean inversion operations.

What is the updated/expected behavior with this PR?

There are no changes to behaviour, but:

• Fixed obscure cases where NullReferenceException could be thrown if a template hasn't been applied yet, or where it provides only some optional items.
• Replaced repeated string literals with nameof.
• Relaxed template part requirements: RepeatButton to Button, Rectancle to Control.

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Addresses one of the complaints made in #19210.

[maxkatz6]

Can you revert back to blocked namespaces? This change is something we want, but it needs to be done at least in a separated commit that can be added to .git-blame-ignore-revs.

[maxkatz6]

There is a need to rewrite styling of these two controls. See #17515

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057692-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[TomEdwardsEnscape]

I removed the namespace change, but do you know that you can do git blame -w to automatically ignore this sort of thing?

There is a need to rewrite styling of these two controls. See #17515

I've struggled with these controls for this exact reason. Any improvements ought not to break existing themes, so these changes are still worthwhile.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057694-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[robloo]

You are doing something new here: creating a struct for the template parts. Naming the controls exactly the same as the XAML template part name and then using nameof() has been done before -- it's the struct that's new.

I think this should be considered as part of the overall discussions below. All ideas essentially converged to code generators to solve this problem.

• Generate properties for templates using TemplatePartAttribute #11934
• Further TemplatePart Cleanup in Code-Behind and XAML #9093 (comment)

[MrJul]

Thank you! As mentioned by Max, this control really needs a rewrite, there are really too many template parts. This doesn't prevent fixing the nullability issues in the meantime :)

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058106-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM, thank you!