Made TabControl observe changes which affect SelectedContent/Template, plus various analyzer warning fixes

[TomEdwardsEnscape]

While fixing analyzer warnings, I noticed that TabControl and TabItems only synchronise content and content templates when the selected item changes. I switched them to bindings/observables, so that changes are applied immediately, e.g. in response to a style trigger.

I also noticed that TabControl.ContentTemplate was either not used at all (for a user-provided TabItem) or always used (for a generated TabItem). WPF evaluates both every time, and TabItem.ContentTemplate wins if not null. This PR introduces the same precedence rules.

The PR also contains the set of fixes for / suppressions of analyzer warnings that I originally set out to make, plus an update to Avalonia.Desktop.slnf which corrects the paths to some projects.

TabControl specifics

TabItem.TabStripPlacement is now a DirectProperty. I applied the same pattern that I used in ScrollBar to subscribe to TabControl.TabStripPlacement and forward the value. The value also is now nullable, so won't incorrectly return Dock.Left when there isn't a TabControl around.

TabControl.SelectedContent and TabControl.SelectedContentTemplate are now direct and read-only. These previously had analyzer suppression attributes saying that they were "supposed to be a styled readonly property", but this doesn't make sense, as they are set with local priority by TabControl and thus can never be styled.

Additionally, both of those properties are now set by observable subscribers. This means that changes to TabControl.ContentTemplate, TabItem.ContentTemplate, and TabItem.Content are now reflected immediately, instead of when the selected tab next changes.

TabControl.UpdateHeader

I didn't change this method despite the warnings it is emitting. It's a weird feature and I don't know what to do with it. Is it really desirable to sometimes automatically set the header when the content changes?

I think we should delete this code, and let users create their own attached behaviour if they actually want the functionality.

Breaking changes

• Some properties have switched from styled to direct.
• TabControl.ContentTemplate is now used if TabItem.ContentTemplate is null. This matches WPF.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0034716-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[Gillibald]

Avalonia/src/Avalonia.Controls/TabItem.cs

Line 85 in 91ff652

if (Header == null)

this method doesn't look symmetrical. In case Header is null there is a check for IHeader for non null Header there isn't.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0034914-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[Mrxx99]

Does this mean TabStripPlacement can't be set in a style? And why the limitation to DirectProperty?

[TomEdwardsEnscape]

TabControl.TabStripPlacement can be set in a style. TabItem.TabStripPlacement just forwards the value of the controlling TabControl, for convenience when writing TabItem templates.

[robloo]

Just ran across TabControl not working with a custom ContentTemplate. Hopefully this makes it in for RC otherwise TabControl doesn't work with MVVM and dynamic tab items set with ItemsSource.

[grokys]

@Gillibald ping.