Warning AVLN2208: Item container in the data template

[maxkatz6]

What does the pull request do?

It's a fairly common mistake to put item container inside of data template.
Typical example can be found even in this repository:

Avalonia/src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml

Lines 74 to 80 in 151d97a

	<ListBox Name="EventsList" ItemsSource="{Binding RecordedEvents}"
	SelectedItem="{Binding SelectedEvent, Mode=TwoWay}">
	<ListBox.ItemTemplate>
	<DataTemplate>
	<ListBoxItem Classes.handled="{Binding IsHandled}">
	<Grid ColumnDefinitions="Auto,Auto,Auto,*,Auto">

The problem with this code is that DataTemplate does not affect how container itself is defined. Instead, it will create a dumb ListBoxItem inside of the proper ListBoxItem. Making it worse, styles are active for both ListBoxItems, but only outer one can be actually selected.

Example of how this code looks in the elements tree:

This problem is not limited by ListBoxItem, but by any items control. I often see this mistake with MenuItem and TabControl.

What is the updated/expected behavior with this PR?

New XAML analyzer is added to XamlX compiler.
Code: "AVLN2208" (can be configured as an error via project settings or editorconfig, or disabled).
Message example (actual text depends on the context):

Unexpected 'TabItem' inside of 'TabControl.DataTemplates'. 'TabControl.DataTemplates' defines template of the container content, not the container itself.

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

By default, this is a warning. No user code will have these errors, unless these projects have all warnings enabled as errors - but these projects typically expect new errors to be found early.

Fixed issues

Contributes towards #13707

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054805-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[robloo]

I seem to recall UWP/WinUI has code to detect when a container is provided as the root of the data template (not sure if WPF does). In that case, no extra container is generated.

There are real-world use cases where is is much easier having direct access to the container by including it in your data template XAML as in your example above.

Bottom line, this seems to be going the wrong direction. We should actually detect and support this case... not throw a warning.

[maxkatz6]

Bottom line, this seems to be going the wrong direction. We should actually detect and support this case... not throw a warning.

It's not about a direction here, but about current state today.
Today containers in data template are not integrated into items control directly.
And this warning is necessary to make finding relevant bugs easier.

If one day we support container inside data templates, this warning can be removed. But nobody requested this feature yet as far as I can remember.

[robloo]

OK, fair enough.

But nobody requested this feature yet as far as I can remember.

I noticed this when I ported code over a while ago and worked around it. During a port it's usually better to just find a work-around and move to the next thing. Still, I should have brought it up before now. Consider this the first unofficial request.

[MrJul]

Should we add an attribute for this at some point? It could be useful. (Not as part of this PR of course.)

[maxkatz6]

I was thinking about it.
And there was a PR for that #13104
The problem is that such attribute won't be followed by third party controls, and it gives a little of benefit otherwise, for built-in controls.

Let's add it to next api review, I suppose. If we will find a better idea.

[MrJul]

LGTM!