Trying to select a non-visible tree node, what's Selection.SelectedIndex?

[BhaaLseN]

I'm currently migrating a WPF application to Avalonia 11.x and got almost everything done. The one thing that's missing is the tree sync after selecting a search result in a flat list.

I basically got a TreeView with node view models bound to it, and a ListBox/TextBox next to it that allow me to search for nodes. Selecting a result in the listbox should select the associated tree node, and expand the ancestors to make it fully visible (all based on the view model behind the TreeViewItem).
In WPF, I had to ask each levels ItemsControl.ItemContainerGenerator if its Status is loaded already, and if not attach to its StatusChanged event to keep selecting once the expansion is complete and UI items are present. Inside this, I simply used ItemsControl.ItemContainerGenerator.ContainerFromIndex to grab the associated container, set IsExpanded and continue until I hit the desired item.
In Avalonia, no such thing exists; it even refers me back from ItemsControl.ItemContainerGenerator.ContainerFromIndex to simply ask the parent ItemsControl.ContainerFromIndex. There's no expansion/visuals/control status, and I can't detect if and when a level is present, so I'm often stuck when the desired node is deeper in the tree (because the containers don't exist yet).

I've tried a few variations of #13475, #11845 and #16485 but none of them really get me where I want to be.
(Don't really worry about my progress there, I might switch to a TreeDataGrid anyways, and do some other things to get this to work.)

Then I stumbled over the remark on TreeView.SelectedItem:

Avalonia/src/Avalonia.Controls/TreeView.cs

Lines 111 to 112 in 18fcfcc

	/// Note that setting this property only currently works if the item is expanded to be visible.
	/// To select non-expanded nodes use `Selection.SelectedIndex`.

And...well...what is even Selection.SelectedIndex? I know my indices per level, and I can keep track of the global index (in a flattened list), but I couldn't figure out what this meant. There is a Avalonia.Controls.Selection namespace and a AvaloniaEdit.Editing.Selection class, but none of those really help me. What seemed more promising was Avalonia.Controls.Selection.ISelectionModel (which has a SelectedIndex property), but no connection to the TreeView.

To get to the point: Is that documentation comment outdated? Should it point us elsewhere, or is it obsolete and doesn't apply anymore?

With the information i currently have on this, I don't know how to proceed, and the only useful results I can find on Google is the source code comment and the documentation; so it feels like something that needs work (one way or another).

[msneijders]

I basically got a TreeView with node view models bound to it

You could give your node viewmodel the properties IsSelected and IsExpanded, using a INotifyPropertyChanged principle. Then setting the correct selected and expanded state using your viewmodel (and expand all parent nodes of the selected one up until the root node). And attach your view model like this to bind your node to the TreeViewItem of the TreeView:

<TreeView
    ItemsSource="{Binding Items}">
    <TreeView.ItemTemplate>
        <TreeDataTemplate ItemsSource="{Binding Children}">
            <TextBlock Text="{Binding Header}" />
        </TreeDataTemplate>
    </TreeView.ItemTemplate>
    <TreeView.ItemContainerTheme>
        <ControlTheme
            x:DataType="viewModels:Node"
            BasedOn="{StaticResource {x:Type TreeViewItem}}"
            TargetType="TreeViewItem">
            <Setter Property="IsSelected" Value="{Binding IsSelected}" />
            <Setter Property="IsExpanded" Value="{Binding IsExpanded}" />
        </ControlTheme>
    </TreeView.ItemContainerTheme>
</TreeView>

[BhaaLseN]

Yeah, I got this part figured out, don't worry about it. But thanks anyways.

This discussions is about potentially missing documentation, an incorrect comment, or some other thing that needs to be addressed (but I didn't want to create an issue for it straight away, I wanted to see if this is actually something to look at or not).

[msneijders]

ok -- about that remark about Selection.SelectedIndex. I think you are right and it is misplaced there, as I suspect it refers to Selection of SelectingItemsControl, but TreeView is not a SelectingItemsControl.

[timunie]

PR to fix documentation are welcome 🤗