TreeDataTemplate does not generate children [11-preview5]

Describe the bug I am using TreeDataTemplate to show a generated tree in a TreeView. Starting in 11-preview5, only the root nodes are shown. Same behavior under nightly (11.0.999-cibuild0030252-beta).

Below is the most relevant XAML. I am using compiled bindings and there are no warnings in the VS Output window. The node Children collections are populated in the debugger. Collections are created once in the constructor and are never replaced. IIRC, there were no TreeViewItem node children in 11-preview5+ in DevTools, only for the root nodes.

<TreeView Items="{Binding Items}">
    <TreeView.DataTemplates>
        <TreeDataTemplate DataType="vm:BranchNodeViewModel" ItemsSource="{Binding Children}">
            <TextBlock Text="{Binding Children.Count, StringFormat=[BranchNode - Children: {0}]}" />
        </TreeDataTemplate>
        <TreeDataTemplate DataType="vm:LeafNodeViewModel" ItemsSource="{Binding Children}">
            <TextBlock Text="[LeafNode]" />
        </TreeDataTemplate>
    </TreeView.DataTemplates>
</TreeView>

To Reproduce

1. Clone this repro
2. Run the app to see the top Grid TreeView works under 11-preview4
3. Update to 11-preview5 (or nightly), fixup IControl -> Control in ViewLocator, run the app, and see it no longer generates children.

Expected behavior I expect that the nodes with children are expandable.

Screenshots The repro has a TreeDataTemplate generated hierarchy on the top pane and the bottom pane has one that defines a hierarchy manually via TreeViewItem.

11-preview4 correctly has expanders and children for the templated-generated pane:

11-preview5 (and nightly) do not:

Desktop (please complete the following information):

• OS: Windows 10
• Version: 11-preview4 (working), 11-preview5 (not working), 11.0.999-cibuild0030252-beta (not working)

[grokys]

Ok, I see what’s happening. If one removes the view locator in App.axaml in the repro then it starts working.

The cause is the following:

• PanelContainerGenerator prepares the item before adding it to the logical tree
• When ItemsControl tries to find the data template, it first tries to find it relative to the TreeViewItem
• If there is no ViewLocator in App.axaml then hic.FindDataTemplate(item) returns null, so it then tries to find it relative to the TreeView (the code after the ??)
• If there is a ViewLocator in App.axaml though, this data template is found and used, meaning that the code after ?? is never executed

I think we need to delay the search for the item template until the item is attached to the tree.

[timunie]

I found a workaround: https://github.com/stevemonaco/AvaloniaTreeDataTemplateRepro/pull/1

The reason seems to be that only ItemTemplate is respected since preview5. If this is a bug or intended, I cannot say.