How to use TemplateBinding within ContentPresenter

[b4ndersnatch]

I'm running into an issue with a templated control i've made, using another control that has a content presenter. It seems I'm using the content presenter in the wrong way and data context is being broken silently. I have a couple combo boxes that i want populated from data, but no matter what i do they always appear empty.

first off, here's the issue I'm seeing with empty combo boxes:

Here's the (simplified) control i'm using as a base, that has the content presenter:

<ControlTheme x:Key="{x:Type controls:ExpanderComboBase}" TargetType="controls:ExpanderComboBase">
  <Setter Property="Template">
      <ControlTemplate>
          <Expander>
              <Expander.Header>
                  <TextBlock Text="{TemplateBinding PersonName}"/>
              </Expander.Header>
              <StackPanel>
                  <TextBox Text="My Siblings"/>
                  <ContentPresenter Content="{TemplateBinding Siblings}"
                                    ContentTemplate="{TemplateBinding SiblingsTemplate}"
                                    Name="SiblingsPresenter"/>
              </StackPanel>
          </Expander>
      </ControlTemplate>
  </Setter>
</ControlTheme>

Here's my custom control that uses the other control (and the contentpresenter), where i try to populate the combo boxes:

<ControlTheme x:Key="{x:Type controls:ExpanderComboVariation}" TargetType="controls:ExpanderComboVariation">
    <Setter Property="Template">
            <ControlTemplate>
                <controls:ExpanderComboBase PersonName="{TemplateBinding MyPersonName}">
                    <controls:ExpanderComboBase.Siblings>
                            <StackPanel>
                                <ComboBox ItemsSource="{TemplateBinding BrothersItemSource}"/>
                                <ComboBox ItemsSource="{TemplateBinding SistersItemSource}"/>
                            </StackPanel>
                    </controls:ExpanderComboBase.Siblings>
                </controls:ExpanderComboBase>
            </ControlTemplate>
    </Setter>
</ControlTheme>

Here's where I use my custom control in the window:

<StackPanel>
        <ItemsControl ItemsSource="{Binding People}">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <controls:ExpanderComboVariation MyPersonName="{Binding Name}"
                                                     BrothersItemSource="{Binding Brothers}"
                                                     SistersItemSource="{Binding Sisters}"/>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
</StackPanel>

And here's the window viewmodel:

public partial class MainWindowViewModel : ViewModelBase
{
    public ObservableCollection<Person> People {get;}
    
    public MainWindowViewModel()
    {
        People = new ObservableCollection<Person>();
        People.Add(new Person("Jack", new List<string> {"Jake", "Josh"}, new List<string> {"Jill", "Jennifer"} ));
        People.Add(new Person("Bob", new List<string> {"Blake", "Brian"}, new List<string> {"Beverly", "Brynn"} ));
    }
}

public partial class Person : ObservableObject
{
    [ObservableProperty]
    private string _name;
    
    public ObservableCollection<string> Brothers { get; } = new ObservableCollection<string>();
    public ObservableCollection<string> Sisters { get; } = new ObservableCollection<string>();

    public Person( string name, List<string> brothers, List<string> sisters )
    {
        Name = name;
        foreach ( string bro in brothers )
            Brothers.Add( bro );
        
        foreach ( string sis in sisters )
            Sisters.Add( sis );
    }
}

[b4ndersnatch]

I'm pretty sure i'm messing up where i define "siblings" and the combo boxes in my control theme, but i'm not sure how to fix it...

<ControlTheme x:Key="{x:Type controls:ExpanderComboVariation}" TargetType="controls:ExpanderComboVariation">
    <Setter Property="Template">
            <ControlTemplate>
                <controls:ExpanderComboBase PersonName="{TemplateBinding MyPersonName}">
                    <controls:ExpanderComboBase.Siblings>
                            <StackPanel>
                                <ComboBox ItemsSource="{TemplateBinding BrothersItemSource}"/>
                                <ComboBox ItemsSource="{TemplateBinding SistersItemSource}"/>
                            </StackPanel>
                    </controls:ExpanderComboBase.Siblings>
                </controls:ExpanderComboBase>
            </ControlTemplate>
    </Setter>
</ControlTheme>

[b4ndersnatch]

Is this issue related maybe? #13230

[b4ndersnatch]

Fixed it. I find avalonia's datacontext/template syntax really confusing, but stepping through the contentpresenter .cs file made it clear i was misunderstanding the template/content relationship. I needed to make the template be the actual controls, and the content be the 'Person' Data.

control theme fix:

<ControlTheme x:Key="{x:Type controls:ExpanderComboVariation}" TargetType="controls:ExpanderComboVariation">
        <Setter Property="Template">
            <ControlTemplate>
                <controls:ExpanderComboBase Name="PART_BaseControl"
                                            PersonName="{TemplateBinding MyPersonName}">
                    <controls:ExpanderComboBase.SiblingsTemplate>
                        <DataTemplate x:DataType="vm:Person">
                            <StackPanel>
                                <ComboBox ItemsSource="{Binding Brothers}"/>
                                <ComboBox ItemsSource="{Binding Sisters}"/>
                            </StackPanel>
                        </DataTemplate>
                    </controls:ExpanderComboBase.SiblingsTemplate>
                </controls:ExpanderComboBase>
            </ControlTemplate>
        </Setter>
    </ControlTheme>

control theme code-behind fix (I set the Datacontext explicitly because i can't figure out the syntax to do it in the xaml)

[TemplatePart("BaseControl", typeof(ExpanderComboBase))]
public class ExpanderComboVariation : TemplatedControl
{
    private ExpanderComboBase? _baseControl;
    
    public static readonly StyledProperty<string?> MyPersonNameProperty =
        AvaloniaProperty.Register<ItemsControl, string?>(nameof(MyPersonName));
    
    public string? MyPersonName
    {
        get => GetValue(MyPersonNameProperty);
        set => SetValue(MyPersonNameProperty, value);
    }

    protected override void OnApplyTemplate(TemplateAppliedEventArgs args)
    {
        base.OnApplyTemplate(args);
        
        _baseControl = args.NameScope.Find<ExpanderComboBase>("PART_BaseControl");
        Debug.Assert(_baseControl != null);
        
        _baseControl.Siblings = DataContext!;
    }
}