Compiled bindings and ContentControl

[pjt33]

I'm trying to port an application from WPF, and I've run into a situation where I'm not sure whether it's possible to provide the type hints required for compiled bindings. The original control is a panel which is designed work with view-models which fetch data asynchronously. It displays progress during the fetch, and either the data or an error message and a "Reload" button after it completes or fails.

Specific view-models implement the following interface:

public interface IAsyncDataSource : INotifyPropertyChanged
{
    object Result { get; }
    bool Loading { get; }
    bool Loaded { get; }
    bool Errored { get; }
    int? Progress { get; }
    string ErrorMessage { get; }
}

Then the panel is

public class AsyncPanel : ContentControl
{
    static AsyncPanel()
    {
        ThemeProperty.OverrideDefaultValue<AsyncPanel>(new AsyncPanelDefaultTheme());
    }

    public static readonly StyledProperty<object> LoadingContentProperty = AvaloniaProperty.Register<AsyncPanel, object>(nameof(LoadingContent));
    public static readonly StyledProperty<object> ErrorContentProperty = AvaloniaProperty.Register<AsyncPanel, object>(nameof(ErrorContent));
    public static readonly StyledProperty<IAsyncDataSource> DataSourceProperty = AvaloniaProperty.Register<AsyncPanel, IAsyncDataSource>(nameof(DataSource));
    public static readonly StyledProperty<ICommand> ReloadCmdProperty = AvaloniaProperty.Register<AsyncPanel, ICommand>(nameof(ReloadCmd));

    public object LoadingContent
    {
        get => GetValue(LoadingContentProperty);
        set => SetValue(LoadingContentProperty, value);
    }

    public object ErrorContent
    {
        get => GetValue(ErrorContentProperty);
        set => SetValue(ErrorContentProperty, value);
    }

    public IAsyncDataSource DataSource
    {
        get => GetValue(DataSourceProperty);
        set => SetValue(DataSourceProperty, value);
    }

    public ICommand ReloadCmd
    {
        get => GetValue(ReloadCmdProperty);
        set => SetValue(ReloadCmdProperty, value);
    }
}

with default theme

<ControlTheme xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:async="using:MyNamespace"
              x:Class="MyNamespace.AsyncPanelDefaultTheme"
              TargetType="async:AsyncPanel">
    <Setter Property="Template">
        <ControlTemplate>
            <Grid>
                <ContentPresenter
                    IsVisible="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataSource.Loading}"
                    Content="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=LoadingContent}"
                    DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataSource, Mode=OneWay}" />

                <ContentPresenter
                    IsVisible="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataSource.Loaded}"
                    Content="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Content}"
                    DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataSource}" />

                <ContentPresenter
                    IsVisible="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataSource.Errored}"
                    Content="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=ErrorContent}"
                    DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataSource, Mode=OneWay}" />
            </Grid>
        </ControlTemplate>
    </Setter>
</ControlTheme>

public class AsyncPanelDefaultTheme : ControlTheme { }

However, when I try to use it:

<async:AsyncPanel DataSource="{Binding PoolsFetcher}" ReloadCmd="{Binding ReloadCmd}" x:Name="domainDetails">
	<DockPanel DataContext="{Binding Result}">
		...
	</DockPanel>
</async:AsyncPanel>

the binding compiler thinks that the DataContext of the DockPanel is relative to the DataContext of the AsyncPanel: it doesn't track through to the theme (and arguably shouldn't, because themes can be replaced) to see that the ContentPresenter which displays that DockPanel is setting its DataContext to DataSource.

I can see some workarounds: e.g. remove the explicit DataContext assignments in the theme and bind to PoolsFetcher.Result. Am I correct in thinking that there's no simple way to tell the compiler "The data type to which the content will be bound is IAsyncDataSource?