Generate properties for templates using TemplatePartAttribute

There is a new attribute ported from WPF. It will be useful to have optional code generation for “parts” of templates. As an example, user code:

[TemplatePart("PART_TextPresenter", typeof(TextPresenter))]
public partial class TextBox
{
    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
    {
        base.OnAttachedToVisualTree(e);
            
        if (IsFocused)
        {
            TextPresenter?.ShowCaret(); // TextPresenter is generated
        }
    }
}

Additional generated code, adds new property and sets value to it:

#nullable enable
public partial class TextBox
{
    private TextPresenter? TextPresenter { get; set; }

    static TextBox()
    {
         TemplateAppliedEvent.AddClassHandler<TextBox>((control, args) =>
         {
             control.TextPresenter = args.NameScope.Get<TextPresenter>("PART_TextPresenter");
         });
    }
}

Problems:

1. Only one static ctor can be created for class, if we want to handle TemplateAppliedEvent to fill these properties.
2. Only one override of OnTemplateApplied method is possible, if we want to use it to fill these properties.

Which means, we most likely will need to modify Avalonia to add yet another way to read namescope after template was applied.

[robloo]

There was a lot of further discussion in https://github.com/AvaloniaUI/Avalonia/issues/9093 related to how to define the string constants and then enforce them statically in XAML. I think that discussion isn’t fully closed yet and it may influence what is done here. Feel free to jump into that conversation as well. That said, the TemplatePart attribute isn’t going to change so however the string name gets into the attribute is irrelevant for you.

Concerning the code generation:

1. I would consider using .Find instead of .Get to fail gracefully if control authors don’t do what they need to do. However, that would require nullable types so maybe isn’t worth it. We definitely need good exception handling and build outputs here to say which template part isn’t found though.
2. Technically instead of TextPresenter the control should be named just Text. Not all template parts currently have good names…

Edit:

1. The implementation you have seems ok to me. However, it also isn’t using any of the partial class ideas. Using a partial class shared with an internal implementation in Control would allow InitializeTemplateParts to be automatically called. Otherwise we are all going to have to remember to call it inside an OnApplyTemplate override-- which I don’t mind since it’s clearly opt-in.

[worldbeater]

Regarding the implementation details of the new generator, I guess it is worth introducing a new class similar to AvaloniaNameGenerator named e.g. TemplatePartGenerator, then adding a new option to GeneratorOptions that would control if the new generator is enabled, and then invoking the generator in the composition root.

The code that scans the assembly and finds all classes annotated with the TemplatePart attribute can be found here https://github.com/reactivemarbles/ObservableEvents/blob/main/src/ReactiveMarbles.ObservableEvents.SourceGenerator/SyntaxReceiver.cs#L17 Most likely we’ll need to implement ISyntaxReceiver that receives all relevant classes with TemplatePart attributes.