Attached properties, general extensibility and the new handlers and property mappers

[GalaxiaGuy]

I've watched most of the early MAUI videos and I've been playing around with the main-handler branch a bit.

How will attached properties fit into the picture? I notice for example there is some StackLayout work, but no Grid work.

The reason I ask is, I use quite a few Effects to handle platform specific behavior. Most of the time I consume them via an attached property that ultimately ends up setting a single property on a native control. That whole process is a lot of boiler plate for basically setting one property in one place (for each platform).

The property mapper along with multi-targetting looks like it should make things easier. But... I could only see how to make it work with new controls.

For example, consider the Xamarin Community Toolkit RemoveBorderEffect: https://github.com/xamarin/XamarinCommunityToolkit/tree/main/src/CommunityToolkit/Xamarin.CommunityToolkit/Effects/RemoveBorder.

It would be easy to create something like the following (assuming a TextInputHandler already existed):

public interface IBorderlessTextInput : ITextInput
{
    bool HasBorder { get; set; }
}

public class BorderlessTextInputHandler : TextInputHandler<IBorderlessTextInput, NativeView>
{
    public static PropertyMapper<IBorderlessTextInput, BorderlessTextInputHandler> BorderlessTexInputMapper = new PropertyMapper<IBorderlessTextInput, BorderlessTextInputHandler>(TextInputHandler.ViewMapper)
    {
        [nameof(IBorderlessTextInput.HasBorder)] = MapHasBorder,
    };

    public static void MapHasBorder(BorderlessTextInputHandler handler, IBorderlessTextInput input)
    {
#if MONOANDROID
        // Pretend there is an Android implementation here
#elif __IOS__
        handler.TypedNativeView.SetBorderStyle(input.HasBorder ? UITextBorderStyle.UITextBorderStyleRoundedRect) : UITextBorderStyle.None;
#endif
    }
}

But that wouldn't be composable with others. Also, I might have a need to add a property somewhere up the inheritance chain, like View.

Has the MAUI team considered this kind of thing at all? Do you hopefully have some secret plan that isn't apparent? Or is the perhaps some way of doing it I just haven't considered yet?

[GalaxiaGuy]

So, after watching a video by @jsuarezruiz (https://www.youtube.com/watch?v=Hfhy_zrVPbI&t=1302s), I think I've answered half of it.

The borderless effect could be added after the fact as:

// Assume an attached property like the following exists:
public static class BorderlessTextInput
{
    public static readonly BindableProperty HasBorderProperty = BindableProperty.CreateAttached...
}

TextInputHandler.PropertyMapper[nameof(BorderlessTextInput.HasBorderProperty.PropertyName)] = MapHasBorder;

Then MapHasBorder could read from an attached property:

    public static void MapHasBorder(BorderlessTextInputHandler handler, IBorderlessTextInput input)
    {
        var hasBorder = BorderlessTextInput.GetHasBorder(input);
#if MONOANDROID
        // Pretend there is an Android implementation here
#elif __IOS__
        handler.TypedNativeView.SetBorderStyle(hasBorder ? UITextBorderStyle.UITextBorderStyleRoundedRect) : UITextBorderStyle.None;
#endif
    }