ClassSetter.cs

using System;

using Avalonia;
using Avalonia.Collections;
using Avalonia.Controls;
using Avalonia.Data;

using ReactiveUI;

namespace Utils
{
    public class ClassSetters : AvaloniaList<ClassSetter>
    {

    }

    public class ClassSetter : AvaloniaObject
    {
        public static readonly DirectProperty<ClassSetter, string> NameProperty
            = AvaloniaProperty.RegisterDirect<ClassSetter, string>(nameof(Name), s => s.Name, (s, v) => s.Name = v, defaultBindingMode: BindingMode.OneTime);

        public static readonly StyledProperty<bool> TriggerProperty
            = AvaloniaProperty.Register<ClassSetter, bool>(nameof(Trigger), false);

        public static readonly AttachedProperty<ClassSetters> ClassesProperty
            = AvaloniaProperty.RegisterAttached<ClassSetter, Control, ClassSetters>("Classes");

        static ClassSetter()
        {
            ClassesProperty.Changed.AddClassHandler<Control>(ClassSettersChanged);
        }

        public static ClassSetters GetClasses(AvaloniaObject obj)
        {
            return obj.GetValue(ClassesProperty);
        }
        public static void SetClasses(AvaloniaObject obj, ClassSetters classes)
        {
            obj.SetValue(ClassesProperty, classes);
        }

        private static void ClassSettersChanged(Control sender, AvaloniaPropertyChangedEventArgs args)
        {
            var items = (IAvaloniaReadOnlyList<ClassSetter>)args.NewValue;
            if (items != null)
            {
                foreach (var item in items)
                {
                    _ = item.WhenAnyValue(i => i.Trigger)
                        .Subscribe(trigger => sender.Classes.Set(item.name, trigger));
                }
            }
        }

        private string name;
        public string Name
        {
            get => name;
            set => SetAndRaise(NameProperty, ref name, value);
        }

        public bool Trigger
        {
            get => GetValue(TriggerProperty);
            set => SetValue(TriggerProperty, value);
        }
    }
}