AddClassBehavior

AddClassBehavior.cs

public class AddClassBehavior : AvaloniaObject, IBehavior
{
    public IAvaloniaObject AssociatedObject { get; private set; }

    public string Class
    {
        get => GetValue(ClassProperty);
        set => SetValue(ClassProperty, value);
    }

    public static readonly StyledProperty<string> ClassProperty = AvaloniaProperty.Register<AddClassBehavior, string>(nameof(Class), null);

    public bool IsEnabled
    {
        get => GetValue(IsEnabledProperty);
        set => SetValue(IsEnabledProperty, value);
    }

    public static readonly StyledProperty<bool> IsEnabledProperty = AvaloniaProperty.Register<AddClassBehavior, bool>(nameof(IsEnabled), false);

    public void Attach(IAvaloniaObject associatedObject)
    {
        if (!(associatedObject is IStyledElement styledElement))
        {
            throw new ArgumentException($"{nameof(AddClassBehavior)} supports only IStyledElement");
        }

        AssociatedObject = associatedObject;

        if (Class is string className)
        {
            styledElement.Classes.Set(className, IsEnabled);
        }
    }

    public void Detach()
    {
        IsEnabled = false;
        AssociatedObject = null;
    }

    protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> e)
    {
        base.OnPropertyChanged(e);

        if (!(AssociatedObject is IStyledElement styledElement))
        {
            return;
        }

        if (e.Property == ClassProperty)
        {
            if (e.OldValue.GetValueOrDefault<string>() is string oldClassName)
            {
                styledElement.Classes.Set(oldClassName, false);
            }
            if (e.NewValue.GetValueOrDefault<string>() is string newClassName)
            {
                styledElement.Classes.Set(newClassName, IsEnabled);
            }
        }
        else if (e.Property == IsEnabledProperty)
        {
            if (Class is string className)
            {
                styledElement.Classes.Set(className, IsEnabled);
            }
        }
    }
}

Updated to work with Avalonia 0.10