Attached Behaviors - works with ICommand but not bool?

[metal450]

I'm trying to setup an attached property. Following the example here: https://docs.avaloniaui.net/docs/next/guides/custom-controls/how-to-create-attached-properties, I'm able set a breakpoint & see when the AddClassHandler callback gets called. However, if I just change the property type in the example from ICommand to bool (and bind to a bool property in my ViewModal rather than an ICommand)...it stops working, and the callback is never called. I'm sure I'm doing something dumb, but I've simplified it as far as I possibly could & just can't get it to work.

Here's the sample code (just simplified/renamed a bit). This works - I can set a breakpoint in HandlePropertyChanged that gets hit:

 public class ScrollViewerBehavior : AvaloniaObject
 {
     public static readonly AttachedProperty<ICommand> HasMoreProperty = AvaloniaProperty.RegisterAttached<ScrollViewerBehavior, ScrollViewer, ICommand>("HasMore");

     static ScrollViewerBehavior()
     {
         HasMoreProperty.Changed.AddClassHandler<ScrollViewer>(HandlePropertyChanged);
     }

     private static void HandlePropertyChanged(ScrollViewer interactElem, AvaloniaPropertyChangedEventArgs args)
     {
         Trace.WriteLine("Test Attached");
     }

     public static void SetHasMore(AvaloniaObject element, ICommand commandValue)
     {
         element.SetValue(HasMoreProperty, commandValue);
     }

     public static ICommand GetHasMore(AvaloniaObject element)
     {
         return element.GetValue(HasMoreProperty);
     }
 }

Xaml:

<ScrollViewer local:ScrollViewerBehavior.HasMore="{Binding OpenFileCommand}" />

Where OpenFileCommand is just a random command in my ViewModel. However, if I simply change each ICommand to a bool above:

 public class ScrollViewerBehavior : AvaloniaObject
 {
     public static readonly AttachedProperty<bool> HasMoreProperty = AvaloniaProperty.RegisterAttached<ScrollViewerBehavior, ScrollViewer, bool>("HasMore");

     static ScrollViewerBehavior()
     {
         HasMoreProperty.Changed.AddClassHandler<ScrollViewer>(HandlePropertyChanged);
     }

     private static void HandlePropertyChanged(ScrollViewer interactElem, AvaloniaPropertyChangedEventArgs args)
     {
         Trace.WriteLine("Test Attached");
     }

     public static void SetHasMore(AvaloniaObject element, bool commandValue)
     {
         element.SetValue(HasMoreProperty, commandValue);
     }

     public static bool GetHasMore(AvaloniaObject element)
     {
         return element.GetValue(HasMoreProperty);
     }
 }

And in xaml, bind to a boolean property:

<ScrollViewer local:ScrollViewerBehavior.HasMore="{Binding HasMore}" />

In my ViewModel:

public bool HasMore
{
    get => _hasMore;
    set => this.RaiseAndSetIfChanged(ref _hasMore, value);
}
private bool _hasMore;

Then HandlePropertyChanged never gets called.

What am I missing? Any pointers would be greatly appreciated.

[timunie]

HasMore is false and thus the default value. So no change. If you set it to true, it should trigger.

[metal450]

Duh, that was it. Thanks!!