How is a routed event executed?

[alvahtin]

I'm trying to understand how routed events work in Avalonia. Because everything is a little different than in WPF. I make some code. I placed a buttion with an Ellipse in StackPanel In form:

     <StackPanel Name="stackPanel" Background="LightGray" >
         <Button Width="80" Height="50" Name="button">
             <Ellipse Width="30" Height="30" Fill="Red" Name="ellipse" />
         </Button>
     </StackPanel> 

And set events with tunnel strategy:

public partial class MainWindow : Window
{   
    public MainWindow()
    {
        InitializeComponent();
        
        stackPanel.AddHandler(StackPanel.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Tunnel);
        button.AddHandler(Button.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Tunnel);
        ellipse.AddHandler(Ellipse.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Tunnel);
    }

    private void PointerPressedHandler(object? sender, PointerPressedEventArgs e)
    {
    
       Debug.WriteLine("Sender: " + sender!.ToString() +"\nSource: " + e.Source!.ToString() + "\n");
    }
}

This works as expected. If you click on the ellipse, you'll see:

Sender: Avalonia.Controls.StackPanel
Source: Avalonia.Controls.Shapes.Ellipse

Sender: Avalonia.Controls.Button
Source: Avalonia.Controls.Shapes.Ellipse

Sender: Avalonia.Controls.Shapes.Ellipse
Source: Avalonia.Controls.Shapes.Ellipse

But when I set buble strategy:

       stackPanel.AddHandler(StackPanel.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Bubble);
        button.AddHandler(Button.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Bubble);
        ellipse.AddHandler(Ellipse.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Bubble);

It works as if it is direct. The event handler of the component where it was called. If you click on the ellipse, you'll see:

Sender: Avalonia.Controls.Shapes.Ellipse
Source: Avalonia.Controls.Shapes.Ellipse

When I set direct strategy:

        stackPanel.AddHandler(StackPanel.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Direct);
        button.AddHandler(Button.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Direct);
        ellipse.AddHandler(Ellipse.PointerPressedEvent, PointerPressedHandler, RoutingStrategies.Direct);

Events are not caught.

Why? How does this work?

[stevemonaco]

You should use DevTools to trace events instead: https://docs.avaloniaui.net/docs/guides/implementation-guides/developer-tools#events

For the bubbling case, you don't get the event because the Button handles the event as part of its Click interaction implementation. This means that all controls above the Button in the visual tree will not receive the event unless you specify handledEventsToo in the AddHandler registration.

For the direct calls, I believe this is because the PointerPressedEvent is registered only to tunnel and bubble. I don't really use Direct events much. See:

Avalonia/src/Avalonia.Base/Input/InputElement.cs

Lines 153 to 159 in 46d4735

	/// <summary>
	/// Defines the <see cref="PointerPressed"/> event.
	/// </summary>
	public static readonly RoutedEvent<PointerPressedEventArgs> PointerPressedEvent =
	RoutedEvent.Register<InputElement, PointerPressedEventArgs>(
	nameof(PointerPressed),
	RoutingStrategies.Tunnel | RoutingStrategies.Bubble);

Avalonia/src/Avalonia.Base/Interactivity/EventRoute.cs

Lines 77 to 110 in 46d4735

	public void RaiseEvent(Interactive source, RoutedEventArgs e)
	{
	source = source ?? throw new ArgumentNullException(nameof(source));
	e = e ?? throw new ArgumentNullException(nameof(e));
	e.Source = source;
	using var _ = Diagnostic.RaisingRoutedEvent()?
	.AddTag(Diagnostic.Tags.Control, e.Source)
	.AddTag(Diagnostic.Tags.RoutedEvent, e.RoutedEvent);
	if (_event.RoutingStrategies == RoutingStrategies.Direct)
	{
	e.Route = RoutingStrategies.Direct;
	RaiseEventImpl(e);
	_event.InvokeRouteFinished(e);
	}
	else
	{
	if (_event.RoutingStrategies.HasAllFlags(RoutingStrategies.Tunnel))
	{
	e.Route = RoutingStrategies.Tunnel;
	RaiseEventImpl(e);
	_event.InvokeRouteFinished(e);
	}
	if (_event.RoutingStrategies.HasAllFlags(RoutingStrategies.Bubble))
	{
	e.Route = RoutingStrategies.Bubble;
	RaiseEventImpl(e);
	_event.InvokeRouteFinished(e);
	}
	}
	}