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Hit testing

Hit testing determines which visual element is located at a given point on the screen. Avalonia uses hit testing internally for pointer events, but you can also perform hit testing programmatically for custom controls and advanced interaction scenarios.

How hit testing works​

When a pointer event occurs, Avalonia walks the visual tree from the topmost element downward. For each element, it checks whether the point falls within the element's bounds and its rendered content. The first element that passes the test becomes the event target.

The hit test considers:

  1. Visibility: Elements with IsVisible="False" are skipped.
  2. IsHitTestVisible: Elements with IsHitTestVisible="False" are skipped, but their children may still be tested.
  3. Bounds: The point must fall within the element's layout bounds.
  4. Rendered content: For shapes and custom-rendered controls, the test checks actual rendered pixels, not just the bounding box.

IsHitTestVisible​

Set IsHitTestVisible="False" to make a control "transparent" to pointer events. The control still renders normally, but pointer events pass through to the control behind it:

<!-- This overlay displays text but passes clicks through to controls underneath -->
<Panel>
    <Button Content="Click Me" />
    <TextBlock Text="Overlay label" IsHitTestVisible="False"
               HorizontalAlignment="Right" VerticalAlignment="Top"
               Foreground="Gray" Margin="8" />
</Panel>

Common uses:

  • Decorative overlays that should not intercept clicks
  • Watermark or status text over interactive content
  • Animation layers

Background and hit testing​

A control without a Background (or with Background set to null) does not participate in hit testing for its empty area. Only the child content receives pointer events.

To make the entire area of a panel respond to pointer events, set Background="Transparent":

<!-- This panel does NOT receive clicks in empty areas -->
<StackPanel PointerPressed="OnPressed">
    <TextBlock Text="Only this text is clickable" />
</StackPanel>

<!-- This panel receives clicks anywhere within its bounds -->
<StackPanel PointerPressed="OnPressed" Background="Transparent">
    <TextBlock Text="Click anywhere in the panel" />
</StackPanel>

This rule applies at every level, including the Window itself. Setting Background="{x:Null}" with TransparencyLevelHint="Transparent" lets pointer events pass through empty areas to OS windows underneath. Setting Background="Transparent" looks the same but captures all input. See Transparent click-through window for a complete example.

Programmatic hit testing​

InputHitTest​

Use InputHitTest on any control to find the element at a specific point:

// Point is relative to the control you call InputHitTest on
var result = myPanel.InputHitTest(new Point(50, 30));

if (result is Control hitControl)
{
    Debug.WriteLine($"Hit: {hitControl.GetType().Name}");
}

Finding the element under the pointer​

In a pointer event handler, the Source property of the event args tells you the original element:

private void OnPointerPressed(object? sender, PointerPressedEventArgs e)
{
    // e.Source is the element that was directly hit
    if (e.Source is Border border)
    {
        border.Background = Brushes.Yellow;
    }
}

Hit testing in custom controls​

When building custom controls that render their own content with DrawingContext, you may need to override hit testing to match the rendered shapes.

Custom hit test geometry​

Override the HitTestCore method to define a custom hit region:

public class CircleControl : Control
{
    public override void Render(DrawingContext context)
    {
        var radius = Math.Min(Bounds.Width, Bounds.Height) / 2;
        var center = new Point(Bounds.Width / 2, Bounds.Height / 2);
        context.DrawEllipse(Brushes.Blue, null, center, radius, radius);
    }

    protected override bool HitTest(Point point)
    {
        // Only hit test within the circle, not the full bounding box
        var radius = Math.Min(Bounds.Width, Bounds.Height) / 2;
        var center = new Point(Bounds.Width / 2, Bounds.Height / 2);
        var distance = Point.Distance(point, center);
        return distance <= radius;
    }
}

With this override, pointer events only fire when the user clicks inside the circle, not in the corners of the bounding rectangle.

Hit testing order​

When multiple controls overlap at the same point, the hit test returns the topmost control in the visual tree. The order is determined by:

  1. ZIndex: Higher ZIndex values are tested first.
  2. Visual tree order: Later children in the same panel are rendered (and hit tested) on top of earlier children.
<Panel>
    <Border Background="Red" Width="100" Height="100" />
    <!-- This border is on top and receives the click -->
    <Border Background="Blue" Width="100" Height="100" Margin="30" />
</Panel>

Practical patterns​

Click-through overlay​

Create a visual overlay that does not block interaction:

<Grid>
    <ListBox ItemsSource="{Binding Items}" />

    <!-- Semi-transparent loading overlay, clicks pass through when hidden -->
    <Border Background="#80000000"
            IsVisible="{Binding IsLoading}"
            IsHitTestVisible="{Binding IsLoading}">
        <ProgressBar IsIndeterminate="True" Width="200"
                     HorizontalAlignment="Center" VerticalAlignment="Center" />
    </Border>
</Grid>

Detecting clicks on a canvas drawing​

private void OnCanvasPointerPressed(object? sender, PointerPressedEventArgs e)
{
    var pos = e.GetPosition((Visual)sender!);

    // Check against your drawn shapes
    foreach (var shape in _shapes)
    {
        if (shape.Bounds.Contains(pos))
        {
            SelectShape(shape);
            e.Handled = true;
            return;
        }
    }
}

Performance with many elements​

Avalonia's hit-testing walks the visual tree and tests each element individually. There is no built-in spatial partitioning (such as a quadtree). For panels with a small number of children, this is fast. When you have hundreds or thousands of interactive elements on a Canvas or Panel, the linear walk becomes noticeable, especially for pointer press events where every candidate element must be tested.

Symptoms​

  • A delay between clicking and the PointerPressed event firing, growing linearly with the number of children.
  • The delay is input-related, not a rendering or layout problem. Frame rates remain normal.

Strategies​

Disable hit testing on individual elements and use an overlay. Place a transparent overlay on top of all the child elements. Handle PointerPressed on the overlay and use your own logic to determine which element was clicked. Set IsHitTestVisible="False" on the children so Avalonia skips them during the tree walk:

<Panel>
    <!-- All items have IsHitTestVisible="False" -->
    <Canvas x:Name="ItemsCanvas" IsHitTestVisible="False">
        <!-- Hundreds of child controls -->
    </Canvas>

    <!-- Transparent overlay catches all pointer events -->
    <Border Background="Transparent" PointerPressed="OnOverlayPointerPressed" />
</Panel>
private void OnOverlayPointerPressed(object? sender, PointerPressedEventArgs e)
{
    var pos = e.GetPosition(ItemsCanvas);

    // Use your own spatial lookup to find the item at this position
    var item = FindItemAt(pos);
    if (item != null)
    {
        SelectItem(item);
        e.Handled = true;
    }
}

Your FindItemAt method can use any lookup strategy that fits your data. For a grid-like arrangement, a simple coordinate calculation may suffice. For irregular shapes, consider a spatial index such as a quadtree or R-tree.

Switch to custom rendering. Instead of creating a separate control for each element, render all elements in a single control's Render override. This eliminates per-element hit testing entirely, since only the single parent control participates in the hit test. You then handle pointer events on that control and determine which logical element was clicked based on the pointer position. See Custom Rendering for details.

Reduce the number of hit-testable elements. If only some elements need to be interactive, set IsHitTestVisible="False" on the rest. For example, in a diagram editor, background grid lines and labels can be excluded from hit testing while only the draggable nodes remain interactive.

See also​

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