WPF keyboard events not firing for FrameworkElement subclass with only visuals

Question

I created a subclass of FrameworkElement that has a collection of Visuals :

public class GameElement : FrameworkElement
{
    private VisualCollection Visuals { get; }

    public GameElement()
    {
        this.KeyDown += this.OnKeyDown;
        this.MouseDown += this.OnMouseDown;
    }

    private void OnKeyDown(object sender, KeyEventArgs keyEventArgs)
    {
        ... // Does not get fired.
    }

    private void OnMouseDown(object sender, MouseButtonEventArgs e)
    {
        ... // Does get fired.
    }

    protected override void OnRender(DrawingContext drawingContext)
    {
        // Draw a transparent background to catch mouse events (otherwise hit testing won't hit anything).
        drawingContext.DrawRectangle(Brushes.Transparent, null, new Rect(0, 0, RenderSize.Width, RenderSize.Height));
    }

    protected override int VisualChildrenCount
    {
        get
        {
            return this.Visuals.Count;
        }
    }

    protected override Visual GetVisualChild(int index)
    {
        if (index < 0 || index >= this.Visuals.Count)
        {
            throw new ArgumentOutOfRangeException();
        }

        return this.Visuals[index];
    }
}

I display this element in XAML with the following code :

<UserControl
    x:Class="..."
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Background="Black">

    <Grid Margin="10">
        <local:GameElement x:Name="GameElement" ClipToBounds="True" />
    </Grid>
</UserControl>

I have tried everything I can think of, but I just cannot get the KeyDown event to fire. The mostly used comment I find online has to do with focus. I have tried every combination of Focusable="True" and calling this.Focus(), but nothing works.

Anyone got any idea how to do this? Thanks!

Answer 1

I finally got it to work by registering a class handler that also handles handled events.

EventManager.RegisterClassHandler(typeof(Window), Keyboard.KeyDownEvent, new KeyEventHandler(OnKeyDown), true);

Answer 2

To be able to handle keys pressing your element should be focused. Also try to derive it from control instead of FramworkElement if you can do that.

public class GameElement : Control
{
    private VisualCollection Visuals { get; }

    public GameElement()
    {
        this.KeyDown += this.OnKeyDown;
        this.MouseDown += this.OnMouseDown;
    }

    private void OnKeyDown(object sender, KeyEventArgs keyEventArgs)
    {
        // Does get fired.
    }

    private void OnMouseDown(object sender, MouseButtonEventArgs e)
    {
        Focus();
    }

    protected override void OnRender(DrawingContext drawingContext)
    {
        // Draw a transparent background to catch mouse events (otherwise hit testing won't hit anything).
        drawingContext.DrawRectangle(Brushes.Transparent, null, new Rect(0, 0, RenderSize.Width, RenderSize.Height));
    }

    protected override int VisualChildrenCount
    {
        get
        {
            return this.Visuals.Count;
        }
    }

    protected override Visual GetVisualChild(int index)
    {
        if (index < 0 || index >= this.Visuals.Count)
        {
            throw new ArgumentOutOfRangeException();
        }

        return this.Visuals[index];
    }
}