feat: setup demo.

src/Ursa/Controls/Dialog/OverlayDialogHost.cs

@@ -0,0 +1,61 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+using Avalonia.Media;
+using Avalonia.Utilities;
+
+namespace Ursa.Controls;
+
+public class OverlayDialogHost: Canvas
+{
+    public static readonly StyledProperty<string> HostIdProperty = AvaloniaProperty.Register<OverlayDialogHost, string>(
+        nameof(HostId));
+
+    public string HostId
+    {
+        get => GetValue(HostIdProperty);
+        set => SetValue(HostIdProperty, value);
+    }
+    
+    private Point _lastPoint;
+    
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        OverlayDialogManager.RegisterOverlayDialogHost(this, HostId);
+    }
+
+    protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        OverlayDialogManager.UnregisterOverlayDialogHost(HostId);
+        base.OnDetachedFromVisualTree(e);
+    }
+
+    protected override void OnPointerMoved(PointerEventArgs e)
+    {
+        base.OnPointerMoved(e);
+        if (e.Source is Control item)
+        {
+            if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            {
+                var p = e.GetPosition(this);
+                var left=  p.X - _lastPoint.X;
+                var top = p.Y - _lastPoint.Y;
+                left = MathUtilities.Clamp(left, 0, Bounds.Width - item.Bounds.Width);
+                top = MathUtilities.Clamp(top, 0, Bounds.Height - item.Bounds.Height);
+                Canvas.SetLeft(item, left);
+                Canvas.SetTop(item, top);
+            }
+        }
+    }
+
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        base.OnPointerPressed(e);
+        if (e.Source is Control item)
+        {
+            _lastPoint = e.GetPosition(item);
+        }
+    }
+}