feat: setup demo.

src/Ursa/Controls/Dialog/DialogBox.cs

@@ -1,4 +1,7 @@
+using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Controls.Shapes;
+using Avalonia.Media;
 
 namespace Ursa.Controls;
 
@@ -15,6 +18,20 @@ public static class DialogBox
     {
         TView t = new TView();
         t.DataContext = vm;
-        return;
+    }
+
+    public static async Task<object?> ShowOverlayAsync<TView, TViewModel>(TViewModel vm, string hostId)
+        where TView : Control, new()
+        where TViewModel: new()
+    {
+        var t = new Border()
+        {
+            Width = 100, Height = 100, Background = Brushes.Aqua, BorderBrush = Brushes.Yellow,
+            BorderThickness = new Thickness(1)
+        };
+        t.DataContext = vm;
+        var host = OverlayDialogManager.GetOverlayDialogHost(hostId);
+        host?.Children.Add(t);
+        return null;
     }
 }

src/Ursa/Controls/Dialog/OverlayDialogHost.cs

@@ -2,18 +2,40 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
+using Avalonia.Media;
 using Avalonia.Utilities;
 
 namespace Ursa.Controls;
 
-public class DialogHost: Canvas
+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 DialogControl item)
+        if (e.Source is Control item)
         {
             if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
             {
@@ -31,7 +53,7 @@ public class DialogHost: Canvas
     protected override void OnPointerPressed(PointerPressedEventArgs e)
     {
         base.OnPointerPressed(e);
-        if (e.Source is DialogControl item)
+        if (e.Source is Control item)
         {
             _lastPoint = e.GetPosition(item);
         }

src/Ursa/Controls/Dialog/OverlayDialogManager.cs

@@ -0,0 +1,23 @@
+using System.Collections.Concurrent;
+
+namespace Ursa.Controls;
+
+internal static class OverlayDialogManager
+{
+    private static ConcurrentDictionary<string, OverlayDialogHost> _hosts = new();
+    
+    public static void RegisterOverlayDialogHost(OverlayDialogHost host, string id)
+    {
+        _hosts.TryAdd(id, host);
+    }
+    
+    public static void UnregisterOverlayDialogHost(string id)
+    {
+        _hosts.TryRemove(id, out _);
+    }
+    
+    public static OverlayDialogHost? GetOverlayDialogHost(string id)
+    {
+        return _hosts.TryGetValue(id, out var host) ? host : null;
+    }
+}