feat: add overlay dialog sample.

demo/Ursa.Demo/Dialogs/DialogWithAction.axaml

@@ -0,0 +1,18 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:local="clr-namespace:Ursa.Demo.Dialogs"
+             x:DataType="local:DialogWithActionViewModel"
+             x:CompileBindings="True"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             x:Class="Ursa.Demo.Dialogs.DialogWithAction">
+    <StackPanel Margin="0 16 0 0">
+        <TextBlock Text="{Binding Title}"></TextBlock>
+        <Calendar SelectedDate="{Binding Date}" ></Calendar>
+        <StackPanel HorizontalAlignment="Right" Orientation="Horizontal" Spacing="20">
+            <Button Content="OK" Command="{Binding OKCommand}"></Button>
+            <Button Content="Cancel" Command="{Binding CancelCommand}"></Button>
+        </StackPanel>
+    </StackPanel>
+</UserControl>

demo/Ursa.Demo/Dialogs/DialogWithAction.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class DialogWithAction : UserControl
+{
+    public DialogWithAction()
+    {
+        InitializeComponent();
+    }
+}

demo/Ursa.Demo/Dialogs/DialogWithActionViewModel.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Windows.Input;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using Ursa.Controls;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class DialogWithActionViewModel: ObservableObject, IDialogContext
+{
+    [ObservableProperty] private string _title;
+    [ObservableProperty] private DateTime _date;
+    public object? DefaultCloseResult { get; set; } = true;
+    public event EventHandler<object>? Closed;
+    
+    public ICommand OKCommand { get; set; }
+    public ICommand CancelCommand { get; set; }
+    
+    public DialogWithActionViewModel()
+    {
+        OKCommand = new RelayCommand(OK);
+        CancelCommand = new RelayCommand(Cancel);
+        Title = "Please select a date";
+        Date = DateTime.Now;
+    }
+
+    private void OK()
+    {
+        Closed?.Invoke(this, true);
+    }
+
+    private void Cancel()
+    {
+        Closed?.Invoke(this, false);
+    }
+}

demo/Ursa.Demo/Dialogs/PlainDialog.axaml

@@ -0,0 +1,8 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             x:Class="Ursa.Demo.Dialogs.PlainDialog">
+    
+</UserControl>

demo/Ursa.Demo/Dialogs/PlainDialog.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class PlainDialog : UserControl
+{
+    public PlainDialog()
+    {
+        InitializeComponent();
+    }
+}

demo/Ursa.Demo/Dialogs/PlainDialogViewModel.cs

@@ -0,0 +1,8 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Ursa.Demo.Dialogs;
+
+public class PlainDialogViewModel: ObservableObject
+{
+    
+}

demo/Ursa.Demo/Pages/DialogDemo.axaml

@@ -13,6 +13,10 @@
     mc:Ignorable="d">
     <Grid ColumnDefinitions="Auto, *">
         <StackPanel Grid.Column="0">
+            <TextBlock>
+                <Run Text="Result: "></Run>
+                <Run Text="{Binding Result}"></Run>
+            </TextBlock>
             <Button Command="{Binding ShowGlobalDialogCommand}">Show Dialog</Button>
             <Button Command="{Binding ShowLocalOverlayDialogCommand}">Show Local Overlay Dialog</Button>
             <Button Command="{Binding ShowGlobalOverlayDialogCommand}"> Show Global Overlay Dialog </Button>

demo/Ursa.Demo/ViewModels/DialogDemoViewModel.cs

@@ -4,6 +4,7 @@ using System.Windows.Input;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using Ursa.Controls;
+using Ursa.Demo.Dialogs;
 using Ursa.Demo.Pages;
 
 namespace Ursa.Demo.ViewModels;
@@ -14,6 +15,14 @@ public class DialogDemoViewModel: ObservableObject
     public ICommand ShowGlobalOverlayDialogCommand { get; }
     public ICommand ShowGlobalDialogCommand { get; }
 
+    private object? _result;
+
+    public object? Result
+    {
+        get => _result;
+        set => SetProperty(ref _result, value);
+    }
+
     public DialogDemoViewModel()
     {
         ShowLocalOverlayDialogCommand = new AsyncRelayCommand(ShowLocalOverlayDialog);
@@ -23,16 +32,19 @@ public class DialogDemoViewModel: ObservableObject
 
     private async Task ShowGlobalDialog()
     {
-        var result = await DialogBox.ShowAsync<ButtonGroupDemo, ButtonGroupDemoViewModel, string>(new ButtonGroupDemoViewModel());
+        var result = await DialogBox.ShowAsync<DialogWithAction, DialogWithActionViewModel, bool>(new DialogWithActionViewModel());
+        Result = result;
     }
 
     private async Task ShowGlobalOverlayDialog()
     {
-        await DialogBox.ShowOverlayAsync<ButtonGroupDemo, ButtonGroupDemoViewModel>(new ButtonGroupDemoViewModel(), "GlobalHost");
+        await DialogBox.ShowOverlayAsync<DialogWithAction, DialogWithActionViewModel, bool>(new DialogWithActionViewModel(), "GlobalHost");
     }
 
     private async Task ShowLocalOverlayDialog()
     {
-        await DialogBox.ShowOverlayAsync<ButtonGroupDemo, ButtonGroupDemoViewModel>(new ButtonGroupDemoViewModel(), "LocalHost");
+        var result = await DialogBox.ShowOverlayAsync<DialogWithAction, DialogWithActionViewModel, bool>(
+            new DialogWithActionViewModel(), "LocalHost");
+        Result = result;
     }
 }

src/Ursa/Controls/Dialog/DialogBox.cs

@@ -47,7 +47,7 @@ public static class DialogBox
     }
     
 
-    public static async Task<object?> ShowOverlayAsync<TView, TViewModel>(TViewModel vm, string hostId)
+    public static Task<TResult> ShowOverlayAsync<TView, TViewModel, TResult>(TViewModel vm, string hostId)
         where TView : Control, new()
         where TViewModel: new()
     {
@@ -59,6 +59,6 @@ public static class DialogBox
         t.DataContext = vm;
         var host = OverlayDialogManager.GetOverlayDialogHost(hostId);
         host?.AddDialog(t);
-        return null;
+        return t.ShowAsync<TResult>();
     }
 }

src/Ursa/Controls/Dialog/DialogControl.cs

@@ -27,18 +27,36 @@ public class DialogControl: ContentControl
         {
             _closeButton.Click += Close;
         }
-    }
 
-    public void Show()
+        if (this.DataContext is IDialogContext context)
+        {
+            context.Closed += Close;
+        }
+    }
+    
+
+    public Task<T> ShowAsync<T>()
     {
-        
+        var tcs = new TaskCompletionSource<T>();
+        this.OnClose+= (sender, args) =>
+        {
+            if (args is T result)
+            {
+                tcs.SetResult(result);
+            }
+            else
+            {
+                tcs.SetResult(default);
+            }
+        };
+        return tcs.Task;
     }
 
     private void Close(object sender, object args)
     {
         if (this.DataContext is IDialogContext context)
         {
-            OnClose?.Invoke(this, context.DefaultCloseResult);
+            OnClose?.Invoke(this, args);
         }
         else
         {