feat: add CanClose hook to easily block window closing.

demo/Ursa.Demo/Views/MainWindow.axaml.cs

@@ -1,3 +1,4 @@
+using System.Threading.Tasks;
 using Ursa.Controls;
 
 namespace Ursa.Demo.Views;
@@ -8,4 +9,10 @@ public partial class MainWindow : UrsaWindow
     {
         InitializeComponent();
     }
+
+    protected override async Task<bool> CanClose()
+    {
+        var result = await MessageBox.ShowOverlayAsync("Are you sure you want to exit?", "Exit", button: MessageBoxButton.YesNo);
+        return result == MessageBoxResult.Yes;
+    }
 }

src/Ursa/Windows/UrsaWindow.cs

@@ -1,3 +1,4 @@
+using System.ComponentModel;
 using Avalonia;
 using Avalonia.Controls;
 
@@ -100,5 +101,24 @@ public class UrsaWindow: Window
         set => SetValue(TitleBarMarginProperty, value);
     }
     
-    
+    protected virtual async Task<bool> CanClose()
+    {
+        return await Task.FromResult(true);
+    }
+    
+    private bool _canClose = false;
+    protected override async void OnClosing(WindowClosingEventArgs e)
+    {
+        VerifyAccess();
+        if (!_canClose)
+        {
+            e.Cancel = true;
+            _canClose = await CanClose();
+            if (_canClose)
+            {
+                Close();
+            }
+        }
+        base.OnClosing(e);
+    }
 }