remove messy install step.

samples/ControlCatalog/MainWindow.xaml.cs

@@ -21,7 +21,7 @@ namespace ControlCatalog
             //Renderer.DrawFps = true;
             //Renderer.DrawDirtyRects = Renderer.DrawFps = true;
 
-            _notificationArea = new WindowNotificationManager
+            _notificationArea = new WindowNotificationManager(this)
             {
                 Position = NotificationPosition.TopRight,
                 MaxItems = 3
@@ -39,12 +39,5 @@ namespace ControlCatalog
             theme.TryGetResource("Button", out _);
             AvaloniaXamlLoader.Load(this);
         }
-
-        protected override void OnTemplateApplied(TemplateAppliedEventArgs e)
-        {
-            base.OnTemplateApplied(e);
-
-            _notificationArea.Install(this);
-        }
     }
 }

src/Avalonia.Controls/Notifications/WindowNotificationManager.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Linq;
+using System.Reactive.Linq;
 using System.Threading.Tasks;
 using Avalonia.Controls.Primitives;
 using Avalonia.Interactivity;
@@ -50,6 +51,26 @@ namespace Avalonia.Controls.Notifications
             set { SetValue(MaxItemsProperty, value); }
         }
 
+        public WindowNotificationManager(Window host)
+        {
+            if (VisualChildren.Count != 0)
+            {
+                Install(host);
+            }
+            else
+            {
+                IDisposable stopListening = null;
+
+                stopListening = Observable.FromEventPattern<TemplateAppliedEventArgs>(host, nameof(host.TemplateApplied))
+                    .Subscribe(_ =>
+                    {
+                        stopListening.Dispose();
+
+                        Install(host);
+                    });
+            }
+        }
+
         static WindowNotificationManager()
         {
             PseudoClass<WindowNotificationManager, NotificationPosition>(PositionProperty, x => x == NotificationPosition.TopLeft, ":topleft");
@@ -125,7 +146,7 @@ namespace Avalonia.Controls.Notifications
             _items.Remove(notification);
         }
 
-        public void Install(Window host)
+        private void Install(Window host)
         {
             var adornerLayer = host.GetVisualDescendants()
                 .OfType<AdornerDecorator>()