Added TopLevel.Opened event.

And raise the event when a window is opened.

src/Avalonia.Controls/TopLevel.cs

@@ -136,6 +136,11 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Fired when the window is opened.
+        /// </summary>
+        public event EventHandler Opened;
+
         /// <summary>
         /// Fired when the window is closed.
         /// </summary>
@@ -311,6 +316,12 @@ namespace Avalonia.Controls
                 $"Control '{GetType().Name}' is a top level control and cannot be added as a child.");
         }
 
+        /// <summary>
+        /// Raises the <see cref="Opened"/> event.
+        /// </summary>
+        /// <param name="e">The event args.</param>
+        protected virtual void OnOpened(EventArgs e) => Opened?.Invoke(this, e);
+
         /// <summary>
         /// Tries to get a service from an <see cref="IAvaloniaDependencyResolver"/>, logging a
         /// warning if not found.

src/Avalonia.Controls/Window.cs

@@ -389,6 +389,7 @@ namespace Avalonia.Controls
                 Renderer?.Start();
             }
             SetWindowStartupLocation(Owner?.PlatformImpl);
+            OnOpened(EventArgs.Empty);
         }
 
         /// <summary>
@@ -458,6 +459,7 @@ namespace Avalonia.Controls
                         owner.Activate();
                         result.SetResult((TResult)(_dialogResult ?? default(TResult)));
                     });
+                OnOpened(EventArgs.Empty);
             }
 
             SetWindowStartupLocation(owner);

src/Avalonia.Controls/WindowBase.cs

@@ -163,7 +163,7 @@ namespace Avalonia.Controls
         }
 
         /// <summary>
-        /// Shows the popup.
+        /// Shows the window.
         /// </summary>
         public virtual void Show()
         {
@@ -181,6 +181,7 @@ namespace Avalonia.Controls
                 }
                 PlatformImpl?.Show();
                 Renderer?.Start();
+                OnOpened(EventArgs.Empty);
             }
             finally
             {