add some more menu implementation.

src/Avalonia.Native/AvaloniaNativeMenuExporter.cs

@@ -1,25 +1,86 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Specialized;
 using System.Text;
 using Avalonia.Controls;
 using Avalonia.Controls.Platform;
+using Avalonia.Threading;
 
 namespace Avalonia.Native
 {
     class AvaloniaNativeMenuExporter : ITopLevelNativeMenuExporter
     {
+        private NativeMenu _menu;
+        private bool _resetQueued;
+        private Dictionary<int, NativeMenuItem> _idsToItems = new Dictionary<int, NativeMenuItem>();
+        private Dictionary<NativeMenuItem, int> _itemsToIds = new Dictionary<NativeMenuItem, int>();
+        private uint _revision = 1;
+
         public bool IsNativeMenuExported => throw new NotImplementedException();
 
         public event EventHandler OnIsNativeMenuExportedChanged;
 
+        private event Action<(uint revision, int parent)> LayoutUpdated;
+
         public void SetNativeMenu(NativeMenu menu)
         {
-            throw new NotImplementedException();
+            if (menu == null)
+                menu = new NativeMenu();
+
+            if (_menu != null)
+                ((INotifyCollectionChanged)_menu.Items).CollectionChanged -= OnMenuItemsChanged;
+            _menu = menu;
+            ((INotifyCollectionChanged)_menu.Items).CollectionChanged += OnMenuItemsChanged;
+
+            DoLayoutReset();
         }
 
         public void SetPrependApplicationMenu(bool prepend)
         {
             throw new NotImplementedException();
         }
+
+        private void OnItemPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        {
+            QueueReset();
+        }
+
+        private void OnMenuItemsChanged(object sender, NotifyCollectionChangedEventArgs e)
+        {
+            QueueReset();
+        }
+
+        /*
+                 This is basic initial implementation, so we don't actually track anything and
+                 just reset the whole layout on *ANY* change
+                 
+                 This is not how it should work and will prevent us from implementing various features,
+                 but that's the fastest way to get things working, so...
+             */
+        void DoLayoutReset()
+        {
+            _resetQueued = false;
+            foreach (var i in _idsToItems.Values)
+            {
+                i.PropertyChanged -= OnItemPropertyChanged;
+                if (i.Menu != null)
+                    ((INotifyCollectionChanged)i.Menu.Items).CollectionChanged -= OnMenuItemsChanged;
+            }
+
+            _idsToItems.Clear();
+            _itemsToIds.Clear();
+
+            _revision++;
+
+            LayoutUpdated?.Invoke((_revision, 0));
+        }
+
+        private void QueueReset()
+        {
+            if (_resetQueued)
+                return;
+            _resetQueued = true;
+            Dispatcher.UIThread.Post(DoLayoutReset, DispatcherPriority.Background);
+        }
     }
 }