Implemented a11y focus on OSX.

native/Avalonia.Native/src/OSX/automation.mm

@@ -36,7 +36,12 @@ public:
                 break;
         }
     }
-    
+
+    virtual void FocusChanged(IAvnAutomationPeer* peer) override
+    {
+        // Only implemented in top-level nodes, i.e. AvnWindow.
+    }
+
     virtual NSObject* GetNSAccessibility() override
     {
         return _node;

native/Avalonia.Native/src/OSX/window.mm

@@ -518,6 +518,11 @@ public:
     {
     }
     
+    virtual void FocusChanged(IAvnAutomationPeer* peer) override
+    {
+        NSAccessibilityPostNotification(Window, NSAccessibilityFocusedUIElementChangedNotification);
+    }
+    
 protected:
     virtual NSWindowStyleMask GetStyle()
     {
@@ -2301,6 +2306,18 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
     return GetAccessibilityElement(hit);
 }
 
+- (id)accessibilityFocusedUIElement
+{
+    auto peer = [self getAutomationPeer];
+    
+    if (peer->IsRootProvider())
+    {
+        return GetAccessibilityElement(peer->RootProvider_GetFocus());
+    }
+    
+    return [super accessibilityFocusedUIElement];
+}
+
 - (IAvnAutomationPeer* _Nonnull) getAutomationPeer
 {
     if (_automationPeer == nullptr)

src/Avalonia.Native/AutomationNode.cs

@@ -38,9 +38,6 @@ namespace Avalonia.Native
             Native.PropertyChanged(p);
         }
 
-        public void FocusChanged(AutomationPeer? focus)
-        {
-            // TODO
-        }
+        public void FocusChanged(AutomationPeer? focus) => Native.FocusChanged(AvnAutomationPeer.Wrap(focus));
     }
 }

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -52,6 +52,7 @@ namespace Avalonia.Native
         }
 
         public int IsRootProvider() => (_inner is IRootProvider).AsComBool();
+        public IAvnAutomationPeer? RootProvider_GetFocus() => Wrap(((IRootProvider)_inner).GetFocus());
 
         public IAvnAutomationPeer? RootProvider_GetPeerFromPoint(AvnPoint point)
         {

src/Avalonia.Native/avn.idl

@@ -812,6 +812,7 @@ interface IAvnAutomationPeer : IUnknown
      IAvnAutomationPeer* GetRootPeer();
      
      bool IsRootProvider();
+     IAvnAutomationPeer* RootProvider_GetFocus();
      IAvnAutomationPeer* RootProvider_GetPeerFromPoint(AvnPoint point);
      
      bool IsInvokeProvider();
@@ -844,4 +845,5 @@ interface IAvnAutomationNode : IUnknown
 {
     void ChildrenChanged();
     void PropertyChanged(AvnAutomationProperty property);
+    void FocusChanged(IAvnAutomationPeer* peer);
 }