Expose tree item selection to macOS accessibility (#21598)

TreeViewItemAutomationPeer implements ISelectionItemProvider, and AvnAccessibilityElement maps setAccessibilitySelected to it, so AX clients can select a TreeViewItem.

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

@@ -474,6 +474,12 @@
     return NO;
 }
 
+- (void)setAccessibilitySelected:(BOOL)accessibilitySelected
+{
+    if (accessibilitySelected && _peer->IsSelectionItemProvider())
+        _peer->SelectionItemProvider_Select();
+}
+
 - (BOOL)isAccessibilitySelectorAllowed:(SEL)selector
 {
     if (selector == @selector(setAccessibilityValue:))
@@ -492,6 +498,10 @@
     {
         return _peer->IsInvokeProvider() || _peer->IsExpandCollapseProvider() || _peer->IsToggleProvider();
     }
+    else if (selector == @selector(setAccessibilitySelected:))
+    {
+        return _peer->IsSelectionItemProvider();
+    }
     else if (selector == @selector(accessibilityPerformIncrement) ||
              selector == @selector(accessibilityPerformDecrement) ||
              selector == @selector(accessibilityMinValue) ||

src/Avalonia.Controls/Automation/Peers/TreeViewItemAutomationPeer.cs

@@ -1,14 +1,59 @@
+using Avalonia.Automation.Provider;
 using Avalonia.Controls;
 
 namespace Avalonia.Automation.Peers;
 
-public class TreeViewItemAutomationPeer : ItemsControlAutomationPeer
+public class TreeViewItemAutomationPeer : ItemsControlAutomationPeer,
+    ISelectionItemProvider
 {
     public TreeViewItemAutomationPeer(TreeViewItem owner)
         : base(owner)
     {
     }
-   
+
+    public bool IsSelected => Owner.GetValue(TreeViewItem.IsSelectedProperty);
+
+    public ISelectionProvider? SelectionContainer
+    {
+        get
+        {
+            if (Owner is TreeViewItem { TreeViewOwner: { } treeView })
+            {
+                var parentPeer = GetOrCreate(treeView);
+                return parentPeer.GetProvider<ISelectionProvider>();
+            }
+
+            return null;
+        }
+    }
+
+    public void Select()
+    {
+        EnsureEnabled();
+
+        if (Owner is TreeViewItem item)
+        {
+            item.TreeViewOwner?.SelectedItems.Clear();
+            item.IsSelected = true;
+        }
+    }
+
+    void ISelectionItemProvider.AddToSelection()
+    {
+        EnsureEnabled();
+
+        if (Owner is TreeViewItem item)
+            item.IsSelected = true;
+    }
+
+    void ISelectionItemProvider.RemoveFromSelection()
+    {
+        EnsureEnabled();
+
+        if (Owner is TreeViewItem item)
+            item.IsSelected = false;
+    }
+
     protected override AutomationControlType GetAutomationControlTypeCore()
     {
         return AutomationControlType.TreeItem;

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -187,6 +187,7 @@ namespace Avalonia.Native
 
         public int IsSelectionItemProvider() => IsProvider<ISelectionItemProvider>();
         public int SelectionItemProvider_IsSelected() => SelectionItemProvider.IsSelected.AsComBool();
+        public void SelectionItemProvider_Select() => SelectionItemProvider.Select();
 
         public IAvnAutomationPeer? ScrollProvider_GetHorizontalScrollBar()
             => _inner is ScrollViewerAutomationPeer scrollViewer ? Wrap(scrollViewer.GetHorizontalScrollBarPeer()) : null;

src/Avalonia.Native/avn.idl

@@ -1318,6 +1318,7 @@ interface IAvnAutomationPeer : IUnknown
      
      bool IsSelectionItemProvider();
      bool SelectionItemProvider_IsSelected();
+     void SelectionItemProvider_Select();
 
      IAvnAutomationPeer* ScrollProvider_GetHorizontalScrollBar();
      IAvnAutomationPeer* ScrollProvider_GetVerticalScrollBar();