Added automation peer for Expander control (#20475)

* Added `ExpanderAutomationPeer` for `Expander` control

* Use Group/"group" on UIA and NSAccessibilityDisclosureTriangleRole on AX

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

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

@@ -126,6 +126,7 @@
         case AutomationHeaderItem:  return NSAccessibilityButtonRole;
         case AutomationTable: return NSAccessibilityTableRole;
         case AutomationTitleBar: return NSAccessibilityGroupRole;
+        case AutomationExpander: return NSAccessibilityDisclosureTriangleRole;
         // Treat unknown roles as generic group container items. Returning
         // NSAccessibilityUnknownRole is also possible but makes the screen
        // reader focus on the item instead of passing focus to child items.

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

@@ -47,6 +47,7 @@ namespace Avalonia.Automation.Peers
         Table,
         TitleBar,
         Separator,
+        Expander,
     }
 
     public enum AutomationLandmarkType
@@ -536,6 +537,7 @@ namespace Avalonia.Automation.Peers
                 AutomationControlType.SplitButton => "split button",
                 AutomationControlType.HeaderItem => "header item",
                 AutomationControlType.TitleBar => "title bar",
+                AutomationControlType.Expander => "group",
                 AutomationControlType.None => (GetLandmarkType()?.ToString() ?? controlType.ToString()).ToLowerInvariant(),
                 _ => controlType.ToString().ToLowerInvariant(),
             };

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

@@ -0,0 +1,47 @@
+using Avalonia.Automation;
+using Avalonia.Automation.Peers;
+using Avalonia.Automation.Provider;
+
+namespace Avalonia.Controls.Automation.Peers
+{
+    public class ExpanderAutomationPeer : ControlAutomationPeer,
+        IExpandCollapseProvider
+    {
+        public ExpanderAutomationPeer(Control owner)
+            : base(owner)
+        {
+            owner.PropertyChanged += OwnerPropertyChanged;
+        }
+
+        public new Expander Owner => (Expander)base.Owner;
+
+        public ExpandCollapseState ExpandCollapseState => ToState(Owner.IsExpanded);
+        public bool ShowsMenu => false;
+        public void Collapse() => Owner.IsExpanded = false;
+        public void Expand() => Owner.IsExpanded = true;
+
+        protected override AutomationControlType GetAutomationControlTypeCore()
+        {
+            return AutomationControlType.Expander;
+        }
+
+        private void OwnerPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.Property == Expander.IsExpandedProperty)
+            {
+                RaisePropertyChangedEvent(
+                    ExpandCollapsePatternIdentifiers.ExpandCollapseStateProperty,
+                    ToState((bool)e.OldValue!),
+                    ToState((bool)e.NewValue!));
+            }
+        }
+
+        private static ExpandCollapseState ToState(bool value)
+        {
+            return value ? ExpandCollapseState.Expanded : ExpandCollapseState.Collapsed;
+        }
+
+        protected override bool IsContentElementCore() => true;
+        protected override bool IsControlElementCore() => true;
+    }
+}

src/Avalonia.Controls/Expander.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Threading;
 using Avalonia.Animation;
+using Avalonia.Automation.Peers;
+using Avalonia.Controls.Automation.Peers;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 using Avalonia.Data;
@@ -274,6 +276,11 @@ namespace Avalonia.Controls
             }
         }
 
+        protected override AutomationPeer OnCreateAutomationPeer()
+        {
+            return new ExpanderAutomationPeer(this);
+        }
+
         /// <summary>
         /// Updates the visual state of the control by applying latest PseudoClasses.
         /// </summary>

src/Avalonia.Native/avn.idl

@@ -652,6 +652,7 @@ enum AvnAutomationControlType
     AutomationTable,
     AutomationTitleBar,
     AutomationSeparator,
+    AutomationExpander,
 }
 
 enum AvnLandmarkType

src/Windows/Avalonia.Win32.Automation/AutomationNode.cs

@@ -357,6 +357,7 @@ namespace Avalonia.Win32.Automation
                 AutomationControlType.Table => UiaControlTypeId.Table,
                 AutomationControlType.TitleBar => UiaControlTypeId.TitleBar,
                 AutomationControlType.Separator => UiaControlTypeId.Separator,
+                AutomationControlType.Expander => UiaControlTypeId.Group,
                 _ => UiaControlTypeId.Custom,
             };
         }