ExpanderAutomationPeer WIP

samples/Sandbox/MainWindow.axaml

@@ -1,4 +1,9 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         x:Class="Sandbox.MainWindow">
+  <Expander Header="Expand Up" ExpandDirection="Up">
+    <StackPanel>
+      <TextBlock>Expanded content</TextBlock>
+    </StackPanel>
+  </Expander>
 </Window>

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

@@ -0,0 +1,64 @@
+using System;
+using System.Collections.Generic;
+using Avalonia.Automation;
+using Avalonia.Automation.Peers;
+using Avalonia.Automation.Provider;
+
+namespace Avalonia.Controls.Automation.Peers
+{
+    public class ExpanderAutomationPeer : ControlAutomationPeer, IExpandCollapseProvider
+    {
+        public ExpanderAutomationPeer(Expander owner) : base(owner)
+        {
+            owner.PropertyChanged += Owner_PropertyChanged;
+        }
+
+        protected override string? GetNameCore()
+        {
+            return base.GetNameCore();
+        }
+        protected override string GetClassNameCore()
+        {
+            return "Expander";
+        }
+        protected override bool IsContentElementCore() => true;
+        protected override bool IsControlElementCore() => true;
+        protected override AutomationControlType GetAutomationControlTypeCore()
+        {
+            return AutomationControlType.Group;
+        }
+
+        public ExpandCollapseState ExpandCollapseState => ((Expander)Owner).IsExpanded ? ExpandCollapseState.Expanded : ExpandCollapseState.Collapsed;
+
+        public bool ShowsMenu => throw new NotImplementedException();
+
+        public void Collapse()
+        {
+            if (!IsEnabled())
+                throw new ElementNotEnabledException();
+
+            ((Expander)Owner).IsExpanded = false;
+        }
+
+        public void Expand()
+        {
+            if (!IsEnabled())
+                throw new ElementNotEnabledException();
+
+            ((Expander)Owner).IsExpanded = true;
+        }
+
+
+        private void Owner_PropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.Property == Expander.IsExpandedProperty)
+            {
+                RaisePropertyChangedEvent(
+                    ExpandCollapsePatternIdentifiers.ExpandCollapseStateProperty,
+                    (bool)e.OldValue! ? ExpandCollapseState.Expanded : ExpandCollapseState.Collapsed,
+                    (bool)e.NewValue! ? ExpandCollapseState.Expanded : ExpandCollapseState.Collapsed);
+            }
+        }
+
+    }
+}

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>