Implement -accessibilityPlaceholderValue on macOS

macOS uses a unique -accessibilityPlaceholderValue property for the
placeholder of a text field; -accessibilityHelp has different meaning from
HelpText on Windows. Map TextBox.PlaceholderText to be returned by
-accessibilityPlaceholderValue.

Ref: https://www.w3.org/TR/core-aam-1.2/#ariaPlaceholder

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

@@ -220,6 +220,11 @@
     return GetNSStringAndRelease(_peer->GetHelpText()); 
 }
 
+- (NSString *)accessibilityPlaceholderValue
+{
+    return GetNSStringAndRelease(_peer->GetPlaceholderText());
+}
+
 - (id)accessibilityValue
 {
     if (_peer->IsRangeValueProvider())

samples/IntegrationTestApp/Pages/AutomationPage.axaml

@@ -10,7 +10,7 @@
       TextBlockWithNameAndAutomationId
     </TextBlock>
     <TextBlock Name="TextBlockAsLabel">Label for TextBox</TextBlock>
-    <TextBox Name="LabeledByTextBox" AutomationProperties.LabeledBy="{Binding #TextBlockAsLabel}">
+    <TextBox Name="LabeledByTextBox" AutomationProperties.LabeledBy="{Binding #TextBlockAsLabel}" PlaceholderText="Placeholder for TextBox">
       Foo
     </TextBox>
     <TextBlock Name="TextBlockWithHeader1" AutomationProperties.ControlTypeOverride="Header" AutomationProperties.HeadingLevel="1">

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

@@ -283,6 +283,23 @@ namespace Avalonia.Automation.Peers
         /// </remarks>
         public string GetHelpText() => GetHelpTextCore() ?? string.Empty;
 
+        /// <summary>
+        /// Gets text that provides a placeholder for the element that is associated with this automation peer.
+        /// </summary>
+        /// <remarks>
+        /// <list type="table">
+        ///   <item>
+        ///     <term>Windows</term>
+        ///     <description>No mapping.</description>
+        ///   </item>
+        ///   <item>
+        ///     <term>macOS</term>
+        ///     <description><c>NSAccessibilityProtocol.accessibilityPlaceholderValue</c></description>
+        ///   </item>
+        /// </list>
+        /// </remarks>
+        public string GetPlaceholderText() => GetPlaceholderTextCore() ?? string.Empty;
+
         /// <summary>
         /// Gets the control type for the element that is associated with the UI Automation peer.
         /// </summary>
@@ -595,6 +612,7 @@ namespace Avalonia.Automation.Peers
         protected abstract AutomationPeer? GetLabeledByCore();
         protected abstract string? GetNameCore();
         protected virtual string? GetHelpTextCore() => null;
+        protected virtual string? GetPlaceholderTextCore() => null;
         protected virtual AutomationLandmarkType? GetLandmarkTypeCore() => null;
         protected virtual int GetHeadingLevelCore() => 0;
         protected virtual string? GetItemTypeCore() => null;

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

@@ -29,6 +29,7 @@ internal class InteropAutomationPeer : AutomationPeer
     protected override AutomationPeer? GetLabeledByCore() => throw new NotImplementedException();
     protected override string? GetNameCore() => throw new NotImplementedException();
     protected override string? GetHelpTextCore() => throw new NotImplementedException();
+    protected override string? GetPlaceholderTextCore() => throw new NotImplementedException();
     protected override IReadOnlyList<AutomationPeer> GetOrCreateChildrenCore() => throw new NotImplementedException();
     protected override AutomationPeer? GetParentCore() => _parent;
     protected override bool HasKeyboardFocusCore() => throw new NotImplementedException();

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

@@ -21,6 +21,8 @@ namespace Avalonia.Automation.Peers
             return AutomationControlType.Edit;
         }
 
+        protected override string? GetPlaceholderTextCore() => Owner.PlaceholderText;
+
         protected virtual void OwnerPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if(e.Property == TextBox.TextProperty)

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -46,6 +46,7 @@ namespace Avalonia.Native
         public IAvnAutomationPeer? LabeledBy => Wrap(_inner.GetLabeledBy());
         public IAvnString? Name => _inner.GetName().ToAvnString();
         public IAvnString? HelpText => _inner.GetHelpText().ToAvnString();
+        public IAvnString? PlaceholderText => _inner.GetPlaceholderText().ToAvnString();
         public AvnLandmarkType LandmarkType => (AvnLandmarkType?)_inner.GetLandmarkType() ?? AvnLandmarkType.LandmarkNone;
         public int HeadingLevel => _inner.GetHeadingLevel();
         public IAvnAutomationPeer? Parent => Wrap(_inner.GetParent());

src/Avalonia.Native/avn.idl

@@ -1328,6 +1328,7 @@ interface IAvnAutomationPeer : IUnknown
      void ValueProvider_SetValue([const] char* value);
 
      IAvnString* GetHelpText();
+     IAvnString* GetPlaceholderText();
 
      AvnLandmarkType GetLandmarkType();
      int GetHeadingLevel();