Fix bug in ExpressionObserver.

Ensure `rootGetter` is evaluated each time the observer is initialized; plus failing and now passing test.

src/Avalonia.Base/Data/Core/ExpressionObserver.cs

@@ -52,6 +52,7 @@ namespace Avalonia.Data.Core
         private static readonly object UninitializedValue = new object();
         private readonly ExpressionNode _node;
         private object? _root;
+        private Func<object?>? _rootGetter;
         private IDisposable? _rootSubscription;
         private WeakReference<object?>? _value;
         private IReadOnlyList<ITransformNode>? _transformNodes;
@@ -109,11 +110,9 @@ namespace Avalonia.Data.Core
             IObservable<Unit> update,
             string? description)
         {
-            _ = rootGetter ?? throw new ArgumentNullException(nameof(rootGetter));
-
             Description = description;
-            _node = node ?? throw new ArgumentNullException(nameof(rootGetter));
-            _node.Target = new WeakReference<object?>(rootGetter());
+            _rootGetter = rootGetter ?? throw new ArgumentNullException(nameof(rootGetter));
+            _node = node ?? throw new ArgumentNullException(nameof(node));
             _root = update.Select(x => rootGetter());
         }
 
@@ -263,6 +262,8 @@ namespace Avalonia.Data.Core
         protected override void Initialize()
         {
             _value = null;
+            if (_rootGetter is not null)
+                _node.Target = new WeakReference<object?>(_rootGetter());
             _node.Subscribe(ValueChanged);
             StartRoot();
         }

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Property.cs

@@ -3,14 +3,13 @@ using System.Collections.Generic;
 using System.Reactive;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
-using Microsoft.Reactive.Testing;
+using System.Threading.Tasks;
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Threading;
 using Avalonia.UnitTests;
+using Microsoft.Reactive.Testing;
 using Xunit;
-using System.Threading.Tasks;
-using Avalonia.Markup.Parsers;
-using Avalonia.Threading;
 
 namespace Avalonia.Base.UnitTests.Data.Core
 {
@@ -636,7 +635,25 @@ namespace Avalonia.Base.UnitTests.Data.Core
             
             target.Subscribe(x => result.Add(x));
         }
-        
+
+        [Fact]
+        public void RootGetter_Is_Reevaluated_On_Subscribe()
+        {
+            var data = "foo";
+            var target = new ExpressionObserver(() => data, new EmptyExpressionNode(), new Subject<Unit>(), null);
+            var result = new List<object>();
+            var sub = target.Subscribe(x => result.Add(x));
+
+            Assert.Equal(new object[] { "foo" }, result);
+
+            sub.Dispose();
+            data = "bar";
+
+            target.Subscribe(x => result.Add(x));
+
+            Assert.Equal(new object[] { "foo", "bar" }, result);
+        }
+
         public class MyViewModelBase { public object Name => "Name"; }
         
         public class MyViewModel : MyViewModelBase { public new string Name => "NewName"; }