Made OneTime bindings update on DataContext changes (#17683)

* Add failing tests for OneTime and null data context bindings

* Made OneTime bindings update on DataContext changes

Also allows null as a valid value for bindings without path

* Remove now obsolete test

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

@@ -30,6 +30,7 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
     private readonly List<ExpressionNode> _nodes;
     private readonly TargetTypeConverter? _targetTypeConverter;
     private readonly UncommonFields? _uncommon;
+    private bool _shouldUpdateOneTimeBindingTarget;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="BindingExpression"/> class.
@@ -83,6 +84,7 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         _mode = mode;
         _nodes = nodes ?? s_emptyExpressionNodes;
         _targetTypeConverter = targetTypeConverter;
+        _shouldUpdateOneTimeBindingTarget = _mode == BindingMode.OneTime;
 
         if (converter is not null ||
             converterCulture is not null ||
@@ -231,10 +233,14 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
 
         if (nodeIndex == _nodes.Count - 1)
         {
-            // If the binding source is a data context without any path and is currently null, treat it as an invalid
-            // value. This allows bindings to DataContext and DataContext.Property to share the same behavior.
-            if (value is null && _nodes[nodeIndex] is DataContextNodeBase)
-                value = AvaloniaProperty.UnsetValue;
+            if (_mode == BindingMode.OneTime)
+            {
+                // In OneTime mode, only changing the data context updates the binding.
+                if (!_shouldUpdateOneTimeBindingTarget && _nodes[nodeIndex] is not DataContextNodeBase)
+                    return;
+
+                _shouldUpdateOneTimeBindingTarget = false;
+            }
 
             // The leaf node has changed. If the binding mode is not OneWayToSource, publish the
             // value to the target.
@@ -245,10 +251,6 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
                     null;
                 ConvertAndPublishValue(value, error);
             }
-
-            // If the binding mode is OneTime, then stop the binding if a valid value was published.
-            if (_mode == BindingMode.OneTime && GetValue() != AvaloniaProperty.UnsetValue)
-                Stop();
         }
         else if (_mode == BindingMode.OneWayToSource && nodeIndex == _nodes.Count - 2 && value is not null)
         {
@@ -260,6 +262,9 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         }
         else
         {
+            if (_mode == BindingMode.OneTime && _nodes[nodeIndex] is DataContextNodeBase)
+                _shouldUpdateOneTimeBindingTarget = true;
+
             _nodes[nodeIndex + 1].SetSource(value, dataValidationError);
         }
     }

tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.GetValue.cs

@@ -95,13 +95,13 @@ public abstract partial class BindingExpressionTests
     }
 
     [Fact]
-    public void TargetNullValue_Should_Not_Be_Used_When_Source_Is_Data_Context_And_Null()
+    public void TargetNullValue_Should_Be_Used_When_Source_Is_Data_Context_And_Null()
     {
         var target = CreateTarget<string?, string?>(
             o => o,
             targetNullValue: "bar");
 
-        Assert.Equal(null, target.String);
+        Assert.Equal("bar", target.String);
     }
 
     [Fact]
@@ -151,4 +151,16 @@ public abstract partial class BindingExpressionTests
 
         Assert.Equal("fooBar", target.String);
     }
+
+    [Fact]
+    public void Should_Use_Converter_For_Null_DataContext_Without_Path()
+    {
+        var converter = new PrefixConverter();
+        var target = CreateTarget<string?, string?>(
+            o => o,
+            converter: converter,
+            converterParameter: "foo");
+
+        Assert.Equal("foo", target.String);
+    }
 }

tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.Mode.cs

@@ -1,6 +1,5 @@
 using Avalonia.Data;
 using Xunit;
-using Xunit.Sdk;
 
 #nullable enable
 
@@ -9,21 +8,64 @@ namespace Avalonia.Base.UnitTests.Data.Core;
 public partial class BindingExpressionTests
 {
     [Fact]
-    public void OneTime_Binding_Sets_Target_Only_Once()
+    public void OneTime_Binding_Sets_Target_Only_Once_If_Data_Context_Does_Not_Change()
     {
-        var data = new ViewModel { StringValue = "foo" };
-        var target = CreateTargetWithSource(data, x => x.StringValue, mode: BindingMode.OneTime);
+        var data = new ViewModel { Next = new ViewModel { StringValue = "foo" } };
+        var target = CreateTarget<ViewModel, string?>(x => x.Next!.StringValue, mode: BindingMode.OneTime);
+        target.DataContext = data;
 
         Assert.Equal("foo", target.String);
 
-        data.StringValue = "bar";
+        data.Next!.StringValue = "bar";
+        Assert.Equal("foo", target.String);
+
+        data.Next = new ViewModel { StringValue = "baz" };
+        Assert.Equal("foo", target.String);
+    }
+
+    [Fact]
+    public void OneTime_Binding_With_Simple_Path_Sets_Target_When_Data_Context_Changes()
+    {
+        var data1 = new ViewModel { StringValue = "foo" };
+        var target = CreateTarget<ViewModel, string?>(x => x.StringValue, mode: BindingMode.OneTime);
+        target.DataContext = data1;
+
         Assert.Equal("foo", target.String);
+
+        var data2 = new ViewModel { StringValue = "bar" };
+        target.DataContext = data2;
+        Assert.Equal("bar", target.String);
+    }
+
+    [Fact]
+    public void OneTime_Binding_With_Complex_Path_Sets_Target_When_Data_Context_Changes()
+    {
+        var data1 = new ViewModel { Next = new ViewModel { StringValue = "foo" } };
+        var target = CreateTarget<ViewModel, string?>(x => x.Next!.StringValue, mode: BindingMode.OneTime);
+        target.DataContext = data1;
+
+        Assert.Equal("foo", target.String);
+
+        var data2 = new ViewModel { Next = new ViewModel { StringValue = "bar" } };
+        target.DataContext = data2;
+        Assert.Equal("bar", target.String);
+    }
+
+    [Fact]
+    public void OneTime_Binding_Without_Path_Sets_Target_When_Data_Context_Changes()
+    {
+        var target = CreateTarget<string, string?>(x => x, mode: BindingMode.OneTime);
+        target.DataContext = "foo";
+
+        Assert.Equal("foo", target.String);
+
+        target.DataContext = "bar";
+        Assert.Equal("bar", target.String);
     }
 
     [Fact]
     public void OneTime_Binding_Waits_For_DataContext()
     {
-        var data = new ViewModel { StringValue = "foo" };
         var target = CreateTarget<ViewModel, string?>(
             x => x.StringValue,
             mode: BindingMode.OneTime);

tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs

@@ -151,21 +151,6 @@ namespace Avalonia.Markup.UnitTests.Data
             Assert.Equal("bar", source.Foo);
         }
 
-        [Fact]
-        public void OneTime_Binding_Releases_Subscription_If_DataContext_Set_Later()
-        {
-            var target = new TextBlock();
-            var source = new Source { Foo = "foo" };
-
-            target.Bind(TextBlock.TextProperty, new Binding("Foo", BindingMode.OneTime));
-            target.DataContext = source;
-
-            // Forces WeakEvent compact
-            Dispatcher.UIThread.RunJobs();
-
-            Assert.Equal(0, source.SubscriberCount);
-        }
-
         [Fact]
         public void OneWayToSource_Binding_Should_Be_Set_Up()
         {