From 58db340d9a72454eba86828cdfef8dcadc26e72c Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Fri, 24 Nov 2023 17:06:41 +0100
Subject: [PATCH] Added BindingExpressionBase.UpdateTarget.

---
 .../Data/BindingExpressionBase.cs             |  6 +++++
 .../Data/Core/BindingExpression.cs            | 13 +++++++++++
 .../Core/ExpressionNodes/ExpressionNode.cs    |  5 ++++-
 .../Data/Core/BindingExpressionTests.cs       | 22 +++++++++++++++++++
 4 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Base/Data/BindingExpressionBase.cs b/src/Avalonia.Base/Data/BindingExpressionBase.cs
index ad411130cf..24bed9ddd5 100644
--- a/src/Avalonia.Base/Data/BindingExpressionBase.cs
+++ b/src/Avalonia.Base/Data/BindingExpressionBase.cs
@@ -14,8 +14,14 @@ public abstract class BindingExpressionBase : IDisposable, ISetterInstance
 
     public virtual void Dispose()
     {
+        GC.SuppressFinalize(this);
     }
 
+    /// <summary>
+    /// Forces a data transfer from the binding source to the binding target.
+    /// </summary>
+    public virtual void UpdateTarget() { }
+
     /// <summary>
     /// When overridden in a derived class, attaches the binding expression to a value store but
     /// does not start it.
diff --git a/src/Avalonia.Base/Data/Core/BindingExpression.cs b/src/Avalonia.Base/Data/Core/BindingExpression.cs
index 1203b3d0e3..f4fd076f90 100644
--- a/src/Avalonia.Base/Data/Core/BindingExpression.cs
+++ b/src/Avalonia.Base/Data/Core/BindingExpression.cs
@@ -131,6 +131,19 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
     public ExpressionNode LeafNode => _nodes[_nodes.Count - 1];
     public string? StringFormat => _uncommon?._stringFormat;
 
+    public override void UpdateTarget()
+    {
+        if (_nodes.Count == 0)
+            return;
+
+        var source = _nodes[0].Source;
+
+        for (var i = 0; i < _nodes.Count; ++i)
+            _nodes[i].SetSource(null, null);
+
+        _nodes[0].SetSource(source, null);
+    }
+
     /// <summary>
     /// Creates an <see cref="BindingExpression"/> from an expression tree.
     /// </summary>
diff --git a/src/Avalonia.Base/Data/Core/ExpressionNodes/ExpressionNode.cs b/src/Avalonia.Base/Data/Core/ExpressionNodes/ExpressionNode.cs
index fc86213373..150502d67a 100644
--- a/src/Avalonia.Base/Data/Core/ExpressionNodes/ExpressionNode.cs
+++ b/src/Avalonia.Base/Data/Core/ExpressionNodes/ExpressionNode.cs
@@ -127,6 +127,9 @@ internal abstract class ExpressionNode
         if (source == AvaloniaProperty.UnsetValue)
             source = null;
 
+        if (source == oldSource)
+            return;
+
         if (oldSource is not null)
             Unsubscribe(oldSource);
 
@@ -139,7 +142,7 @@ internal abstract class ExpressionNode
             // binding change.
             _value = BindingExpression._nullReference;
         }
-        else if (source != oldSource)
+        else
         {
             try { OnSourceChanged(source, dataValidationError); }
             catch (Exception e) { SetError(e); }
diff --git a/tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.cs b/tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.cs
index 094c3fc708..a7707c526d 100644
--- a/tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.cs
+++ b/tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.cs
@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Globalization;
 using System.Reactive.Linq;
 using System.Threading.Tasks;
+using Avalonia.Controls;
 using Avalonia.Data;
 using Avalonia.Data.Converters;
 using Avalonia.Data.Core;
@@ -271,6 +272,22 @@ namespace Avalonia.Base.UnitTests.Data.Core
             GC.KeepAlive(data);
         }
 
+        [Fact]
+        public void Can_Use_UpdateTarget_To_Update_From_Non_INPC_Data()
+        {
+            var data = new NonInpcClass { StringValue = "foo" };
+            var target = new Control { DataContext = data };
+            var expression = target.Bind(Control.TagProperty, new Binding(nameof(data.StringValue)));
+
+            Assert.Equal("foo", target.Tag);
+
+            data.StringValue = "bar";
+            Assert.Equal("foo", target.Tag);
+
+            expression.UpdateTarget();
+            Assert.Equal("bar", target.Tag);
+        }
+
         private class Class1 : NotifyingBase
         {
             private string _stringValue;
@@ -289,6 +306,11 @@ namespace Avalonia.Base.UnitTests.Data.Core
             }
         }
 
+        private class NonInpcClass
+        {
+            public string StringValue { get; set; }
+        }
+
         private class TargetProperties : AvaloniaObject
         {
             public static readonly StyledProperty<double> DoubleProperty =