Remove hack from PropertySetterBindingInstance.

src/Avalonia.Base/PropertyStore/BindingEntry.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Diagnostics;
 using System.Reactive.Disposables;
 using Avalonia.Data;
 
@@ -10,7 +9,6 @@ namespace Avalonia.PropertyStore
         IDisposable
     {
         private readonly ValueFrame _frame;
-        private readonly IObservable<object?> _source;
         private IDisposable? _subscription;
         private bool _hasValue;
         private object? _value;
@@ -21,7 +19,7 @@ namespace Avalonia.PropertyStore
             IObservable<object?> source)
         {
             _frame = frame;
-            _source = source;
+            Source = source;
             Property = property;
         }
 
@@ -35,6 +33,7 @@ namespace Avalonia.PropertyStore
         }
 
         public AvaloniaProperty Property { get; }
+        protected IObservable<object?> Source { get; }
 
         public void Dispose()
         {
@@ -69,6 +68,18 @@ namespace Avalonia.PropertyStore
             _subscription = null;
         }
 
+        protected virtual void Start(bool produceValue)
+        {
+            if (_subscription is not null)
+                return;
+
+            // Will only produce a new value when subscription isn't null.
+            if (produceValue)
+                _subscription = Disposable.Empty;
+
+            _subscription = Source.Subscribe(this);
+        }
+
         private void ClearValue()
         {
             if (_hasValue)
@@ -123,17 +134,5 @@ namespace Avalonia.PropertyStore
             _subscription = null;
             _frame.OnBindingCompleted(this);
         }
-
-        private void Start(bool produceValue)
-        {
-            if (_subscription is not null)
-                return;
-
-            // Will only produce a new value when subscription isn't null.
-            if (produceValue)
-                _subscription = Disposable.Empty;
-
-            _subscription = _source.Subscribe(this);
-        }
     }
 }

src/Avalonia.Base/Styling/PropertySetterBindingInstance.cs

@@ -7,7 +7,9 @@ namespace Avalonia.Styling
 {
     internal class PropertySetterBindingInstance : BindingEntry, ISetterInstance
     {
-        private readonly IDisposable? _twoWaySubscription;
+        private readonly AvaloniaObject _target;
+        private readonly BindingMode _mode;
+        private IDisposable? _twoWaySubscription;
 
         public PropertySetterBindingInstance(
             AvaloniaObject target,
@@ -17,18 +19,14 @@ namespace Avalonia.Styling
             IObservable<object?> source)
             : base(instance, property, source)
         {
-            if (mode == BindingMode.TwoWay)
+            _target = target;
+            _mode = mode;
+
+            if (mode == BindingMode.TwoWay &&
+                source is not IObserver<object?>)
             {
-                // TODO: HUGE HACK FIXME
+                throw new NotSupportedException(
-                if (source is IObserver<object?> observer)
+                    "Attempting to bind two-way with a binding source which doesn't support it.");
-                {
-                    _twoWaySubscription = target.GetObservable(property).Skip(1).Subscribe(observer);
-                }
-                else
-                {
-                    throw new NotSupportedException(
-                        "Attempting to bind two-way with a binding source which doesn't support it.");
-                }
             }
         }
 
@@ -37,5 +35,16 @@ namespace Avalonia.Styling
             _twoWaySubscription?.Dispose();
             base.Unsubscribe();
         }
+
+        protected override void Start(bool produceValue)
+        {
+            if (_mode == BindingMode.TwoWay)
+            {
+                var observer = (IObserver<object?>)Source;
+                _twoWaySubscription = _target.GetObservable(Property).Skip(1).Subscribe(observer);
+            }
+
+            base.Start(produceValue);
+        }
     }
 }