Use custom rx for observing properties.

src/Avalonia.Base/AvaloniaObjectExtensions.cs

@@ -41,27 +41,7 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(o != null);
             Contract.Requires<ArgumentNullException>(property != null);
 
-            return new AvaloniaObservable<object>(
-                observer =>
-                {
-                    EventHandler<AvaloniaPropertyChangedEventArgs> handler = (s, e) =>
-                    {
-                        if (e.Property == property)
-                        {
-                            observer.OnNext(e.NewValue);
-                        }
-                    };
-
-                    observer.OnNext(o.GetValue(property));
-
-                    o.PropertyChanged += handler;
-
-                    return Disposable.Create(() =>
-                    {
-                        o.PropertyChanged -= handler;
-                    });
-                },
-                GetDescription(o, property));
+            return new AvaloniaPropertyObservable<object>(o, property);
         }
 
         /// <summary>
@@ -79,7 +59,7 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(o != null);
             Contract.Requires<ArgumentNullException>(property != null);
 
-            return o.GetObservable((AvaloniaProperty)property).Cast<T>();
+            return new AvaloniaPropertyObservable<T>(o, property);
         }
 
         /// <summary>
@@ -100,25 +80,9 @@ namespace Avalonia
             Contract.Requires<ArgumentNullException>(o != null);
             Contract.Requires<ArgumentNullException>(property != null);
 
-            return new AvaloniaObservable<Tuple<T, T>>(
-                observer =>
-                {
-                    EventHandler<AvaloniaPropertyChangedEventArgs> handler = (s, e) =>
-                    {
-                        if (e.Property == property)
-                        {
-                            observer.OnNext(Tuple.Create((T)e.OldValue, (T)e.NewValue));
-                        }
-                    };
-
-                    o.PropertyChanged += handler;
-
-                    return Disposable.Create(() =>
-                    {
-                        o.PropertyChanged -= handler;
-                    });
-                },
-                GetDescription(o, property));
+            return new AvaloniaPropertyObservable<T>(o, property)
+                .Buffer(2, 1)
+                .Select(x => Tuple.Create(x[0], x[1]));
         }
 
         /// <summary>

src/Avalonia.Base/Reactive/AvaloniaObservable.cs

@@ -1,42 +0,0 @@
-// Copyright (c) The Avalonia Project. All rights reserved.
-// Licensed under the MIT license. See licence.md file in the project root for full license information.
-
-using System;
-using System.Reactive;
-using System.Reactive.Disposables;
-
-namespace Avalonia.Reactive
-{
-    /// <summary>
-    /// An <see cref="IObservable{T}"/> with an additional description.
-    /// </summary>
-    /// <typeparam name="T">The type of the elements in the sequence.</typeparam>
-    public class AvaloniaObservable<T> : ObservableBase<T>, IDescription
-    {
-        private readonly Func<IObserver<T>, IDisposable> _subscribe;
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="AvaloniaObservable{T}"/> class.
-        /// </summary>
-        /// <param name="subscribe">The subscribe function.</param>
-        /// <param name="description">The description of the observable.</param>
-        public AvaloniaObservable(Func<IObserver<T>, IDisposable> subscribe, string description)
-        {
-            Contract.Requires<ArgumentNullException>(subscribe != null);            
-
-            _subscribe = subscribe;
-            Description = description;
-        }
-
-        /// <summary>
-        /// Gets the description of the observable.
-        /// </summary>
-        public string Description { get; }
-
-        /// <inheritdoc/>
-        protected override IDisposable SubscribeCore(IObserver<T> observer)
-        {
-            return _subscribe(observer) ?? Disposable.Empty;
-        }
-    }
-}

src/Avalonia.Base/Reactive/AvaloniaPropertyObservable.cs

@@ -0,0 +1,46 @@
+using System;
+
+namespace Avalonia.Reactive
+{
+    public class AvaloniaPropertyObservable<T> : LightweightObservableBase<T>, IDescription
+    {
+        private readonly IAvaloniaObject _target;
+        private readonly AvaloniaProperty _property;
+        private T _value;
+
+        public AvaloniaPropertyObservable(
+            IAvaloniaObject target,
+            AvaloniaProperty property)
+        {
+            _target = target;
+            _property = property;
+        }
+
+        public string Description => $"{_target.GetType().Name}.{_property.Name}";
+
+        protected override void Initialize()
+        {
+            _value = (T)_target.GetValue(_property);
+            _target.PropertyChanged += PropertyChanged;
+        }
+
+        protected override void Deinitialize()
+        {
+            _target.PropertyChanged -= PropertyChanged;
+        }
+
+        protected override void Subscribed(IObserver<T> observer, bool first)
+        {
+            observer.OnNext(_value);
+        }
+
+        private void PropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.Property == _property)
+            {
+                _value = (T)e.NewValue;
+                PublishNext(_value);
+            }
+        }
+    }
+}