Make AvaloniaPropertyObservable use weak refs.

And remove `WeakPropertyChangedObservable`/`GetWeakObservable`.

src/Avalonia.Base/AvaloniaObjectExtensions.cs

@@ -36,6 +36,9 @@ namespace Avalonia
         /// An observable which fires immediately with the current value of the property on the
         /// object and subsequently each time the property value changes.
         /// </returns>
+        /// <remarks>
+        /// The subscription to <paramref name="o"/> is created using a weak reference.
+        /// </remarks>
         public static IObservable<object> GetObservable(this IAvaloniaObject o, AvaloniaProperty property)
         {
             Contract.Requires<ArgumentNullException>(o != null);
@@ -54,6 +57,9 @@ namespace Avalonia
         /// An observable which fires immediately with the current value of the property on the
         /// object and subsequently each time the property value changes.
         /// </returns>
+        /// <remarks>
+        /// The subscription to <paramref name="o"/> is created using a weak reference.
+        /// </remarks>
         public static IObservable<T> GetObservable<T>(this IAvaloniaObject o, AvaloniaProperty<T> property)
         {
             Contract.Requires<ArgumentNullException>(o != null);
@@ -130,23 +136,6 @@ namespace Avalonia
                 o.GetObservable(property));
         }
 
-        /// <summary>
-        /// Gets a weak observable for a <see cref="AvaloniaProperty"/>.
-        /// </summary>
-        /// <param name="o">The object.</param>
-        /// <param name="property">The property.</param>
-        /// <returns>An observable.</returns>
-        public static IObservable<object> GetWeakObservable(this IAvaloniaObject o, AvaloniaProperty property)
-        {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
-
-            return new WeakPropertyChangedObservable(
-                new WeakReference<IAvaloniaObject>(o), 
-                property, 
-                GetDescription(o, property));
-        }
-
         /// <summary>
         /// Binds a property on an <see cref="IAvaloniaObject"/> to an <see cref="IBinding"/>.
         /// </summary>

src/Avalonia.Base/Data/Core/Plugins/AvaloniaPropertyAccessorPlugin.cs

@@ -153,7 +153,7 @@ namespace Avalonia.Data.Core.Plugins
 
             protected override void SubscribeCore(IObserver<object> observer)
             {
-                _subscription = Instance?.GetWeakObservable(_property).Subscribe(observer);
+                _subscription = Instance?.GetObservable(_property).Subscribe(observer);
             }
         }
     }

src/Avalonia.Base/Reactive/AvaloniaPropertyObservable.cs

@@ -1,10 +1,11 @@
 using System;
+using Avalonia.Utilities;
 
 namespace Avalonia.Reactive
 {
     public class AvaloniaPropertyObservable<T> : LightweightObservableBase<T>, IDescription
     {
-        private readonly IAvaloniaObject _target;
+        private readonly WeakReference<IAvaloniaObject> _target;
         private readonly AvaloniaProperty _property;
         private T _value;
 
@@ -12,7 +13,7 @@ namespace Avalonia.Reactive
             IAvaloniaObject target,
             AvaloniaProperty property)
         {
-            _target = target;
+            _target = new WeakReference<IAvaloniaObject>(target);
             _property = property;
         }
 
@@ -20,13 +21,19 @@ namespace Avalonia.Reactive
 
         protected override void Initialize()
         {
-            _value = (T)_target.GetValue(_property);
-            _target.PropertyChanged += PropertyChanged;
+            if (_target.TryGetTarget(out var target))
+            {
+                _value = (T)target.GetValue(_property);
+                target.PropertyChanged += PropertyChanged;
+            }
         }
 
         protected override void Deinitialize()
         {
-            _target.PropertyChanged -= PropertyChanged;
+            if (_target.TryGetTarget(out var target))
+            {
+                target.PropertyChanged -= PropertyChanged;
+            }
         }
 
         protected override void Subscribed(IObserver<T> observer, bool first)

src/Avalonia.Base/Reactive/WeakPropertyChangedObservable.cs

@@ -1,85 +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;
-using System.Reactive.Linq;
-using System.Reactive.Subjects;
-using Avalonia.Utilities;
-
-namespace Avalonia.Reactive
-{
-    internal class WeakPropertyChangedObservable : ObservableBase<object>, 
-        IWeakSubscriber<AvaloniaPropertyChangedEventArgs>, IDescription
-    {
-        private WeakReference<IAvaloniaObject> _sourceReference;
-        private readonly AvaloniaProperty _property;
-        private readonly Subject<object> _changed = new Subject<object>();
-
-        private int _count;
-
-        public WeakPropertyChangedObservable(
-            WeakReference<IAvaloniaObject> source, 
-            AvaloniaProperty property, 
-            string description)
-        {
-            _sourceReference = source;
-            _property = property;
-            Description = description;
-        }
-
-        public string Description { get; }
-
-        public void OnEvent(object sender, AvaloniaPropertyChangedEventArgs e)
-        {
-            if (e.Property == _property)
-            {
-                _changed.OnNext(e.NewValue);
-            }
-        }
-
-        protected override IDisposable SubscribeCore(IObserver<object> observer)
-        {
-            IAvaloniaObject instance;
-
-            if (_sourceReference.TryGetTarget(out instance))
-            {
-                if (_count++ == 0)
-                {
-                    WeakSubscriptionManager.Subscribe(
-                        instance, 
-                        nameof(instance.PropertyChanged), 
-                        this);
-                }
-
-                observer.OnNext(instance.GetValue(_property));
-
-                return Observable.Using(() => Disposable.Create(DecrementCount), _ => _changed)
-                    .Subscribe(observer);
-            }
-            else
-            {
-                _changed.OnCompleted();
-                observer.OnCompleted();
-                return Disposable.Empty;
-            }
-        }
-
-        private void DecrementCount()
-        {
-            if (--_count == 0)
-            {
-                IAvaloniaObject instance;
-
-                if (_sourceReference.TryGetTarget(out instance))
-                {
-                    WeakSubscriptionManager.Unsubscribe(
-                    instance,
-                    nameof(instance.PropertyChanged),
-                    this);
-                }
-            }
-        }
-    }
-}