From 7b9a6f29c98e1da7241e7a46213c8a7866dbca4e Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Sun, 6 Mar 2016 16:01:25 +0100
Subject: [PATCH] Added GetWeakObservable().

And use in in bindings.
---
 .../Plugins/PerspexPropertyAccessorPlugin.cs  |  2 +-
 src/Perspex.Base/PerspexObjectExtensions.cs   | 17 ++++
 .../PerspexPropertyChangedEventArgs.cs        |  3 +-
 .../Reactive/PerspexObservable.cs             |  2 +-
 .../Reactive/WeakPropertyChangedObservable.cs | 85 +++++++++++++++++++
 5 files changed, 106 insertions(+), 3 deletions(-)
 create mode 100644 src/Perspex.Base/Reactive/WeakPropertyChangedObservable.cs

diff --git a/src/Markup/Perspex.Markup/Data/Plugins/PerspexPropertyAccessorPlugin.cs b/src/Markup/Perspex.Markup/Data/Plugins/PerspexPropertyAccessorPlugin.cs
index b8e7cadfc1..61f05ce4f2 100644
--- a/src/Markup/Perspex.Markup/Data/Plugins/PerspexPropertyAccessorPlugin.cs
+++ b/src/Markup/Perspex.Markup/Data/Plugins/PerspexPropertyAccessorPlugin.cs
@@ -74,7 +74,7 @@ namespace Perspex.Markup.Data.Plugins
 
                 _reference = reference;
                 _property = property;
-                _subscription = Instance.GetObservable(property).Skip(1).Subscribe(changed);
+                _subscription = Instance.GetWeakObservable(property).Skip(1).Subscribe(changed);
             }
 
             public PerspexObject Instance
diff --git a/src/Perspex.Base/PerspexObjectExtensions.cs b/src/Perspex.Base/PerspexObjectExtensions.cs
index dc0a6b6465..fcb4e8dc32 100644
--- a/src/Perspex.Base/PerspexObjectExtensions.cs
+++ b/src/Perspex.Base/PerspexObjectExtensions.cs
@@ -164,6 +164,23 @@ namespace Perspex
             return result;
         }
 
+        /// <summary>
+        /// Gets a weak observable for a <see cref="PerspexProperty"/>.
+        /// </summary>
+        /// <param name="o">The object.</param>
+        /// <param name="property">The property.</param>
+        /// <returns>An observable.</returns>
+        public static IObservable<object> GetWeakObservable(this IPerspexObject o, PerspexProperty property)
+        {
+            Contract.Requires<ArgumentNullException>(o != null);
+            Contract.Requires<ArgumentNullException>(property != null);
+
+            return new WeakPropertyChangedObservable(
+                new WeakReference<IPerspexObject>(o), 
+                property, 
+                GetDescription(o, property));
+        }
+
         /// <summary>
         /// Binds a property on an <see cref="IPerspexObject"/> to an <see cref="IBinding"/>.
         /// </summary>
diff --git a/src/Perspex.Base/PerspexPropertyChangedEventArgs.cs b/src/Perspex.Base/PerspexPropertyChangedEventArgs.cs
index 1136b5ce2a..4563278acc 100644
--- a/src/Perspex.Base/PerspexPropertyChangedEventArgs.cs
+++ b/src/Perspex.Base/PerspexPropertyChangedEventArgs.cs
@@ -1,6 +1,7 @@
 // Copyright (c) The Perspex Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
 using Perspex.Data;
 
 namespace Perspex
@@ -8,7 +9,7 @@ namespace Perspex
     /// <summary>
     /// Provides information for a perspex property change.
     /// </summary>
-    public class PerspexPropertyChangedEventArgs
+    public class PerspexPropertyChangedEventArgs : EventArgs
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="PerspexPropertyChangedEventArgs"/> class.
diff --git a/src/Perspex.Base/Reactive/PerspexObservable.cs b/src/Perspex.Base/Reactive/PerspexObservable.cs
index bd76f544d4..a939bf33b5 100644
--- a/src/Perspex.Base/Reactive/PerspexObservable.cs
+++ b/src/Perspex.Base/Reactive/PerspexObservable.cs
@@ -11,7 +11,7 @@ namespace Perspex.Reactive
     /// An <see cref="IObservable{T}"/> with an additional description.
     /// </summary>
     /// <typeparam name="T">The type of the elements in the sequence.</typeparam>
-    public sealed class PerspexObservable<T> : ObservableBase<T>, IDescription
+    public class PerspexObservable<T> : ObservableBase<T>, IDescription
     {
         private readonly Func<IObserver<T>, IDisposable> _subscribe;
 
diff --git a/src/Perspex.Base/Reactive/WeakPropertyChangedObservable.cs b/src/Perspex.Base/Reactive/WeakPropertyChangedObservable.cs
new file mode 100644
index 0000000000..117fcc65ad
--- /dev/null
+++ b/src/Perspex.Base/Reactive/WeakPropertyChangedObservable.cs
@@ -0,0 +1,85 @@
+// Copyright (c) The Perspex 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 Perspex.Utilities;
+
+namespace Perspex.Reactive
+{
+    internal class WeakPropertyChangedObservable : ObservableBase<object>, 
+        IWeakSubscriber<PerspexPropertyChangedEventArgs>, IDescription
+    {
+        private WeakReference<IPerspexObject> _sourceReference;
+        private readonly PerspexProperty _property;
+        private readonly Subject<object> _changed = new Subject<object>();
+
+        private int _count;
+
+        public WeakPropertyChangedObservable(
+            WeakReference<IPerspexObject> source, 
+            PerspexProperty property, 
+            string description)
+        {
+            _sourceReference = source;
+            _property = property;
+            Description = description;
+        }
+
+        public string Description { get; }
+
+        public void OnEvent(object sender, PerspexPropertyChangedEventArgs e)
+        {
+            if (e.Property == _property)
+            {
+                _changed.OnNext(e.NewValue);
+            }
+        }
+
+        protected override IDisposable SubscribeCore(IObserver<object> observer)
+        {
+            IPerspexObject 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)
+            {
+                IPerspexObject instance;
+
+                if (_sourceReference.TryGetTarget(out instance))
+                {
+                    WeakSubscriptionManager.Unsubscribe(
+                    instance,
+                    nameof(instance.PropertyChanged),
+                    this);
+                }
+            }
+        }
+    }
+}