Make template bindings complete on Template change.

This fixes a leak - needs to be done in XAML as well though.

src/Perspex.Base/BindingDescriptor.cs

@@ -78,6 +78,18 @@ namespace Perspex
             set;
         }
 
+        /// <summary>
+        /// Gets or sets the source observable.
+        /// </summary>
+        /// <remarks>
+        /// If null, then <see cref="Source"/>.<see cref="Property"/> will be used.
+        /// </remarks>
+        public IObservable<object> SourceObservable
+        {
+            get;
+            set;
+        }
+
         /// <summary>
         /// Gets a description of the binding.
         /// </summary>
@@ -128,7 +140,7 @@ namespace Perspex
         /// <inheritdoc/>
         protected override IDisposable SubscribeCore(IObserver<object> observer)
         {
-            return Source.GetObservable(Property).Subscribe(observer);
+            return (SourceObservable ?? Source.GetObservable(Property)).Subscribe(observer);
         }
     }
 }

src/Perspex.Base/PerspexObject.cs

@@ -163,13 +163,7 @@ namespace Perspex
         {
             get
             {
-                return new BindingDescriptor
-                {
-                    Mode = binding.Mode,
-                    Priority = binding.Priority,
-                    Property = binding.Property,
-                    Source = this,
-                };
+                return CreateBindingDescriptor(binding);
             }
 
             set
@@ -203,6 +197,17 @@ namespace Perspex
             }
         }
 
+        protected virtual BindingDescriptor CreateBindingDescriptor(BindingDescriptor source)
+        {
+            return new BindingDescriptor
+            {
+                Mode = source.Mode,
+                Priority = source.Priority,
+                Property = source.Property,
+                Source = this,
+            };
+        }
+
         public bool CheckAccess() => Dispatcher.UIThread.CheckAccess();
 
         public void VerifyAccess() => Dispatcher.UIThread.VerifyAccess();

src/Perspex.Controls/Primitives/TemplatedControl.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Linq;
+using System.Reactive.Linq;
 using Perspex.Controls.Presenters;
 using Perspex.Controls.Templates;
 using Perspex.Media;
@@ -211,6 +212,22 @@ namespace Perspex.Controls.Primitives
             }
         }
 
+        protected sealed override BindingDescriptor CreateBindingDescriptor(BindingDescriptor source)
+        {
+            var result = base.CreateBindingDescriptor(source);
+
+            // If the binding is a template binding, then complete when the Template changes.
+            if (source.Priority == BindingPriority.TemplatedParent)
+            {
+                var templateChanged = GetObservable(TemplateProperty).Skip(1);
+
+                result.SourceObservable = result.Source.GetObservable(result.Property)
+                    .TakeUntil(templateChanged);
+            }
+
+            return result;
+        }
+
         /// <summary>
         /// Called when the control's template is applied.
         /// </summary>