Update binding anchor to be plain object.

Because e.g. a Style can be an anchor but is not an IPerspexObject.

src/Markup/Perspex.Markup.Xaml/Data/Binding.cs

@@ -61,20 +61,11 @@ namespace Perspex.Markup.Xaml.Data
         /// </summary>
         public object Source { get; set; }
 
-        /// <summary>
-        /// Creates a subject that can be used to get and set the value of the binding.
-        /// </summary>
-        /// <param name="target">The target instance.</param>
-        /// <param name="targetProperty">The target property. May be null.</param>
-        /// <param name="treeAnchor">
-        /// For `ElementName` bindings to elements that are not themselves controls, describes
-        /// where in the logical tree to begin searching for the named element.
-        /// </param>
-        /// <returns>An <see cref="ISubject{Object}"/>.</returns>
+        /// <inheritdoc/>
         public ISubject<object> CreateSubject(
             IPerspexObject target, 
             PerspexProperty targetProperty,
-            IPerspexObject treeAnchor = null)
+            object anchor = null)
         {
             Contract.Requires<ArgumentNullException>(target != null);
 
@@ -86,7 +77,7 @@ namespace Perspex.Markup.Xaml.Data
             if (pathInfo.ElementName != null || ElementName != null)
             {
                 observer = CreateElementObserver(
-                    (target as IControl) ?? (treeAnchor as IControl),
+                    (target as IControl) ?? (anchor as IControl),
                     pathInfo.ElementName ?? ElementName, 
                     pathInfo.Path);
             }

src/Markup/Perspex.Markup.Xaml/Data/MultiBinding.cs

@@ -49,35 +49,11 @@ namespace Perspex.Markup.Xaml.Data
         /// </summary>
         public RelativeSource RelativeSource { get; set; }
 
-        /// <summary>
-        /// Applies the binding to a property on an instance.
-        /// </summary>
-        /// <param name="instance">The target instance.</param>
-        /// <param name="property">The target property.</param>
-        public void Bind(IPerspexObject instance, PerspexProperty property)
-        {
-            var subject = CreateSubject(instance, property);
-
-            if (subject != null)
-            {
-                Bind(instance, property, subject);
-            }
-        }
-
-        /// <summary>
-        /// Creates a subject that can be used to get and set the value of the binding.
-        /// </summary>
-        /// <param name="target">The target instance.</param>
-        /// <param name="targetProperty">The target property. May be null.</param>
-        /// <param name="treeAnchor">
-        /// For `ElementName` bindings to elements that are not themselves controls, describes
-        /// where in the logical tree to begin searching for the named element.
-        /// </param>
-        /// <returns>An <see cref="ISubject{Object}"/>.</returns>
+        /// <inheritdoc/>
         public ISubject<object> CreateSubject(
             IPerspexObject target,
             PerspexProperty targetProperty,
-            IPerspexObject treeAnchor = null)
+            object anchor = null)
         {
             if (Converter == null)
             {

src/Markup/Perspex.Markup.Xaml/Data/StyleResourceBinding.cs

@@ -37,22 +37,22 @@ namespace Perspex.Markup.Xaml.Data
         public ISubject<object> CreateSubject(
             IPerspexObject target,
             PerspexProperty targetProperty,
-            IPerspexObject treeAnchor = null)
+            object anchor = null)
         {
-            return new Subject(target, Name, treeAnchor);
+            return new Subject(target, Name, anchor);
         }
 
         private class Subject : ISubject<object>
         {
             private IPerspexObject _target;
             private string _name;
-            private IPerspexObject _treeAnchor;
+            private object _anchor;
 
-            public Subject(IPerspexObject target, string name, IPerspexObject treeAnchor)
+            public Subject(IPerspexObject target, string name, object anchor)
             {
                 _target = target;
                 _name = name;
-                _treeAnchor = treeAnchor;
+                this._anchor = anchor;
             }
 
             public void OnCompleted()
@@ -69,7 +69,7 @@ namespace Perspex.Markup.Xaml.Data
 
             public IDisposable Subscribe(IObserver<object> observer)
             {
-                var host = (_target as IControl) ?? (_treeAnchor as IControl);
+                var host = (_target as IControl) ?? (_anchor as IControl);
 
                 if (host != null)
                 {

src/Perspex.Base/Data/IBinding.cs

@@ -25,14 +25,16 @@ namespace Perspex.Data
         /// </summary>
         /// <param name="target">The target instance.</param>
         /// <param name="targetProperty">The target property. May be null.</param>
-        /// <param name="treeAnchor">
-        /// For `ElementName` bindings to elements that are not themselves controls, describes
-        /// where in the logical tree to begin searching for the named element.
-        /// </param>
+        /// <param name="anchor">An optional anchor from which to locate required context.</param>
         /// <returns>An <see cref="ISubject{Object}"/>.</returns>
+        /// <remarks>
+        /// When binding to objects that are not in the logical tree, certain types of binding need
+        /// an anchor into the tree in order to locate named controls or resources. The
+        /// <paramref name="anchor"/> parameter can be used to provice this context.
+        /// </remarks>
         ISubject<object> CreateSubject(
             IPerspexObject target, 
             PerspexProperty targetProperty,
-            IPerspexObject treeAnchor = null);
+            object anchor = null);
     }
 }