Another tweak to the binding system.

Binding levels with a an even number priority use the last fired binding
for the current value, those with odd number priorities use the latest
added.

Perspex.Base/PerspexObject.cs

@@ -6,6 +6,7 @@
 
 namespace Perspex
 {
+    using Perspex.Reactive;
     using Splat;
     using System;
     using System.Collections.Generic;
@@ -14,8 +15,6 @@ namespace Perspex
     using System.Reactive.Disposables;
     using System.Reactive.Linq;
     using System.Reflection;
-    using Perspex.Diagnostics;
-    using Perspex.Reactive;
 
     /// <summary>
     /// The priority of a binding.
@@ -25,12 +24,12 @@ namespace Perspex
         /// <summary>
         /// A value that comes from an animation.
         /// </summary>
-        Animation = -1,
+        Animation = -2,
 
         /// <summary>
         /// A local value.
         /// </summary>
-        LocalValue,
+        LocalValue = 0,
 
         /// <summary>
         /// A triggered style binding.

Perspex.Base/PriorityLevel.cs

@@ -10,6 +10,22 @@ namespace Perspex
     using System.Collections.Generic;
     using System.Reactive.Disposables;
 
+    /// <summary>
+    /// Determines how the current binding is selected for a <see cref="PriorityLevel"/>.
+    /// </summary>
+    internal enum LevelPrecedenceMode
+    {
+        /// <summary>
+        /// The latest fired binding is used as the current value.
+        /// </summary>
+        Latest,
+
+        /// <summary>
+        /// The latest added binding is used as the current value.
+        /// </summary>
+        Newest,
+    }
+
     /// <summary>
     /// Stores bindings for a priority level in a <see cref="PriorityValue"/>.
     /// </summary>
@@ -49,6 +65,8 @@ namespace Perspex
         /// </summary>
         private int nextIndex;
 
+        private LevelPrecedenceMode mode;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="PriorityLevel"/> class.
         /// </summary>
@@ -56,10 +74,12 @@ namespace Perspex
         /// <param name="changed">A method to be called when the current value changes.</param>
         public PriorityLevel(
             int priority, 
+            LevelPrecedenceMode mode,
             Action<PriorityLevel> changed)
         {
             Contract.Requires<ArgumentNullException>(changed != null);
 
+            this.mode = mode;
             this.changed = changed;
             this.Priority = priority;
             this.Value = this.directValue = PerspexProperty.UnsetValue;
@@ -136,7 +156,7 @@ namespace Perspex
         /// <param name="entry">The entry that changed.</param>
         private void Changed(PriorityBindingEntry entry)
         {
-            if (entry.Index >= this.ActiveBindingIndex)
+            if (mode == LevelPrecedenceMode.Latest || entry.Index >= this.ActiveBindingIndex)
             {
                 if (entry.Value != PerspexProperty.UnsetValue)
                 {

Perspex.Base/PriorityValue.cs

@@ -195,7 +195,7 @@ namespace Perspex
                 b.AppendLine(level.Value.Value?.ToString() ?? "(null)");
                 b.AppendLine("--------");
                 b.Append("Direct: ");
-                b.AppendLine(level.Value.DirectValue.ToString());
+                b.AppendLine(level.Value.DirectValue?.ToString() ?? "(null)");
 
                 foreach (var binding in level.Value.Bindings)
                 {
@@ -239,7 +239,8 @@ namespace Perspex
 
             if (!this.levels.TryGetValue(priority, out result))
             {
-                result = new PriorityLevel(priority, this.ValueChanged);
+                var mode = (LevelPrecedenceMode)(priority % 2);
+                result = new PriorityLevel(priority, mode, this.ValueChanged);
                 this.levels.Add(priority, result);
             }
 

Perspex.Themes.Default/DropDownStyle.cs

@@ -98,9 +98,8 @@ namespace Perspex.Themes.Default
                             },
                             PlacementTarget = control,
                             StaysOpen = false,
-                            [!Popup.IsOpenProperty] = control[!DropDown.IsDropDownOpenProperty],
-                            [~Popup.MinWidthProperty] = control[~DropDown.BoundsProperty].Cast<Rect>().Select(x => (object)x.Width),
-                        }
+                            [~~Popup.IsOpenProperty] = control[~~DropDown.IsDropDownOpenProperty],
+                            [~Popup.MinWidthProperty] = control[~DropDown.BoundsProperty].Cast<Rect>().Select(x => (object)x.Width),                       }
                     },
                 },
             };