Supply UnsetValue for DirectProperty.

When a direct property is set to UnsetValue, previously it used
default(TValue). Allow that value to be overridden and make it be -1 for
SelectingItemsControl.SelectedIndex.

src/Perspex.Base/DirectPropertyMetadata`1.cs

@@ -0,0 +1,52 @@
+// 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 Perspex.Data;
+
+namespace Perspex
+{
+    /// <summary>
+    /// Metadata for direct perspex properties.
+    /// </summary>
+    public class DirectPropertyMetadata<TValue> : PropertyMetadata, IDirectPropertyMetadata
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="StyledPropertyMetadata{TValue}"/> class.
+        /// </summary>
+        /// <param name="unsetValue">
+        /// The value to use when the property is set to <see cref="PerspexProperty.UnsetValue"/>
+        /// </param>
+        /// <param name="defaultBindingMode">The default binding mode.</param>
+        public DirectPropertyMetadata(
+            TValue unsetValue = default(TValue),
+            BindingMode defaultBindingMode = BindingMode.Default)
+                : base(defaultBindingMode)
+        {
+            UnsetValue = unsetValue;
+        }
+
+        /// <summary>
+        /// Gets the to use when the property is set to <see cref="PerspexProperty.UnsetValue"/>.
+        /// </summary>
+        public TValue UnsetValue { get; private set; }
+
+        /// <inheritdoc/>
+        object IDirectPropertyMetadata.UnsetValue => UnsetValue;
+
+        /// <inheritdoc/>
+        public override void Merge(PropertyMetadata baseMetadata, PerspexProperty property)
+        {
+            base.Merge(baseMetadata, property);
+
+            var src = baseMetadata as DirectPropertyMetadata<TValue>;
+
+            if (src != null)
+            {
+                if (UnsetValue == null)
+                {
+                    UnsetValue = src.UnsetValue;
+                }
+            }
+        }
+    }
+}

src/Perspex.Base/IDirectPropertyMetadata.cs

@@ -0,0 +1,16 @@
+// 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.
+
+namespace Perspex
+{
+    /// <summary>
+    /// Untyped interface to <see cref="DirectPropertyMetadata{TValue}"/>
+    /// </summary>
+    public interface IDirectPropertyMetadata
+    {
+        /// <summary>
+        /// Gets the to use when the property is set to <see cref="PerspexProperty.UnsetValue"/>.
+        /// </summary>
+        object UnsetValue { get; }
+    }
+}

src/Perspex.Base/Perspex.Base.csproj

@@ -56,6 +56,7 @@
     <Compile Include="IDirectPropertyAccessor.cs" />
     <Compile Include="DirectProperty.cs" />
     <Compile Include="IPerspexObject.cs" />
+    <Compile Include="IDirectPropertyMetadata.cs" />
     <Compile Include="IStyledPropertyMetadata.cs" />
     <Compile Include="ISupportInitialize.cs" />
     <Compile Include="Metadata\DependsOnAttribute.cs" />
@@ -92,6 +93,7 @@
     <Compile Include="Reactive\AnonymousSubject`1.cs" />
     <Compile Include="Reactive\AnonymousSubject`2.cs" />
     <Compile Include="Reactive\PerspexObservable.cs" />
+    <Compile Include="DirectPropertyMetadata`1.cs" />
     <Compile Include="StyledPropertyMetadata`1.cs" />
     <Compile Include="Threading\Dispatcher.cs" />
     <Compile Include="Threading\DispatcherPriority.cs" />

src/Perspex.Base/PerspexObject.cs

@@ -268,14 +268,7 @@ namespace Perspex
         {
             Contract.Requires<ArgumentNullException>(property != null);
 
-            if (property.IsDirect)
-            {
-                return (T)((IDirectPropertyAccessor)GetRegistered(property)).GetValue(this);
-            }
-            else
-            {
-                return (T)GetValue((PerspexProperty)property);
-            }
+            return (T)GetValue((PerspexProperty)property);
         }
 
         /// <summary>
@@ -315,7 +308,7 @@ namespace Perspex
             {
                 var accessor = (IDirectPropertyAccessor)GetRegistered(property);
                 LogPropertySet(property, value, priority);
-                accessor.SetValue(this, UnsetToDefault(value, property));
+                accessor.SetValue(this, DirectUnsetToDefault(value, property));
             }
             else
             {
@@ -567,19 +560,6 @@ namespace Perspex
             }
         }
 
-        /// <summary>
-        /// Converts an unset value to the default value for a property type.
-        /// </summary>
-        /// <param name="value">The value.</param>
-        /// <param name="property">The property.</param>
-        /// <returns>The value.</returns>
-        private static object UnsetToDefault(object value, PerspexProperty property)
-        {
-            return value == PerspexProperty.UnsetValue ?
-                TypeUtilities.Default(property.PropertyType) :
-                value;
-        }
-
         /// <summary>
         /// Creates a <see cref="PriorityValue"/> for a <see cref="PerspexProperty"/>.
         /// </summary>
@@ -626,6 +606,19 @@ namespace Perspex
             return result;
         }
 
+        /// <summary>
+        /// Converts an unset value to the default value for a direct property.
+        /// </summary>
+        /// <param name="value">The value.</param>
+        /// <param name="property">The property.</param>
+        /// <returns>The value.</returns>
+        private object DirectUnsetToDefault(object value, PerspexProperty property)
+        {
+            return value == PerspexProperty.UnsetValue ?
+                ((IDirectPropertyMetadata)property.GetMetadata(GetType())).UnsetValue :
+                value;
+        }
+
         /// <summary>
         /// Gets the default value for a property.
         /// </summary>

src/Perspex.Base/PerspexProperty.cs

@@ -342,18 +342,25 @@ namespace Perspex
         /// <param name="name">The name of the property.</param>
         /// <param name="getter">Gets the current value of the property.</param>
         /// <param name="setter">Sets the value of the property.</param>
+        /// <param name="unsetValue">
+        /// The value to use when the property is set to <see cref="PerspexProperty.UnsetValue"/>
+        /// </param>
         /// <param name="defaultBindingMode">The default binding mode for the property.</param>
         /// <returns>A <see cref="PerspexProperty{TValue}"/></returns>
         public static DirectProperty<TOwner, TValue> RegisterDirect<TOwner, TValue>(
             string name,
             Func<TOwner, TValue> getter,
             Action<TOwner, TValue> setter = null,
+            TValue unsetValue = default(TValue),
             BindingMode defaultBindingMode = BindingMode.OneWay)
                 where TOwner : IPerspexObject
         {
             Contract.Requires<ArgumentNullException>(name != null);
 
-            var metadata = new PropertyMetadata(defaultBindingMode);
+            var metadata = new DirectPropertyMetadata<TValue>(
+                unsetValue: unsetValue,
+                defaultBindingMode: defaultBindingMode);
+
             var result = new DirectProperty<TOwner, TValue>(name, getter, setter, metadata);
             PerspexPropertyRegistry.Instance.Register(typeof(TOwner), result);
             return result;

src/Perspex.Controls/Primitives/SelectingItemsControl.cs

@@ -53,7 +53,8 @@ namespace Perspex.Controls.Primitives
             PerspexProperty.RegisterDirect<SelectingItemsControl, int>(
                 nameof(SelectedIndex),
                 o => o.SelectedIndex,
-                (o, v) => o.SelectedIndex = v);
+                (o, v) => o.SelectedIndex = v,
+                unsetValue: -1);
 
         /// <summary>
         /// Defines the <see cref="SelectedItem"/> property.

src/Perspex.Controls/Primitives/ToggleButton.cs

@@ -13,7 +13,8 @@ namespace Perspex.Controls.Primitives
             PerspexProperty.RegisterDirect<ToggleButton, bool>(
                 "IsChecked",
                 o => o.IsChecked,
-                (o,v) => o.IsChecked = v, BindingMode.TwoWay);
+                (o,v) => o.IsChecked = v,
+                defaultBindingMode: BindingMode.TwoWay);
 
         private bool _isChecked;
 

tests/Perspex.Base.UnitTests/PerspexObjectTests_Direct.cs

@@ -62,7 +62,7 @@ namespace Perspex.Base.UnitTests
 
             target.SetValue((PerspexProperty)Class1.BazProperty, PerspexProperty.UnsetValue);
 
-            Assert.Equal(0, target.Baz);
+            Assert.Equal(-1, target.Baz);
         }
 
         [Fact]
@@ -160,7 +160,7 @@ namespace Perspex.Base.UnitTests
         }
 
         [Fact]
-        public void Bind_NonGeneric_Coerces_UnsetValue()
+        public void Bind_NonGeneric_Uses_UnsetValue()
         {
             var target = new Class1();
             var source = new Subject<object>();
@@ -171,7 +171,7 @@ namespace Perspex.Base.UnitTests
             source.OnNext(6);
             Assert.Equal(6, target.Baz);
             source.OnNext(PerspexProperty.UnsetValue);
-            Assert.Equal(0, target.Baz);
+            Assert.Equal(-1, target.Baz);
         }
 
         [Fact]
@@ -282,6 +282,17 @@ namespace Perspex.Base.UnitTests
             Assert.Equal("newvalue", target.Foo);
         }
 
+
+        [Fact]
+        public void UnsetValue_Is_Used_On_AddOwnered_Property()
+        {
+            var target = new Class2();
+
+            target.SetValue((PerspexProperty)Class1.FooProperty, PerspexProperty.UnsetValue);
+
+            Assert.Equal("unset", target.Foo);
+        }
+
         [Fact]
         public void Bind_Binds_AddOwnered_Property_Value()
         {
@@ -342,13 +353,21 @@ namespace Perspex.Base.UnitTests
         private class Class1 : PerspexObject
         {
             public static readonly DirectProperty<Class1, string> FooProperty =
-                PerspexProperty.RegisterDirect<Class1, string>("Foo", o => o.Foo, (o, v) => o.Foo = v);
+                PerspexProperty.RegisterDirect<Class1, string>(
+                    "Foo", 
+                    o => o.Foo, 
+                    (o, v) => o.Foo = v,
+                    unsetValue: "unset");
 
             public static readonly DirectProperty<Class1, string> BarProperty =
                 PerspexProperty.RegisterDirect<Class1, string>("Bar", o => o.Bar);
 
             public static readonly DirectProperty<Class1, int> BazProperty =
-                PerspexProperty.RegisterDirect<Class1, int>("Bar", o => o.Baz, (o,v) => o.Baz = v);
+                PerspexProperty.RegisterDirect<Class1, int>(
+                    "Bar", 
+                    o => o.Baz, 
+                    (o,v) => o.Baz = v,
+                    unsetValue: -1);
 
             private string _foo = "initial";
             private readonly string _bar = "bar";