Implemented two-way binding.

And hooked up the TreeViewItem's expander button. The hit testing on it
is a bit off though...

Perspex.UnitTests/PerspexObjectTests.cs

@@ -353,6 +353,37 @@ namespace Perspex.UnitTests
             target.Bind((PerspexProperty)Class1.FooProperty, Observable.Return((object)123));
         }
 
+        [TestMethod]
+        public void BindTwoWay_Gets_Initial_Value_From_Source()
+        {
+            Class1 source = new Class1();
+            Class1 target = new Class1();
+
+            source.SetValue(Class1.FooProperty, "initial");
+            target.BindTwoWay(Class1.FooProperty, source, Class1.FooProperty);
+
+            Assert.AreEqual("initial", target.GetValue(Class1.FooProperty));
+        }
+
+        [TestMethod]
+        public void BindTwoWay_Updates_Values()
+        {
+            Class1 source = new Class1();
+            Class1 target = new Class1();
+
+            System.Diagnostics.Debug.WriteLine("source: " + source.GetHashCode());
+            System.Diagnostics.Debug.WriteLine("target: " + target.GetHashCode());
+
+            source.SetValue(Class1.FooProperty, "first");
+            target.BindTwoWay(Class1.FooProperty, source, Class1.FooProperty);
+
+            Assert.AreEqual("first", target.GetValue(Class1.FooProperty));
+            source.SetValue(Class1.FooProperty, "second");
+            Assert.AreEqual("second", target.GetValue(Class1.FooProperty));
+            target.SetValue(Class1.FooProperty, "third");
+            Assert.AreEqual("third", source.GetValue(Class1.FooProperty));
+        }
+
         [TestMethod]
         public void Setting_UnsetValue_Reverts_To_Default_Value()
         {
@@ -385,6 +416,79 @@ namespace Perspex.UnitTests
             Assert.AreEqual("newvalue", target.GetValue(Class1.FooProperty));
         }
 
+        [TestMethod]
+        public void this_Operator_Returns_Value_Property()
+        {
+            Class1 target = new Class1();
+
+            target.SetValue(Class1.FooProperty, "newvalue");
+
+            Assert.AreEqual("newvalue", target[Class1.FooProperty]);
+        }
+
+        [TestMethod]
+        public void this_Operator_Sets_Value_Property()
+        {
+            Class1 target = new Class1();
+
+            target[Class1.FooProperty] = "newvalue";
+
+            Assert.AreEqual("newvalue", target.GetValue(Class1.FooProperty));
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(InvalidOperationException))]
+        public void this_Operator_Doesnt_Accept_Observable()
+        {
+            Class1 target = new Class1();
+
+            target[Class1.FooProperty] = Observable.Return("newvalue");
+        }
+
+        [TestMethod]
+        public void this_Operator_Binds_One_Way()
+        {
+            Class1 target1 = new Class1();
+            Class1 target2 = new Class1();
+            Binding binding = Class1.FooProperty.Bind().WithMode(BindingMode.OneWay);
+
+            target1.SetValue(Class1.FooProperty, "first");
+            target2[binding] = target1[!Class1.FooProperty];
+            target1.SetValue(Class1.FooProperty, "second");
+
+            Assert.AreEqual("second", target2.GetValue(Class1.FooProperty));
+        }
+
+        [TestMethod]
+        public void this_Operator_Binds_Two_Way()
+        {
+            Class1 target1 = new Class1();
+            Class1 target2 = new Class1();
+            Binding binding = Class1.FooProperty.Bind().WithMode(BindingMode.TwoWay);
+
+            target1.SetValue(Class1.FooProperty, "first");
+            target2[binding] = target1[!Class1.FooProperty];
+            Assert.AreEqual("first", target2.GetValue(Class1.FooProperty));
+            target1.SetValue(Class1.FooProperty, "second");
+            Assert.AreEqual("second", target2.GetValue(Class1.FooProperty));
+            target2.SetValue(Class1.FooProperty, "third");
+            Assert.AreEqual("third", target1.GetValue(Class1.FooProperty));
+        }
+
+        [TestMethod]
+        public void this_Operator_Binds_One_Time()
+        {
+            Class1 target1 = new Class1();
+            Class1 target2 = new Class1();
+            Binding binding = Class1.FooProperty.Bind().WithMode(BindingMode.OneTime);
+
+            target1.SetValue(Class1.FooProperty, "first");
+            target2[binding] = target1[!Class1.FooProperty];
+            target1.SetValue(Class1.FooProperty, "second");
+
+            Assert.AreEqual("first", target2.GetValue(Class1.FooProperty));
+        }
+
         /// <summary>
         /// Returns an observable that returns a single value but does not complete.
         /// </summary>

Perspex.UnitTests/PerspexPropertyTests.cs

@@ -19,7 +19,8 @@ namespace Perspex.UnitTests
                 "test",
                 typeof(Class1),
                 "Foo",
-                false);
+                false,
+                BindingMode.OneWay);
 
             Assert.AreEqual("test", target.Name);
             Assert.AreEqual(typeof(string), target.PropertyType);
@@ -34,7 +35,8 @@ namespace Perspex.UnitTests
                 "test",
                 typeof(Class1),
                 "Foo",
-                false);
+                false,
+                BindingMode.OneWay);
 
             Assert.AreEqual("Foo", target.GetDefaultValue<Class1>());
         }
@@ -46,7 +48,8 @@ namespace Perspex.UnitTests
                 "test",
                 typeof(Class1),
                 "Foo",
-                false);
+                false,
+                BindingMode.OneWay);
 
             Assert.AreEqual("Foo", target.GetDefaultValue<Class2>());
         }
@@ -58,7 +61,8 @@ namespace Perspex.UnitTests
                 "test",
                 typeof(Class3),
                 "Foo",
-                false);
+                false,
+                BindingMode.OneWay);
 
             Assert.AreEqual("Foo", target.GetDefaultValue<Class2>());
         }
@@ -70,7 +74,8 @@ namespace Perspex.UnitTests
                 "test",
                 typeof(Class1),
                 "Foo",
-                false);
+                false,
+                BindingMode.OneWay);
 
             target.OverrideDefaultValue(typeof(Class2), "Bar");
 

Perspex/Binding.cs

@@ -0,0 +1,66 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Perspex
+{
+    public enum BindingMode
+    {
+        Default,
+        OneWay,
+        TwoWay,
+        OneTime,
+        OneWayToSource,
+    }
+
+    public struct Binding
+    {
+        public BindingMode Mode
+        {
+            get;
+            set;
+        }
+
+        public BindingPriority Priority
+        {
+            get;
+            set;
+        }
+
+        public PerspexProperty Property
+        {
+            get;
+            set;
+        }
+
+        public PerspexObject Source
+        {
+            get;
+            set;
+        }
+
+        public static Binding operator !(Binding binding)
+        {
+            return binding.WithMode(BindingMode.TwoWay);
+        }
+
+        public static Binding operator ~(Binding binding)
+        {
+            return binding.WithMode(BindingMode.TwoWay);
+        }
+
+        public Binding WithMode(BindingMode mode)
+        {
+            this.Mode = mode;
+            return this;
+        }
+
+        public Binding WithPriority(BindingPriority priority)
+        {
+            this.Priority = priority;
+            return this;
+        }
+    }
+}

Perspex/Perspex.csproj

@@ -69,6 +69,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="Application.cs" />
+    <Compile Include="Binding.cs" />
     <Compile Include="BindingExtensions.cs" />
     <Compile Include="Controls\TreeDataTemplate.cs" />
     <Compile Include="Controls\HeaderedContentControl.cs" />

Perspex/PerspexObject.cs

@@ -140,13 +140,45 @@ namespace Perspex
         }
 
         /// <summary>
-        /// Gets or sets the binding for a <see cref="PerspexProperty"/>.
+        /// Gets or sets a binding for a <see cref="PerspexProperty"/>.
         /// </summary>
-        /// <param name="property">The property.</param>
-        public IObservable<object> this[PerspexProperty.BindingAccessor property]
+        /// <param name="binding">The binding information.</param>
+        public Binding this[Binding binding]
         {
-            get { return this.GetObservable(property.Property); }
-            set { this.Bind(property.Property, value, property.Priority); }
+            get
+            {
+                return new Binding
+                {
+                    Mode = binding.Mode,
+                    Priority = binding.Priority,
+                    Property = binding.Property,
+                    Source = this,
+                };
+            }
+
+            set
+            {
+                BindingMode mode = (binding.Mode == BindingMode.Default) ? 
+                    binding.Property.DefaultBindingMode : 
+                    binding.Mode;
+
+                switch (mode)
+                {
+                    case BindingMode.Default:
+                    case BindingMode.OneWay:
+                        this.Bind(binding.Property, value.Source.GetObservable(value.Property), binding.Priority);
+                        break;
+                    case BindingMode.OneTime:
+                        this.SetValue(binding.Property, value.Source.GetValue(value.Property));
+                        break;
+                    case BindingMode.OneWayToSource:
+                        value.Source.Bind(value.Property, this.GetObservable(binding.Property), binding.Priority);
+                        break;
+                    case BindingMode.TwoWay:
+                        this.BindTwoWay(binding.Property, value.Source, value.Property);
+                        break;
+                }
+            }
         }
 
         /// <summary>
@@ -218,8 +250,8 @@ namespace Perspex
         /// <summary>
         /// Gets an observable for a <see cref="PerspexProperty"/>.
         /// </summary>
-        /// <param name="property"></param>
-        /// <returns></returns>
+        /// <param name="property">The property.</param>
+        /// <returns>An observable.</returns>
         public IObservable<object> GetObservable(PerspexProperty property)
         {
             Contract.Requires<NullReferenceException>(property != null);
@@ -505,6 +537,28 @@ namespace Perspex
             return this.Bind((PerspexProperty)property, (IObservable<object>)source, priority);
         }
 
+        /// <summary>
+        /// Initialites a two-way bind between <see cref="PerspexProperty"/>s.
+        /// </summary>
+        /// <param name="property">The property on this object.</param>
+        /// <param name="source">The source object.</param>
+        /// <param name="sourceProperty">The property on the source object.</param>
+        /// <returns>
+        /// A disposable which can be used to terminate the binding.
+        /// </returns>
+        /// <remarks>
+        /// The binding is first carried out from <paramref name="source"/> to this. Two-way 
+        /// bindings are always at the LocalValue priority.
+        /// </remarks>
+        public void BindTwoWay(
+            PerspexProperty property, 
+            PerspexObject source, 
+            PerspexProperty sourceProperty)
+        {
+            source.GetObservable(sourceProperty).Subscribe(x => this.SetValue(property, x));
+            this.GetObservable(property).Subscribe(x => source.SetValue(sourceProperty, x));
+        }
+
         private PriorityValue CreatePriorityValue(PerspexProperty property)
         {
             PriorityValue result = new PriorityValue(property.Name, property.PropertyType);
@@ -535,6 +589,11 @@ namespace Perspex
             return result;
         }
 
+        /// <summary>
+        /// Gets the default value for a property.
+        /// </summary>
+        /// <param name="property">The property.</param>
+        /// <returns>The default value.</returns>
         private object GetDefaultValue(PerspexProperty property)
         {
             if (property.Inherits && this.inheritanceParent != null)

Perspex/PerspexProperty.cs

@@ -45,12 +45,14 @@ namespace Perspex
         /// <param name="ownerType">The type of the class that registers the property.</param>
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
+        /// <param name="defaultBindingMode">The default binding mode for the property.</param>
         public PerspexProperty(
             string name,
             Type valueType,
             Type ownerType,
             object defaultValue,
-            bool inherits)
+            bool inherits,
+            BindingMode defaultBindingMode)
         {
             Contract.Requires<NullReferenceException>(name != null);
             Contract.Requires<NullReferenceException>(valueType != null);
@@ -60,6 +62,7 @@ namespace Perspex
             this.PropertyType = valueType;
             this.OwnerType = ownerType;
             this.Inherits = inherits;
+            this.DefaultBindingMode = defaultBindingMode;
             this.defaultValues.Add(ownerType, defaultValue);
         }
 
@@ -83,6 +86,12 @@ namespace Perspex
         /// </summary>
         public bool Inherits { get; private set; }
 
+        /// <summary>
+        /// Gets the default binding mode for the property.
+        /// </summary>
+        /// <returns></returns>
+        public BindingMode DefaultBindingMode { get; private set; }
+
         /// <summary>
         /// Gets an observable that is fired when this property changes on any 
         /// <see cref="PerspexObject"/> instance.
@@ -100,11 +109,13 @@ namespace Perspex
         /// <param name="name">The name of the property.</param>
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
-        /// <returns></returns>
+        /// <param name="defaultBindingMode">The default binding mode for the property.</param>
+        /// <returns>A <see cref="PerspexProperty{TValue}"/></returns>
         public static PerspexProperty<TValue> Register<TOwner, TValue>(
             string name,
             TValue defaultValue = default(TValue),
-            bool inherits = false)
+            bool inherits = false,
+            BindingMode defaultBindingMode = BindingMode.OneWay)
             where TOwner : PerspexObject
         {
             Contract.Requires<NullReferenceException>(name != null);
@@ -113,7 +124,8 @@ namespace Perspex
                 name,
                 typeof(TOwner),
                 defaultValue,
-                inherits);
+                inherits,
+                defaultBindingMode);
 
             PerspexObject.Register(typeof(TOwner), result);
 
@@ -129,11 +141,13 @@ namespace Perspex
         /// <param name="name">The name of the property.</param>
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
-        /// <returns></returns>
+        /// <param name="defaultBindingMode">The default binding mode for the property.</param>
+        /// <returns>A <see cref="PerspexProperty{TValue}"/></returns>
         public static PerspexProperty<TValue> RegisterAttached<TOwner, THost, TValue>(
             string name,
             TValue defaultValue = default(TValue),
-            bool inherits = false)
+            bool inherits = false,
+            BindingMode defaultBindingMode = BindingMode.OneWay)
             where TOwner : PerspexObject
         {
             Contract.Requires<NullReferenceException>(name != null);
@@ -142,7 +156,8 @@ namespace Perspex
                 name,
                 typeof(TOwner),
                 defaultValue,
-                inherits);
+                inherits,
+                defaultBindingMode);
 
             PerspexObject.Register(typeof(THost), result);
 
@@ -154,10 +169,14 @@ namespace Perspex
         /// indexer.
         /// </summary>
         /// <param name="property">The property.</param>
-        /// <returns>A <see cref="BindingAccessor"/> describing the binding.</returns>
-        public static BindingAccessor operator!(PerspexProperty property)
+        /// <returns>A <see cref="Binding"/> describing the binding.</returns>
+        public static Binding operator!(PerspexProperty property)
         {
-            return new BindingAccessor(property, BindingPriority.LocalValue);
+            return new Binding
+            {
+                Priority = BindingPriority.LocalValue,
+                Property = property,
+            };
         }
 
         /// <summary>
@@ -165,10 +184,30 @@ namespace Perspex
         /// indexer.
         /// </summary>
         /// <param name="property">The property.</param>
-        /// <returns>A <see cref="BindingAccessor"/> describing the binding.</returns>
-        public static BindingAccessor operator ~(PerspexProperty property)
+        /// <returns>A <see cref="Binding"/> describing the binding.</returns>
+        public static Binding operator ~(PerspexProperty property)
         {
-            return new BindingAccessor(property, BindingPriority.TemplatedParent);
+            return new Binding
+            {
+                Priority = BindingPriority.TemplatedParent,
+                Property = property,
+            };
+        }
+
+        /// <summary>
+        /// Returns a binding accessor that can be passed to <see cref="PerspexObject"/>'s [] 
+        /// operator to initiate a binding.
+        /// </summary>
+        /// <returns>A <see cref="Binding"/>.</returns>
+        /// <remarks>
+        /// The ! and ~ operators are short forms of this.
+        /// </remarks>
+        public Binding Bind()
+        {
+            return new Binding
+            {
+                Property = this,
+            };
         }
 
         /// <summary>
@@ -239,27 +278,6 @@ namespace Perspex
             this.changed.OnNext(e);
         }
 
-        public class BindingAccessor
-        {
-            public BindingAccessor(PerspexProperty property, BindingPriority priority)
-            {
-                this.Property = property;
-                this.Priority = priority;
-            }
-
-            public PerspexProperty Property
-            {
-                get;
-                private set;
-            }
-
-            public BindingPriority Priority
-            {
-                get;
-                private set;
-            }
-        }
-
         private class Unset
         {
             public override string ToString()
@@ -281,12 +299,14 @@ namespace Perspex
         /// <param name="ownerType">The type of the class that registers the property.</param>
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
+        /// <param name="defaultBindingMode">The default binding mode for the property.</param>
         public PerspexProperty(
             string name,
             Type ownerType,
             TValue defaultValue,
-            bool inherits)
-            : base(name, typeof(TValue), ownerType, defaultValue, inherits)
+            bool inherits,
+            BindingMode defaultBindingMode)
+            : base(name, typeof(TValue), ownerType, defaultValue, inherits, defaultBindingMode)
         {
             Contract.Requires<NullReferenceException>(name != null);
             Contract.Requires<NullReferenceException>(ownerType != null);

Perspex/PriorityValue.cs

@@ -239,7 +239,7 @@ namespace Perspex
         }
 
         /// <summary>
-        /// Called when an binding's value changes.
+        /// Called when a binding's value changes.
         /// </summary>
         /// <param name="changed">The changed entry.</param>
         private void EntryChanged(BindingEntry changed)
@@ -251,7 +251,7 @@ namespace Perspex
         }
 
         /// <summary>
-        /// Called when an binding completes.
+        /// Called when a binding completes.
         /// </summary>
         /// <param name="changed">The completed entry.</param>
         private void EntryCompleted(BindingEntry entry)

Perspex/Themes/Default/TreeViewItemStyle.cs

@@ -49,6 +49,7 @@ namespace Perspex.Themes.Default
                             new ToggleButton
                             {
                                 Classes = new Classes("expander"),
+                                [~~ToggleButton.IsCheckedProperty] = control[~TreeViewItem.IsExpandedProperty],
                             },
                             new ContentPresenter
                             {