Merge pull request #600 from marner2/feature/add_twoWayT

Add TwoWayT

src/Elmish.WPF/Binding.fs

@@ -116,6 +116,16 @@ module Binding =
       OneWayToSource.id
       |> createBindingT
 
+  /// <summary>
+  ///   Strongly-typed bindings that update both ways
+  /// </summary>
+  module TwoWayT =
+
+    /// Elemental instance of a two-way binding.
+    let id<'a> : string -> Binding<'a, 'a, 'a> =
+      TwoWay.id
+      |> createBindingT
+
   /// <summary>
   ///   Strongly-typed bindings that dispatch messages from the view.
   /// </summary>

src/Samples/SubModelStatic.Core/Program.fs

@@ -35,11 +35,17 @@ module Counter =
 type [<AllowNullLiteral>] CounterViewModel (args) =
   inherit ViewModelBase<Counter.Model, Counter.Msg>(args)
 
+  let stepSizeBinding =
+    Binding.TwoWayT.id
+    >> Binding.addLazy (=)
+    >> Binding.mapModel (fun (m: Counter.Model) -> m.StepSize)
+    >> Binding.mapMsg Counter.Msg.SetStepSize
+
   new() = CounterViewModel(Counter.init |> ViewModelArgs.simple)
 
   member _.StepSize
-    with get() = base.Get() (Binding.OneWayT.id >> Binding.addLazy (=) >> Binding.mapModel (fun m -> m.StepSize))
-    and set(v) = base.Set(v) (Binding.OneWayToSourceT.id >> Binding.mapMsg Counter.Msg.SetStepSize)
+    with get() = base.Get() stepSizeBinding
+    and set(v) = base.Set(v) stepSizeBinding
   member _.CounterValue = base.Get() (Binding.OneWayT.id >> Binding.addLazy (=) >> Binding.mapModel (fun m -> m.Count))
   member _.Increment = base.Get() (Binding.CmdT.setAlways Counter.Increment)
   member _.Decrement = base.Get() (Binding.CmdT.setAlways Counter.Decrement)