Merge pull request elmish#374 from TysonMN/fix_373_backwards_text

synchronously dispatch msgs from the UI and asynchronously from commands

src/Elmish.WPF/WpfProgram.fs

@@ -57,16 +57,36 @@ module WpfProgram =
     let bindingsLogger = program.LoggerFactory.CreateLogger("Elmish.WPF.Bindings")
     let performanceLogger = program.LoggerFactory.CreateLogger("Elmish.WPF.Performance")
 
-    let setState model dispatch =
+    (*
+     * Capture the dispatch function before wrapping it with Dispatcher.InvokeAsync
+     * so that the UI can sychrononously dispatch messages.
+     * In additional to being slightly more efficient,
+     * it also helps keep WPF in the correct state.
+     * https://github.com/elmish/Elmish.WPF/issues/371
+     * https://github.com/elmish/Elmish.WPF/issues/373
+     *
+     * This is definitely a hack.
+     * Maybe something with Elmish can change so this hack can be avoided.
+     *)
+    let mutable dispatch = Unchecked.defaultof<Dispatch<'msg>>
+
+    let setState model _ =
       match viewModel with
       | None ->
-          let vm = ViewModel<'model, 'msg>(model, dispatch, program.Bindings, program.PerformanceLogThreshold, "main", bindingsLogger, performanceLogger)
+          let uiDispatch msg = element.Dispatcher.Invoke(fun () -> dispatch msg)
+          let vm = ViewModel<'model, 'msg>(model, uiDispatch, program.Bindings, program.PerformanceLogThreshold, "main", bindingsLogger, performanceLogger)
           element.DataContext <- vm
           viewModel <- Some vm
       | Some vm ->
           vm.UpdateModel model
 
-    let uiDispatch (innerDispatch: Dispatch<'msg>) : Dispatch<'msg> =
+    let cmdDispatch (innerDispatch: Dispatch<'msg>) : Dispatch<'msg> =
+      dispatch <- innerDispatch
+      (*
+       * Have commands asychrononously dispatch messages.
+       * This avoids race conditions like can occur when shutting down.
+       * https://github.com/elmish/Elmish.WPF/issues/353
+       *)
       fun msg -> element.Dispatcher.InvokeAsync(fun () -> innerDispatch msg) |> ignore
 
     let logMsgAndModel (msg: 'msg) (model: 'model) = 
@@ -79,7 +99,7 @@ module WpfProgram =
     |> if updateLogger.IsEnabled LogLevel.Trace then Program.withTrace logMsgAndModel else id
     |> Program.withErrorHandler logError
     |> Program.withSetState setState
-    |> Program.withSyncDispatch uiDispatch
+    |> Program.withSyncDispatch cmdDispatch
     |> Program.run