Added a comment giving some instructions on how to get internal method visible to this assembly

README.md

@@ -1,3 +1,33 @@
 # WpfViewModelFirst
 
 This project is for researching Actions that need to call an async/await function.
+
+**Unit Testing:**
+
+This area has been troubling. The MainWindowViewModel needs to set a value 
+of the ObservableObject on the UI thread. Doing a straight unit test will 
+cause a deadlock. 
+
+The first process of testing was learning that the Moq mocking framework
+has the ability to trigger the action passed to Part1ViewModel. This has
+extra setup and makes the test hard to read.
+
+The next step was finding a way to wait for the ContinueWith in the method
+to finish. That was accomplished with the TaskCompletionSource. 
+With the code being triggered by the callback, this would exhibit 3 different
+behaviors. Many times it would pass. Occassionally it would fail one of the 
+asserts. The other times it would frustratingly get into a deadlock.
+
+In searching for a way to test this I learned I can set an attribute on the 
+class to allow internal methods and fields to be seen by the test assembly.
+This helped the test code be easier to read.
+[assembly:System.Runtime.CompilerServices.InternalsVisibleTo("WpfViewModelFirst.Tests")]
+
+
+Using a combination of Task.Run and DispatcherFrame the tests appear to 
+complete successfully every time. 
+
+**Note:** When the Task.Run is awaited the test
+will deadlock.
+
+

WpfViewModelFirst.Tests/MainWindowViewModelTests.cs

@@ -0,0 +1,129 @@
+using Moq;
+using System.Windows.Controls;
+using System.Windows.Threading;
+using WpfViewModelFirst.Part1;
+using WpfViewModelFirst.Part2;
+
+namespace WpfViewModelFirst.Tests
+{
+    public class MainWindowViewModelTests
+    {
+        [Fact]
+        public void Part1WillSetPart1ViewModelToCurrentViewModel()
+        {
+            Part1ViewModel part1ViewModel = new(() => { return Task.CompletedTask; });
+            Mock<IViewModelFactory> mockViewModelFactory = new();
+            mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Func<Task>>()))
+                .Returns(part1ViewModel);
+            MainWindowViewModel main = new(mockViewModelFactory.Object);
+            main.Part1(null);
+            Assert.Equal(part1ViewModel, main.CurrentViewModel);
+        }
+        //[Fact]
+        //public async Task Part1CallbackIsSuccessfull()
+        //{
+        //    TaskCompletionSource<bool> taskCompletionSource = new();
+        //    Mock<IViewModelFactory> mockViewModelFactory = new();
+        //    mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Func<Task>>()))
+        //        .Callback<Func<Task>>(c =>
+        //        {
+        //            Task.Run(() =>
+        //                {
+        //                    c.Invoke();
+        //                });
+        //        });
+        //    mockViewModelFactory.Setup(vmf => vmf.GetPart2ViewModel())
+        //        .Returns(() => new Part2ViewModel());
+        //    MainWindowViewModel main = new(mockViewModelFactory.Object);
+        //    main.ItHappened += () => taskCompletionSource.SetResult(true);
+        //    main.Part1(null);
+        //    await taskCompletionSource.Task.WaitAsync(CancellationToken.None);
+        //    Assert.IsType<Part2ViewModel>(main.CurrentViewModel);
+        //    Assert.Single(main.Strings);
+        //}
+        [Fact]
+        public async Task Part1CallBackExample()
+        {
+            Mock<IViewModelFactory> mockViewModelFactory = new();
+            mockViewModelFactory.Setup(vmf => vmf.GetPart2ViewModel())
+                .Returns(() => new Part2ViewModel());
+            MainWindowViewModel main = new(mockViewModelFactory.Object);
+            TaskCompletionSource<bool> taskCompletionSource = new();
+            main.ItHappened += () => taskCompletionSource.SetResult(true);
+            mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Func<Task>>()))
+                .Callback<Func<Task>>(c =>
+                {
+                    Task.Run(() =>
+                        {
+                            DispatcherFrame frame = new();
+                            frame.Dispatcher.Invoke(
+                                DispatcherPriority.Normal, () =>
+                                {
+                                    c.Invoke();
+                                    frame.Continue = true;
+                                });
+                            Dispatcher.PushFrame(frame);
+                        });
+                });
+            main.Part1(null);
+            await taskCompletionSource.Task.WaitAsync(CancellationToken.None);
+            Assert.IsType<Part2ViewModel>(main.CurrentViewModel);
+            Assert.Single(main.Strings);
+        }
+        // This method requires the Part1Action method to be internal not private
+        // The MainWindowViewModel class also needs to have an attribute allowing
+        // this assembly to see the internals
+        // [assembly:System.Runtime.CompilerServices.InternalsVisibleTo("WpfViewModelFirst.Tests")]
+        [Fact]
+        public async Task Part1ActionWillSetCurrentViewModel()
+        {
+            Mock<IViewModelFactory> mockViewModelFactory = new();
+            mockViewModelFactory.Setup(vmf => vmf.GetPart2ViewModel())
+                .Returns(() => new Part2ViewModel());
+            MainWindowViewModel main = new(mockViewModelFactory.Object);
+            TaskCompletionSource<bool> taskCompletionSource = new();
+            main.ItHappened += () => taskCompletionSource.SetResult(true);
+
+            Task run = new(() =>
+            {
+                DispatcherFrame frame = new();
+                frame.Dispatcher.Invoke(
+                    DispatcherPriority.Normal, () =>
+                    {
+                        main.Part1Action();
+                        frame.Continue = true;
+                    });
+                Dispatcher.PushFrame(frame);
+            });
+            run.Start();
+            await taskCompletionSource.Task.WaitAsync(CancellationToken.None);
+            Assert.IsType<Part2ViewModel>(main.CurrentViewModel);
+        }
+        [Fact]
+        public async Task Part1ActionWillSetSingle()
+        {
+            Mock<IViewModelFactory> mockViewModelFactory = new();
+            mockViewModelFactory.Setup(vmf => vmf.GetPart2ViewModel())
+                .Returns(() => new Part2ViewModel());
+            MainWindowViewModel main = new(mockViewModelFactory.Object);
+            TaskCompletionSource<bool> taskCompletionSource = new();
+            main.ItHappened += () => taskCompletionSource.SetResult(true);
+
+            Task run = new(() =>
+            {
+                DispatcherFrame frame = new();
+                frame.Dispatcher.Invoke(
+                    DispatcherPriority.Normal, () =>
+                    {
+                        main.Part1Action();
+                        frame.Continue = true;
+                    });
+                Dispatcher.PushFrame(frame);
+            });
+            run.Start();
+            await taskCompletionSource.Task.WaitAsync(CancellationToken.None);
+            Assert.Single(main.Strings);
+        }
+
+    }
+}

WpfViewModelFirst.Tests/Part1/Part1ViewModelTests.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WpfViewModelFirst.Tests.Part1
+{
+    public class Part1ViewModelTests
+    {
+
+    }
+}

WpfViewModelFirst.Tests/Part2/Part2ViewModelTests.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WpfViewModelFirst.Tests.Part2
+{
+    public class Part2ViewModelTests
+    {
+
+    }
+}

WpfViewModelFirst.Tests/Usings.cs

@@ -0,0 +1 @@
+global using Xunit;

WpfViewModelFirst.Tests/WpfViewModelFirst.Tests.csproj

@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0-windows</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.3.0" />
+    <PackageReference Include="Moq" Version="4.18.2" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="3.1.2">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\WpfViewModelFirst\WpfViewModelFirst.csproj" />
+  </ItemGroup>
+
+</Project>

WpfViewModelFirst.sln

@@ -3,13 +3,15 @@ Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 17
 VisualStudioVersion = 17.3.32804.467
 MinimumVisualStudioVersion = 10.0.40219.1
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "WpfViewModelFirst", "WpfViewModelFirst\WpfViewModelFirst.csproj", "{E5611330-98DE-4571-A035-65576B2F8D4B}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "WpfViewModelFirst", "WpfViewModelFirst\WpfViewModelFirst.csproj", "{E5611330-98DE-4571-A035-65576B2F8D4B}"
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{90D6F27D-C403-4DF2-A570-D0DEECAC4062}"
 	ProjectSection(SolutionItems) = preProject
 		README.md = README.md
 	EndProjectSection
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "WpfViewModelFirst.Tests", "WpfViewModelFirst.Tests\WpfViewModelFirst.Tests.csproj", "{89753E08-BE0A-4FEA-B457-5486BB46EFEE}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -20,6 +22,10 @@ Global
 		{E5611330-98DE-4571-A035-65576B2F8D4B}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{E5611330-98DE-4571-A035-65576B2F8D4B}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{E5611330-98DE-4571-A035-65576B2F8D4B}.Release|Any CPU.Build.0 = Release|Any CPU
+		{89753E08-BE0A-4FEA-B457-5486BB46EFEE}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{89753E08-BE0A-4FEA-B457-5486BB46EFEE}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{89753E08-BE0A-4FEA-B457-5486BB46EFEE}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{89753E08-BE0A-4FEA-B457-5486BB46EFEE}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

WpfViewModelFirst/App.xaml.cs

@@ -18,7 +18,7 @@ namespace WpfViewModelFirst
             base.OnStartup(e);
             MainWindow mainWindow = new()
             {
-                DataContext = new MainWindowViewModel()
+                DataContext = new MainWindowViewModel(new ViewModelFactory())
             };
             mainWindow.Show();
         }

WpfViewModelFirst/IViewModelFactory.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Threading.Tasks;
+using WpfViewModelFirst.Part1;
+using WpfViewModelFirst.Part2;
+
+namespace WpfViewModelFirst
+{
+    public interface IViewModelFactory
+    {
+        Part1ViewModel GetPart1ViewModel(Func<Task> finishAction);
+        Part2ViewModel GetPart2ViewModel();
+    }
+}

WpfViewModelFirst/MainWindowViewModel.cs

@@ -10,39 +10,46 @@ using System.Windows.Threading;
 using WpfViewModelFirst.Part1;
 using WpfViewModelFirst.Part2;
 
+[assembly:System.Runtime.CompilerServices.InternalsVisibleTo("WpfViewModelFirst.Tests")]
 namespace WpfViewModelFirst
 {
     public class MainWindowViewModel : ViewModelBase
     {
-        public MainWindowViewModel()
+        public MainWindowViewModel(IViewModelFactory viewModelFactory)
         {
+            _viewModelFactory = viewModelFactory;
             StartCommand = new CustomCommand(Part1);
         }
+
+        private readonly IViewModelFactory _viewModelFactory;
+
         public ICommand StartCommand { get; set; }
 
         public void Part1(object? e)
         {
             CurrentViewModel = null;
-            CurrentViewModel = new Part1ViewModel(Part1Action);
+            CurrentViewModel = _viewModelFactory.GetPart1ViewModel(Part1Action);
         }
-        private void Part1Action()
+        internal Task Part1Action()
         {
-            var currentDispatcher = Dispatcher.CurrentDispatcher;
             CurrentViewModel = null;
-            LongDelay().ContinueWith(c =>
+            Dispatcher currentDispatcher = Dispatcher.CurrentDispatcher;
+            return LongDelay().ContinueWith((c) =>
             {
-                System.Diagnostics.Debug.WriteLine(c.Result);
+                CurrentViewModel = _viewModelFactory.GetPart2ViewModel();
-                CurrentViewModel = new Part2ViewModel();
                 currentDispatcher.Invoke(() =>
                 {
                     Strings.Add($"{DateTime.Now}");
-                }
+                });
-                    );
+                 ItHappened?.Invoke();
             }, TaskContinuationOptions.OnlyOnRanToCompletion);
         }
 
+        public event Action? ItHappened;
+            
         private async Task<string> LongDelay()
         {
+            // This would be the Repository call in the production application
             await Task.Delay(500);
             return "Completed";
         }

WpfViewModelFirst/Part1/Part1View.xaml

@@ -12,6 +12,9 @@
             <RowDefinition/>
         </Grid.RowDefinitions>
         <Button Grid.Row="0"
+                HorizontalAlignment="Center"
+                VerticalAlignment="Center"
+                Padding="10"
                     Content="Finish"
                     Command="{Binding FinishCommand}"/>
     </Grid>

WpfViewModelFirst/Part1/Part1ViewModel.cs

@@ -9,9 +9,9 @@ namespace WpfViewModelFirst.Part1
 {
     public class Part1ViewModel : ViewModelBase
     {
-        private readonly Action _finishAction;
+        private readonly Func<Task> _finishAction;
 
-        public Part1ViewModel(Action finishAction)
+        public Part1ViewModel(Func<Task> finishAction)
         {
             _finishAction = finishAction;
             FinishCommand = new CustomCommand(Finish);

WpfViewModelFirst/ViewModelFactory.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WpfViewModelFirst
+{
+    public class ViewModelFactory : IViewModelFactory
+    {
+        public Part1.Part1ViewModel GetPart1ViewModel(Func<Task> finishAction)
+        {
+            return new(finishAction);
+        }
+        public Part2.Part2ViewModel GetPart2ViewModel()
+        {
+            return new();
+        }
+    }
+}