The tests are pasing. The UI is working. The tests sometimes hang.

WpfViewModelFirst.Tests/MainWindowViewModelTests.cs

@@ -9,9 +9,9 @@ namespace WpfViewModelFirst.Tests
         [Fact]
         public void Part1WillSetPart1ViewModelToCurrentViewModel()
         {
-            Part1ViewModel part1ViewModel = new(() => { });
+            Part1ViewModel part1ViewModel = new(() => { return Task.CompletedTask; });
             Mock<IViewModelFactory> mockViewModelFactory = new Mock<IViewModelFactory>();
-            mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Action>()))
+            mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Func<Task>>()))
                 .Returns(part1ViewModel);
             MainWindowViewModel main = new(mockViewModelFactory.Object);
             main.Part1(null);
@@ -22,19 +22,21 @@ namespace WpfViewModelFirst.Tests
         {
             TaskCompletionSource<bool> taskCompletionSource = new TaskCompletionSource<bool>();
             Mock<IViewModelFactory> mockViewModelFactory = new Mock<IViewModelFactory>();
-            mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Action>()))
+            mockViewModelFactory.Setup(vmf => vmf.GetPart1ViewModel(It.IsAny<Func<Task>>()))
 
                 .Returns(() => new Part1ViewModel(() =>
                 {
-                    //taskCompletionSource.SetResult(true);
+                    return Task.CompletedTask;
                     
                 }))
-                .Callback<Action>(c =>
+                .Callback<Func<Task>>(c =>
                 {
                     //taskCompletionSource.SetResult(true);
-                    c.Invoke();
-                })
-                ;
+                    Task.Run(() =>
+                        {
+                            c.Invoke();
+                        });
+                });
             mockViewModelFactory.Setup(vmf => vmf.GetPart2ViewModel())
                 .Returns(() => new Part2ViewModel());
             MainWindowViewModel main = new(mockViewModelFactory.Object);
@@ -42,6 +44,10 @@ namespace WpfViewModelFirst.Tests
             main.Part1(null);
             await taskCompletionSource.Task.WaitAsync(CancellationToken.None);
             Assert.IsType<Part2ViewModel>(main.CurrentViewModel);
+            taskCompletionSource = new();
+            await taskCompletionSource.Task.WaitAsync(CancellationToken.None);
+            Assert.Single(main.Strings);
+
         }
 
         Func<Task> Part1Func()

WpfViewModelFirst/IViewModelFactory.cs

@@ -7,7 +7,7 @@ namespace WpfViewModelFirst
 {
     public interface IViewModelFactory
     {
-        Part1ViewModel GetPart1ViewModel(Action finishAction);
+        Part1ViewModel GetPart1ViewModel(Func<Task> finishAction);
         Part2ViewModel GetPart2ViewModel();
     }
 }

WpfViewModelFirst/MainWindowViewModel.cs

@@ -29,28 +29,32 @@ namespace WpfViewModelFirst
             CurrentViewModel = null;
             CurrentViewModel = _viewModelFactory.GetPart1ViewModel(Part1Action);
         }
-        private void Part1Action()
+        private Task Part1Action()
         {
             CurrentViewModel = null;
             var currentDispatcher = Dispatcher.CurrentDispatcher; 
-            LongDelay().ContinueWith((c) =>
+            return LongDelay().ContinueWith((c) =>
             {
                 //System.Diagnostics.Debug.WriteLine(c.Result);
                 CurrentViewModel = _viewModelFactory.GetPart2ViewModel();
-                ItHappened?.Invoke();
-                currentDispatcher.Invoke(() =>
+                 ItHappened?.Invoke();
+               currentDispatcher.Invoke(() =>
                 {
                     Strings.Add($"{DateTime.Now}");
                 });
+                 ItHappened?.Invoke();
             }, TaskContinuationOptions.OnlyOnRanToCompletion);
         }
 
-        public event Action ItHappened;
+        public event Action? ItHappened;
             
         private async Task<string> LongDelay()
         {
             // This would be the Repository call in the production application
-            await Task.Delay(500);
+            await Task.Run(async () =>
+            {
+                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

@@ -8,7 +8,7 @@ namespace WpfViewModelFirst
 {
     public class ViewModelFactory : IViewModelFactory
     {
-        public Part1.Part1ViewModel GetPart1ViewModel(Action finishAction)
+        public Part1.Part1ViewModel GetPart1ViewModel(Func<Task> finishAction)
         {
             return new(finishAction);
         }