Test using the Callback and DispatcherFrame

WpfViewModelFirst.Tests/MainWindowViewModelTests.cs

@@ -42,6 +42,35 @@ namespace WpfViewModelFirst.Tests
         //    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);
+        }
+        [Fact]
         public async Task Part1ActionWillSetCurrentViewModel()
         {
             Mock<IViewModelFactory> mockViewModelFactory = new();