Able to return a string from the async method

Fixed ObservableObject

WpfViewModelFirst/App.xaml.cs

@@ -16,7 +16,10 @@ namespace WpfViewModelFirst
         protected override void OnStartup(StartupEventArgs e)
         {
             base.OnStartup(e);
-            MainWindow mainWindow = new() { DataContext = new MainWindowViewModel() };
+            MainWindow mainWindow = new()
+            {
+                DataContext = new MainWindowViewModel()
+            };
             mainWindow.Show();
         }
     }

WpfViewModelFirst/MainWindow.xaml

@@ -4,9 +4,31 @@
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:local="clr-namespace:WpfViewModelFirst"
+        xmlns:part1="clr-namespace:WpfViewModelFirst.Part1"
+        xmlns:part2="clr-namespace:WpfViewModelFirst.Part2"
         mc:Ignorable="d"
+        d:DataContext="{d:DesignInstance Type={x:Type local:MainWindowViewModel}}"
         Title="MainWindow" Height="450" Width="800">
-    <Grid>
-
+    <Grid Background="DarkRed"> 
+        <Grid.RowDefinitions>
+            <RowDefinition/>
+            <RowDefinition Height="auto"/>
+        </Grid.RowDefinitions>
+        <ContentControl Grid.Row="0" 
+                        Visibility="Visible"
+                        Content="{Binding CurrentViewModel}">
+            <ContentControl.Resources>
+                <DataTemplate DataType="{x:Type part1:Part1ViewModel}">
+                    <part1:Part1View/>
+                </DataTemplate>
+                <DataTemplate DataType="{x:Type part2:Part2ViewModel}">
+                    <part2:Part2View/>
+                </DataTemplate>
+            </ContentControl.Resources>
+        </ContentControl>
+        <Button Grid.Row="1" 
+                HorizontalAlignment="Center"
+                Command="{Binding StartCommand}" 
+                Content="Start"/>
     </Grid>
 </Window>

WpfViewModelFirst/MainWindowViewModel.cs

@@ -3,11 +3,45 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using System.Windows.Input;
+using System.Windows.Threading;
+using WpfViewModelFirst.Part1;
+using WpfViewModelFirst.Part2;
 
 namespace WpfViewModelFirst
 {
     public class MainWindowViewModel : ViewModelBase
     {
+        public MainWindowViewModel()
+        {
+            StartCommand = new CustomCommand(Part1);
+        }
+        public ICommand StartCommand { get; set; }
+
+        public void Part1(object? e)
+        {
+            CurrentViewModel = null;
+            CurrentViewModel = new Part1ViewModel(Part1Action);
+//            MainFrameViewModel = new Part2ViewModel();
+        }
+        private void Part1Action()
+        {
+            CurrentViewModel = null;
+            Task.Run(async () => await LongDelay()).ContinueWith(c =>
+                {
+                    System.Diagnostics.Debug.WriteLine(c.Result);
+            CurrentViewModel = new Part2ViewModel();
+            });
+        }
+
+        private async Task<string> LongDelay()
+        {
+            await Task.Delay(500);
+            return "Completed";
+        }
+
+        private ViewModelBase? _mainFrameViewModel;
+        public ViewModelBase? CurrentViewModel { get => _mainFrameViewModel; set => SetProperty(ref _mainFrameViewModel, value); }
 
     }
 }

WpfViewModelFirst/ObservableObject.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text;
@@ -8,7 +9,7 @@ using System.Threading.Tasks;
 
 namespace WpfViewModelFirst
 {
-    public class ObservableObject
+    public class ObservableObject : INotifyPropertyChanged
     {
         public event PropertyChangedEventHandler? PropertyChanged;
         protected virtual void RaisePropertyChanged([CallerMemberName] string propertyName = "")
@@ -16,7 +17,7 @@ namespace WpfViewModelFirst
             PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
         }
 
-        protected virtual bool Set<T>(ref T storage, T value, [CallerMemberName] string propertyName = "")
+        protected virtual bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = "")
         {
             if (EqualityComparer<T>.Default.Equals(storage, value))
             {
@@ -26,6 +27,5 @@ namespace WpfViewModelFirst
             this.RaisePropertyChanged(propertyName);
             return true;
         }
-
     }
 }

WpfViewModelFirst/Part1/Part1View.xaml

@@ -0,0 +1,18 @@
+<UserControl x:Class="WpfViewModelFirst.Part1.Part1View"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
+             xmlns:local="clr-namespace:WpfViewModelFirst.Part1"
+             mc:Ignorable="d" 
+             d:DataContext="{d:DesignInstance Type={x:Type local:Part1ViewModel}}"
+             d:DesignHeight="450" d:DesignWidth="800">
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition/>
+        </Grid.RowDefinitions>
+        <Button Grid.Row="0"
+                    Content="Finish"
+                    Command="{Binding FinishCommand}"/>
+    </Grid>
+</UserControl>

WpfViewModelFirst/Part1/Part1View.xaml.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+using System.Windows.Documents;
+using System.Windows.Input;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using System.Windows.Navigation;
+using System.Windows.Shapes;
+
+namespace WpfViewModelFirst.Part1
+{
+    /// <summary>
+    /// Interaction logic for Part1View.xaml
+    /// </summary>
+    public partial class Part1View : UserControl
+    {
+        public Part1View()
+        {
+            InitializeComponent();
+        }
+    }
+}

WpfViewModelFirst/Part1/Part1ViewModel.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Input;
+
+namespace WpfViewModelFirst.Part1
+{
+    public class Part1ViewModel : ViewModelBase
+    {
+        private readonly Action _finishAction;
+
+        public Part1ViewModel(Action finishAction)
+        {
+            _finishAction = finishAction;
+            FinishCommand = new CustomCommand(Finish);
+        }
+
+        public ICommand FinishCommand { get; set; }
+        private void Finish(object? e)
+        {
+            _finishAction.Invoke();
+        }
+    }
+}

WpfViewModelFirst/Part2/Part2View.xaml

@@ -0,0 +1,13 @@
+<UserControl x:Class="WpfViewModelFirst.Part2.Part2View"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
+             xmlns:local="clr-namespace:WpfViewModelFirst.Part2"
+             mc:Ignorable="d" 
+             d:DataContext="{d:DesignInstance Type={x:Type local:Part2ViewModel}}"
+             d:DesignHeight="450" d:DesignWidth="800">
+    <Grid>
+        <TextBlock Text="Part 2"/>
+    </Grid>
+</UserControl>

WpfViewModelFirst/Part2/Part2View.xaml.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+using System.Windows.Documents;
+using System.Windows.Input;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using System.Windows.Navigation;
+using System.Windows.Shapes;
+
+namespace WpfViewModelFirst.Part2
+{
+    /// <summary>
+    /// Interaction logic for Part2View.xaml
+    /// </summary>
+    public partial class Part2View : UserControl
+    {
+        public Part2View()
+        {
+            InitializeComponent();
+        }
+    }
+}

WpfViewModelFirst/Part2/Part2ViewModel.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WpfViewModelFirst.Part2
+{
+    public class Part2ViewModel : ViewModelBase
+    {
+
+    }
+}