我如何从 MVVM WPF 中的视图 go 到另一个视图？

Question

Here I have a WPF application that is made with the MVVM structure. I am fairly new to C# WPF and am not familiar with this concept. I am attempting to switch to another view through a function in one view via the press of a button.

Here is what the application looks like,

Once the Login button is pressed a function is triggered that will validate the inputs and if valid switch to another view. Which would look like such,

File Structure

How can i switch the views? Below are some code for reference.

MainWindow.xaml

<Window x:Class="QuizAppV2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:QuizAppV2"
        xmlns:viewModel="clr-namespace:QuizAppV2.MVVM.ViewModel"
        mc:Ignorable="d"
        Height="600" Width="920"
        WindowStartupLocation="CenterScreen"
        WindowStyle="None"
        ResizeMode="NoResize"
        Background="Transparent"
        AllowsTransparency="True">
    <Window.DataContext>
        <viewModel:MainViewModel/>
    </Window.DataContext>
    <Border Background="#272537"
            CornerRadius="20">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="75"/>
                <RowDefinition/>
                <RowDefinition Height="25"/>
            </Grid.RowDefinitions>

            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                </Grid.ColumnDefinitions>

                <TextBlock Text="Online Quiz"
                            Grid.Column="1"
                            FontSize="20"
                            Foreground="White"
                            HorizontalAlignment="Center"
                            VerticalAlignment="Center"/>
                <StackPanel Grid.Column="2"
                            Margin="30,20"
                            Orientation="Horizontal"
                            HorizontalAlignment="Right"
                            VerticalAlignment="Top">

                    <Button Content="–"
                            Background="#00CA4E"
                            Style="{StaticResource UserControls}"
                            Click="Minimise"/>
                    <Button Content="▢"
                            Background="#FFBD44"
                            Style="{StaticResource UserControls}"
                            Click="Restore"/>
                    <Button Content="X"
                            Background="#FF605C"
                            Style="{StaticResource UserControls}"
                            Click="Exit"/>
                </StackPanel>
            </Grid>
            <ContentControl Grid.Column="1"
                            Grid.Row="1"
                            Margin="20,10,20,50"
                            Content="{Binding CurrentView}"/>
        </Grid>
    </Border>
</Window>

MainViewModel.cs

using QuizAppV2.Core;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace QuizAppV2.MVVM.ViewModel
{
    class MainViewModel : ObservableObject
    {

        public RelayCommand LoginViewCommand { get; set; }
        public RelayCommand SubjectSelectionViewCommand { get; set; }
        public RelayCommand QuizViewCommand { get; set; }
        public RelayCommand ResultViewCommand { get; set; }

        public LoginViewModel LoginVM { get; set; }
        public SubjectSelectionViewModel SubjectSelectVM { get; set; }
        public QuizViewModel QuizVM { get; set; }
        public ResultViewModel ResultVM { get; set; }


        private object _currentView;

        public object CurrentView
        {
            get { return _currentView; }
            set
            {
                _currentView = value;
                onPropertyChanged();
            }
        }

        public MainViewModel()
        {
            LoginVM = new LoginViewModel();
            SubjectSelectVM = new SubjectSelectionViewModel();
            QuizVM = new QuizViewModel();
            ResultVM = new ResultViewModel();
            CurrentView = SubjectSelectVM;

            LoginViewCommand = new RelayCommand(o =>
            {
                CurrentView = LoginVM;
            });
            SubjectSelectionViewCommand = new RelayCommand(o =>
            {
                CurrentView = SubjectSelectVM;
            });
        }
    }
}

LoginView.xaml

using QuizAppV2.MVVM.ViewModel;
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 QuizAppV2.MVVM.View
{
    /// <summary>
    /// Interaction logic for LoginView.xaml
    /// </summary>
    public partial class LoginView : UserControl
    {
        public LoginView()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (UsrId.Text == "" || UsrName.Text == "")
            {
                UsrIDErrMsg.Visibility = Visibility.Visible;
                UsrNameErrMsg.Visibility = Visibility.Visible;
            }
            else
            {
                UsrIDErrMsg.Visibility = Visibility.Hidden;
                UsrNameErrMsg.Visibility = Visibility.Hidden;
                MainWindow.currentUser = new Student(UsrId.Text, UsrName.Text);
                
            }
        }
    }
}

Thank you

Answer 1

I suggest using "Datatemplate". Put in the main window resources the following:

<DataTemplate DataType="{x:Type viewmodel:QuizViewModel}">
            <local:QuizView/>
 </DataTemplate>
<DataTemplate DataType="{x:Type viewmodel:LoginViewModel}">
            <local:LoginView/>
 </DataTemplate>

and so on with the others... WPF is doing all the work for you, it examine the "CurrentView" prroperty and select how to view it according the the suitable DataTemplate.

Answer 2

Navigation is a tricky topic there are few ways to do this but since you are new to WPF I tried to outline a simple technique, along the lines of the examples you've provided requirement is have to go from page to page, a simple idea would be to swap out the contents. What I mean by that is when the user clicks "Login" we authenticate the user and swap the LoginPage with some other page, in your case a quiz page, when the user selection any option we swap out the view with the next view and so on.

I've coded up a simple solution with Shell mechanism. Essentially we create a empty shell in MainWindow (ie it has no UI) and we load pages into this empty shell using a NavigationService/Helper. I've gone with a singleton class here just for simplicity, there are 3 main Methods in this,

RegisterShell: This has to be the Window where the swapping will happen, this ideally needs to be set once.

Load View: Method which Swaps out old view with the new one, I have gone with user control for this as most of the sub views can be user control in WPF.

LoadViewWithCustomData: Similar to above but has more flexibilty since it allows you to supply extra data.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace Navigation
{
    class NavigationService
{
    /// <summary>
    /// Singleton so we keep on shell which views can use this to navigate to different pages.
    /// </summary>
    public static NavigationService Instance = new NavigationService();
    private MainWindow myShell;


    private NavigationService()
    {
    }

    /// <summary>
    /// Register the main shell so this service know where to swap the data out and in of
    /// </summary>
    /// <param name="theShell"></param>
    public void RegisterShell(MainWindow theShell)
    {
        this.myShell = theShell;
    }

    /// <summary>
    /// Swaps out any view to the shell.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public void LoadView<T>() where T : UserControl, new()
    {
        myShell.TheShell = new T();
    }

    /// <summary>
    /// Swaps out any view to the shell with custom data, here the user responsible to create UserControl with all the reqired data for the view.
    /// We can automate this via reflection if required.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="theNewControl"></param>
    public void LoadViewWithCustomData<T>(UserControl theNewControl) where T : UserControl, new()
    {
        myShell.TheShell = theNewControl;
    }
}

Now here's how my LoginPage looks, the important line here is NavigationService.Instance.LoadView<_4OptionQuizPage>() this essentially sends the user to _4OptionQuizPage.

    public partial class LoginPage : UserControl
        {
            public ICommand LoginClicked { get; }
            public LoginPage()
            {
                InitializeComponent();
                this.DataContext = this;
                LoginClicked = new SimpleCommand(OnLoginClicked);
            }
    
            private void OnLoginClicked()
            {
                // TODO : Authenticate user here.
    
                // Send the user to Quiz Page
                NavigationService.Instance.LoadView<_4OptionQuizPage>();
            }
        }

And in the _4OptionQuizPage we can have something like this, this is where the bulk of business logic may reside, I have 4 buttons here, 2 of them show message box but Button 1 sends you back to LoginPage and Button 2 reloads the same page with different data (ie sending the user to next question)

public partial class _4OptionQuizPage : UserControl, INotifyPropertyChanged
    {
        public ICommand Option1Clicked { get; }
        public ICommand Option2Clicked { get; }
        public ICommand Option3Clicked { get; }
        public ICommand Option4Clicked { get; }
        private string myQuestion;
        public event PropertyChangedEventHandler PropertyChanged;
        public string Question
        {
            get { return myQuestion; }
            set 
            {
                myQuestion = value;
                NotifyPropertyChanged();
            }
        }
        public _4OptionQuizPage() : this($"Question Loaded At {DateTime.Now}, this can be anything.")
        {
        }
        public _4OptionQuizPage(string theCustomData)
        {
            InitializeComponent();
            Question = theCustomData; 
            this.DataContext = this;
            this.Option1Clicked = new SimpleCommand(OnOption1Clicked);
            this.Option2Clicked = new SimpleCommand(OnOption2Clicked);
            this.Option3Clicked = new SimpleCommand(OnOption3Clicked);
            this.Option4Clicked = new SimpleCommand(OnOption4Clicked);
        }
        private void OnOption4Clicked()
        {
            MessageBox.Show("Option 4 selected, Store the results");
        }
        private void OnOption3Clicked()
        {
            MessageBox.Show("Option 3 selected, Store the results");
        }
        private void OnOption1Clicked()
        {
            NavigationService.Instance.LoadView<LoginPage>();
        }
        private void OnOption2Clicked()
        {
            NavigationService.Instance.LoadViewWithCustomData<LoginPage>(new _4OptionQuizPage("A custom question to emulate custom data"));
        }
        private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

Finally your MainWindow would be registering the shell and sending the user to LoginPage, and it's XAML file should not have anything in it

    public partial class MainWindow : Window, INotifyPropertyChanged
{
    private object myShell;

    public object TheShell
    {
        get { return myShell; }

        set 
        { 
            myShell = value;
            this.NotifyPropertyChanged();
        }
    }

    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;
        NavigationService.Instance.RegisterShell(this);
        NavigationService.Instance.LoadView<LoginPage>();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

}

MainWindow.xaml should be empty, essentially a shell for everything else.

<Window x:Class="Navigation.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Navigation"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800" Content="{Binding TheShell}">
</Window>

Answer 3

There are many ways how to allow a view model to participate in page navigation.
In general, each class that participates in navigation has to have access to your navigation API.
For example, you could move the navigation logic to a dedicated class NavigationService and provide a shared reference to every class that should be able to navigate to a different view.

Alternatively (and recommended), you can use routed commands that you handle on the MainWindow , which then delegates the command to the MainViewModel .
In this scenario each button would have to pass the destination as CommandParameter . This solution allows the particular view models to not directly participate in the navigation. You don't need to pollute your view model classes with navigation details.

The following example shows how to navigate from the QuizView to the ResultView using a RoutedCommand .

MainViewModel.cs
The MainViewModel is the only view model class that knows how to navigate and about the related details.
This enables extensibility while keeping the implementation of the view model classes simple.

In general, to enable data validation let the view models implement INotifyDataErrorInfo .
You can then query the INotifyDataErrorInfo.HasErrors property before allowing to leave a page.

class MainViewModel : ObservableObject
{
  public object CurrentView { get; set; }
  private Dictionary<Type, INotifyPropertyChanged> ViewModelMap { get; }

  public MainViewModel()
  {
    this.ViewModelMap = new Dictionary<Type, INotifyPropertyChanged>
    {
      { typeof(QuizVm),  new QuizVm() },
      { typeof(ResultVm),  new ResultVm() },
    };
  }

  // Check if destination type is valid.
  // In case the navigation source implements INotifyDataErrorInfo,
  // check if the source is in a valid state (INotifyDataErrorInfo.HasEWrrors returns 'false').
  // This method is called by the view. It will delegate its ICommand.CanExecute to this method
  // If this method returns 'false' the command source e.g. Button will be disabled.
  public bool CanNavigate(Type navigationSourceType, Type navigationDestinationType)
    => CanNavigateAwayFrom(navigationSourceType) 
      && CanNavigateTo(navigationDestinationType);

  private bool CanNavigateAwayFrom(Type navigationSourceType) 
    => this.ViewModelMap.TryGetValue(navigationSourceType, out INotifyPropertyChanged viewModel)
      && viewModel is INotifyDataErrorInfo notifyDataErrorInfo
        ? !notifyDataErrorInfo.HasErrors
        : true;

  private bool CanNavigateTo(Type navigationDestinationType)
    => this.ViewModelMap.ContainsKey(navigationDestinationType);

  // This method is called by the view. It will delegate its ICommand.Execute to this method
  public void NavigateTo(Type destinationType)
  {
    if (this.ViewModelMap.TryGetValue(destinationType, out INotifyPropertyChanged viewModel))
    {
      this.CurrentView = viewModel;
    }
  }
}

MainWindow.xaml.cs

partial class MainWindow : Window
{
  public static RoutedCommand NavigateCommand { get; } = new RoutedUICommand(
    "Navigate to view command", 
    nameof(NavigateCommand), 
    typeof(MainWindow));

  private MainViewModel MainViewModel { get; }

  public MainWindow()
  {   
    InitializeComponent();

    this.MainViewModel = new MainViewModel();
    this.DataContext = this.MainViewModel;

    var navigateCommandBinding = new CommandBinding(MainWindow.NavigateCommand, ExecuteNavigateCommand, CanExecuteNavigateCommand);
    this.CommandBindings.Add(navigateCommandBinding);
  }

  private void CanExecuteNavigateCommand(object sender, CanExecuteRoutedEventArgs e)
  {
    if (e.Source is not FrameworkElement commandSource)
    {
      return;
    }

    Type navigationSourceType = commandSource.DataContext.GetType();
    Type navigationDestinationType = (Type)e.Parameter;
    e.CanExecute = this.MainViewModel.CanNavigate(navigationSourceType, navigationDestinationType);
  }

  private void ExecuteNavigateCommand(object sender, ExecutedRoutedEventArgs e)
  {
    var destinationViewModelType = (Type)e.Parameter;
    this.MainViewModel.NavigateTo(destinationViewModelType);
  }
}

MainWindow.xaml
To actually render the views (for example a custom Control) you need to define an implicit DataTemplate (without the x:Key directive) that has the associated view model class as DataType . The ContentControl will then automatically pick the correct one that matches the type of the ContentControl.Content property value.

<Window>
  <Window.Resources>
    <DataTemplate DataType="{x:Type local:QuizVM}">
      <QuizView />
    </DataTemplate>

    <DataTemplate DataType="{x:Type local:ResultVM}">
      <ResultView />
    </DataTemplate>
  </Window.Resources>

  <ContentControl Content="{Binding CurrentView}" />
</Window>

If a view needs to navigate, it must use the static routed command (defined and handled in the MainWindow ) and pass the Type of the destination view model as CommandParameter .
This way, navigation will not pollute the view models and stays within the view.

QuizView.xaml

<QuizView>
  <Button Content="Next"
          Command="{x:Static local:MainWindow.NextPageCommand}"
          CommandParameter="{x:Type local:ResultVM}"/>
</QuizView>

ResultView.xaml

<ResultView>
  <Button Content="Back"
          Command="{x:Static local:MainWindow.NextPageCommand}"
          CommandParameter="{x:Type local:QuizVM}"/>
</ResultView>

Because the view model classes generally don't directly participate in the navigation,
they don't have to implement any related commands or depend on any navigation service.
Navigation is completely controlled by the MainWindow and its MainViewModel .

For optional data validation let them implement INotifyDataErrorInfo .

QuizVM.cs

class QuizVM : INotifyPropertyChnaged, INotifyDataErrorInfo
{
}

ResultVM.cs

class ResultVM : INotifyPropertyChnaged, INotifyDataErrorInfo
{
}

Answer 4

This sample demonstrates two approaches to navigation. Often useful since you say want to start by logging in but not show any menus etc until the user is logged in. Then once they log in you want some sort of menu or list of views they can navigate to which remains static.

My mainwindow is purely a shell to contain everything.

It's markup is:

 <Window ......
    Title="{Binding Title}" 
    Content="{Binding}"
    />

This sample uses viewmodel first for all navigation. Viewmodels are templated out into UI.

There is more in the code behind.

public partial class LoginNavigationWindow : Window
{
    public Type ParentViewModel
    {
        get { return (Type)GetValue(ParentViewModelProperty); }
        set { SetValue(ParentViewModelProperty, value); }
    }

    public static readonly DependencyProperty ParentViewModelProperty =
        DependencyProperty.Register(name: "ParentViewModel",
        propertyType: typeof(Type),
        ownerType: typeof(LoginNavigationWindow),
        typeMetadata: new FrameworkPropertyMetadata(
            defaultValue: null,
        propertyChangedCallback: new PropertyChangedCallback(ParentViewModelChanged)
      ));
    private static void ParentViewModelChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var vm = Activator.CreateInstance((Type)e.NewValue);
        ((Window)d).DataContext = vm;
        Task.Run(((IInitiatedViewModel)vm).Initiate);
    }

    public LoginNavigationWindow()
    {
        InitializeComponent();
        WeakReferenceMessenger.Default.Register<ParentNavigate>(this, (r, pn) =>
        {
            this.SetValue(LoginNavigationWindow.ParentViewModelProperty, pn.ParentViewModelType);
        });
    }

The messenger registration will switch out the window's datacontext using a dependency property. The message is just a class with a property to pass a Type

public class ParentNavigate
{
    public Type ParentViewModelType { get; set; }
}

The callback ParentViewModelChanged takes a type, instantiates it and sets datacontext on the window.

Usually, you're not interested in retaining state of a window or parent level piece of view. You already logged in. If you wanted to log back in again then you would start again and input name and password.

The entrypoint is slightly unusual since I handle application startup and rely on that dependency property callback.

    private void Application_Startup(object sender, StartupEventArgs e)
    {
        var mw = new LoginNavigationWindow();
        mw.Show();
        mw.SetValue(LoginNavigationWindow.ParentViewModelProperty, typeof(LoginViewModel));
    }

Instead of a mainwindow full of menus etc I have of course got nothing.

I have a LoginUC is the first thing you will see on start up. This is just illustrative.

We will get input from the user and validate it before navigating in a real app. We're just interested in that navigation here so this version just has a button to navigate to MainViewModel:

<Grid>
    <StackPanel>
        <TextBlock Text="Log in"/>
        <Button Content="Go"
                Command="{Binding LoadMainCommand}"/>
    </StackPanel>
</Grid>
</UserControl>

My LoginViewModel has a command, title and a task.

public partial class LoginViewModel : BaseParentViewModel
{
    [RelayCommand]
    private async Task LoadMain()
    {
        var pn = new ParentNavigate{ ParentViewModelType = typeof(MainViewModel) };
        WeakReferenceMessenger.Default.Send(pn);
    }
    public LoginViewModel()
    {
        Title = "Please Log In first";
    }

    public override async Task Initiate()
    {
        // Get any data for login here
    }
}

BaseParentViewModel

public partial class BaseParentViewModel : ObservableObject, IInitiatedViewModel
{
    [ObservableProperty]
    private string title = string.Empty;

    virtual public async Task Initiate() { }
}

Interface

public interface IInitiatedViewModel
{
    Task Initiate();
}

The purpose of this interface is to give us a generic way for any viewmodel to get any data it requires. By setting datacontext and then starting up a background thread to get that data the view will appear quickly and then fill with any data it needs. If getting that data takes a while then at least the view is "up" and visible quickly whilst the task still carries on working.

In a fuller example we would have IsBusy in a base viewmodel which would start off true and be changed to false. That would drive a "throbber" or busing indicator in the view.

A resource dictionary associates viewmodel datatemplates with usercontrols using datatype:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:LoginNavigation"
                    >
    <DataTemplate DataType="{x:Type local:MainViewModel}">
        <local:MainUC/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type local:LoginViewModel}">
        <local:LoginUC/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type local:SubjectsViewModel}">
        <local:SubjectsView/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type local:ResultViewModel}">
        <local:ResultView/>
    </DataTemplate>
</ResourceDictionary>

That is merged in app.xaml

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                 <ResourceDictionary  Source="/Resources/ViewDataTemplates.xaml"/>
             </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

Once you login, the entire content of the window is replaced. The datacontext is changed from LoginViewModel to MainViewModel, that is then templated out into MainUC:

public partial class MainViewModel : BaseParentViewModel
{

    [ObservableProperty]
    private object currentChildViewModel;

    [ObservableProperty]
    private List<ChildViewModel> childViewModelList;

    [RelayCommand]
    private async Task ChildNavigation(ChildViewModel cvm)
    {
        if (cvm.Instance == null)
        {
            cvm.Instance = Activator.CreateInstance(cvm.ViewModelType);
            if (cvm.Instance is IInitiatedViewModel)
            {
                Task.Run(((IInitiatedViewModel)cvm.Instance).Initiate);
            }
        }
        CurrentChildViewModel = cvm.Instance;
    }

    public override async Task Initiate()
    {
        ChildViewModelList = new List<ChildViewModel>()
        {
            new ChildViewModel{ DisplayName="Subjects", ViewModelType= typeof(SubjectsViewModel) },
            new ChildViewModel{ DisplayName="Results", ViewModelType= typeof(ResultViewModel) }
        };
    }
    public MainViewModel()
    {
        Title = "Quiz";
    }
}

You would probably want to have more views of course and pick one to show initially which would be setup in Initiate.

MainUC:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="100"/>
        <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>
    <ListBox ItemsSource="{Binding ChildViewModelList}"
             HorizontalContentAlignment="Stretch">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <Button Content="{Binding DisplayName}"
                        Command="{Binding DataContext.ChildNavigationCommand, RelativeSource={RelativeSource AncestorType=ListBox}}"
                        CommandParameter="{Binding}"/>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
    <ContentPresenter Content="{Binding CurrentChildViewModel}"
                      Grid.Column="1"/>
</Grid>
</UserControl>

In the view you get a list of buttons in a left column which will allow navigation in the right column. But retaining MainUC of course.

Instead of a listbox this could be a menu or maybe a tabcontrol.

Clicking on a button calls a command in MainViewModel and passes the instance of ChildViewModel as a parameter.

That is then used to instantiate a viewmodel, set CurrentChildViewmodel and cache the instance.

CurrentChildViewmodel will of course itself be templated out into a usercontrol within MainUC.

public partial class ChildViewModel : ObservableObject
{
    public string DisplayName { get; set; }

    public Type ViewModelType { get; set; }

    public object Instance { get; set; }
}

This is rather a simplistic approach and in a real world substantial app you would want dependency injection, factories and the like. But this is already quite a bit of code for a Stack Overflow answer as it is.

The remaining viewmodels and views are just simplistic implementations to prove it all works. eg

public partial class SubjectsViewModel : ObservableObject, IInitiatedViewModel
{
    public async Task Initiate()
    {
        // Get any data for Subjects here
    }
}

and

    <Grid>
        <TextBlock Text="Subjects"/>
    </Grid>
</UserControl>