How to manage multiple windows in MVVM

Question

I am aware there are a couple of questions similar to this one, however I have not quite been able to find a definitive answer. I'm trying to dive in with MVVM, and keep things as pure as possible, but not sure how exactly to go about launching/closing windows while sticking to the pattern.

My original thinking was data bound commands to the ViewModel triggering code to start a new View, with the View's DataContext then set to it's ViewModel via XAML. But this violates pure MVVM I think...

After some googling/reading answers I came across the concept of a WindowManager (like in CaliburnMicro), now if I was to implement one of these in a vanilla MVVM project, does this go in with my ViewModels? or just in the core of my application? I'm currently separating out my project into a Model assembly/project, ViewModel assembly/project and View assembly/project. Should this go into a different, "Core" assembly?

Which leads on a bit to my next question (relates somewhat to the above), how do I launch my application from an MVVM point of view? Initially I would launch my MainView.xaml from App.xaml, and the DataContext in the XAML would attach the assigned ViewModel. If I add a WindowManager, is this the first thing that is launched by my Application? Do I do this from the code behind of App.xaml.cs?

Answer 1

Well it mainly depends on how your application looks like (i.e. how many windows opened at the same time, modal windows or not...etc).

A general recommendation I would give is to not try to do "pure" MVVM ; I often read things like "there should be ZERO code-behind"...etc., I disagree.

I'm currently separating out my project into a Model assembly/project, ViewModel assembly/project and View assembly/project. Should this go into a different, "Core" assembly?

Separating views and ViewModels into different assemblies is the best thing you can do to ensure you won't ever reference something related to the views in your viewModel. You'll be fine with this strong separation.

Separating Model from ViewModel using two different assemblies could be a good idea too, but it depends on what your model looks like. I personally like 3-tier architectures, so generally my model is the WCF client proxies and are indeed stored in their own assembly.

A "Core" assembly is always a good idea anyway (IMHO), but only to expose basic utility methods that can be used in all the layers of your application (such as basic extension methods....etc.).

Now for your questions about views (how to show them...etc), I would say do simple. Personally I like instantiating my ViewModels in the code-behind of my Views. I also often use events in my ViewModels so the associated view is notified it should open another view for example.

For example, the scenario you have a MainWindow that should shows a child window when the user click on a button:

// Main viewModel
public MainViewModel : ViewModelBase
{
    ...
    // EventArgs<T> inherits from EventArgs and contains a EventArgsData property containing the T instance
    public event EventHandler<EventArgs<MyPopupViewModel>> ConfirmationRequested;
    ...
    // Called when ICommand is executed thanks to RelayCommands
    public void DoSomething()
    {
        if (this.ConfirmationRequested != null)
        {
            var vm = new MyPopupViewModel
            {
                // Initializes property of "child" viewmodel depending
                // on the current viewModel state
            };
            this.ConfirmationRequested(this, new EventArgs<MyPopupViewModel>(vm));
        }
    }
}
...
// Main View
public partial class MainWindow : Window
{
    public public MainWindow()
    {
        this.InitializeComponent();

        // Instantiates the viewModel here
        this.ViewModel = new MainViewModel();

        // Attaches event handlers
        this.ViewModel.ConfirmationRequested += (sender, e) =>
        {
            // Shows the child Window here
            // Pass the viewModel in the constructor of the Window
            var myPopup = new PopupWindow(e.EventArgsData);
            myPopup.Show();         
        };
    }

    public MainViewModel ViewModel { get; private set; }
}

// App.xaml, starts MainWindow by setting the StartupUri
<Application x:Class="XXX.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             ...
             StartupUri="Views/MainWindow.xaml">