WPF using MVVM - Whose responsibility is it to open a new window?

Question

In MVVM we have Views, Viewmodels and Models. All the business logic of my application is in models, where I use the viewModels to manage them, in addition to Binds and commands that are sent from the Views.

But now I have a problem, I need to open a new window on the system, and who would be this responsibility?

We know that in the "perfect world" of MVVM, Viewmodel should not communicate with View, but how can I open a window without Viewmodel’s knowledge of View?

I’m about to break this "rule" and open a new window the same way I do in Windows Forms, but that would impact automated testing and go against the MVVM principle, so what’s the solution? It’s only worth breaking that rule?

Answer 1

I will not give my opinion if it is worth breaking the rule or not, even because here in the OS is not a place to keep giving a lot of opinion. I’ll give you an alternative not to break the rule.

Make an interface to encapsulate the View and Jete in Viewmodel.

public interface IView
{
    // no lugar de string poderia ser um enumerador também
    bool Show(string viewName);
}

public class ViewManager : IView
{
    public bool Show(string viewName)
    {
        if (viewName == "ListaUsuarios")
        {
            new ListaUsuarios().Show();
            return true;
        }

        return false;
    }
}

public class ViewModel
{
    private IView _viewManager;
    public ViewModel(IView viewManager)
    {
        _viewManager = viewManager;
    }

    public void AbrirView()
    {
        if(_viewManager.Show("ListaUsuarios"))
            .....
    }
}