M-V-VM Design Question. Calling View from ViewModel

Question

I've just started looking into M-V-VM for a WPF application. Everything makes sense so far besides this particular issue...

I have a ViewModel I'll call Search. This ViewModel binds to a datagrid and lists results of items. Now, I have a command that needs to bring up another view, the item's details.

Putting the logic to show another view in the Search View doesn't seem right, it's not testable at all.

Here is my ViewModel implementation, which is not testable...

public class SearchViewModel
{
   public void SelectItem()
   {
     // I want to call the DetailsView from here
     // this seems wrong, and is untestable
     var detailsView = new DetailsView();
     detailsView.Show();
   }
}

Where does the logic to show a view from a ViewModel method go in this pattern?

Answer 1

As Kiff noted:

Views should never be instantiated anywhere "below" the UI layer. VMs exist below that realm, therefore this is not the place to put that logic (as you've already realized).

There will almost always be some UI level event that will indicate the need to create the view. In your example, it might be a row (double) click event on the datagrid. That would be the place to new-up and show your DetailsView window.

You have to realize that M-V-VM is slightly different than other patterns like MVC or MVP. The ViewModel has no direct knowledge of the UI. Opening another view is a view-specific function. The View Model should care less what or how many views are using it's data. I most likely would never open the view via a command.

Answer 2

Views should never be instantiated anywhere "below" the UI layer. VMs exist below that realm, therefore this is not the place to put that logic (as you've already realized).

There will almost always be some UI level event that will indicate the need to create the view. In your example, it might be a row (double) click event on the datagrid. That would be the place to new-up and show your DetailsView window.