What is the recommended way to open a new window using MVVM in WPF

Question

Hello everyone and thanks in advance for your time.

I'm currently learning MVVM using WPF for a small Biz App that I'm writing. I have read a lot of articles about the MVVM pattern and found that one of the key areas is to decouple the ViewModel from the View as much as possible.

I want to open a new Window in my app but I'm not sure if I should open it from the ViewModel using an ICommand or directly from the view using a standard event. Someone I work with suggested that that I should use commands, but then I thought that this would mean having a reference to a View in my ViewModel, which according to what I understand is precisely what the MVVM pattern focuses on avoiding.

My understanding is that if a window will open for navigation purposes only and the process of opening that new windows has no effect on the Model, then I should keep all of this on the view using standard events.

I know in sw development everything "depends", but guess my question is there a "right"/standard way of doing this?

Best regards, Daniel

Answer 1

Yes, VMs should communicate with Views utilizing the Events that Views can subscribe to...

In VM:

public event EventHandler<NotificationEventArgs<string>> DisplayOptionsNotice;  

In View:

private readonly MainViewModel mvm;
...
mvm = DataContext as MainViewModel;
mvm.DisplayOptionsNotice += DisplayOptionsWindow;
...
private void DisplayOptionsWindow(object sender, NotificationEventArgs<string> e)
{
    ...  
    optionsWindow = new OptionsWindow { Owner = this };
    optionsWindow.ShowDialog();
    ...
}

Answer 2

but then I thought that this would mean having a reference to a View in my ViewModel, which according to what I understand is precisely what the MVVM pattern focuses on avoiding.

In general, the way this is handled is via some form of inversion of control. Most MVVM frameworks will provide a service of some form to open a window, and use a Service Locator or Dependency Injection to provide the service to the ViewModel.

This allows your ViewModel to stay decoupled from the specific view rendering framework - you'd pass the service the new VM and say "Show this VM in a window", and that code would be platform specific.