Open other window from window - wpf

Question

I have some academic question here. I read this question WPF MVVM Get Parent from VIEW MODEL and concluded that ViewModel should not opens any windows itself. So I use Messenger now to send message to ViewModel's Window and Window opens other window - NewWindow. It works fine, but what if NewWindow does something and get some Result has to be passed in MainWindow for further actions? More detailed:
NewWindow opened by button click in Window (OpenNewWindowCommand) and made some calculations.
After calculations NewWindow got some Result (does't matter what exactly is it) and rise a corresponding event - GotSomeResult, where event arg is Result.
This Result has to be passed in MainWindow to further processing, so I bind event handler to GotSomeResult event.
Below you can see all required code to illustrate this scenario.
MainWindow code-behind:
public MainWindow()
{
InitializeComponent();
DataContext = new MainWindowViewModel();
Messenger.Default.Register<NewWindowMessage>(this, OpenNewWindow);
}
private void OpenNewWindow(NewWindowMessage message)
{
var newWindow = new NewWindow();
var newWindowViewModel = (NewWindowViewModel) message.Target;
newWindowViewModel.GotSomeResult += ((MetaWindowViewModel)DataContext).ProcessResult;
newWindow.Owner = this;
newWindow.DataContext = newWindowViewModel;
newWindow.ShowDialog();
}
MainWindow ViewModel:
public void OpenNewWindowCommand()
{
Messenger.Default.Send(new NewWindowMessage(this, new NewWindowViewModel("OpenWindow"), String.Empty));
}
public void ProcessResult(Object someResult)
{
// Any actions with result
}
newWindowViewModel.GotSomeResult += ((MetaWindowViewModel)DataContext).ProcessResult; --- this string seems problem for me. Is it correct to get access to public method of ViewModel right in theView? Does it violent MVVM pattern?

Answer 1

Why don't you hook the handler to GotSomeResult at the VM level, ie :
public void OpenNewWindowCommand()
{
var newWindowViewModel = new NewWindowMessage(this, new NewWindowViewModel("OpenWindow"), String.Empty)
newWindowViewModel.GotSomeResult += this.ProcessResult;
Messenger.Default.Send();
}
It removes the references to your ViewModel in your codebehind (which indeed should be avoided):
private void OpenNewWindow(NewWindowMessage message)
{
var newWindow = new NewWindow();
newWindow.Owner = this;
newWindow.DataContext = message.Target;
newWindow.ShowDialog();
}