title: MVVM-Calling commands on child viewModel
body: <p>Iam using MVVM and  have a main window with Close button and it is bound to the MainWindowViewModel's ICommand command.
  In the main window there are two UserControl1 and UserControl2 which is bound to viewmodels UserControlVM1 and UserControlVM2 respectively.</p>
  
  <p>UserControlVM1 and UserControlVM2 has command named CleanUp  that will clean up the resources.</p>
  
  <p>So Whenever the close button is clicked on the mainwindow, i wanted to call the CleanUp command of Usercontrol viewmodels. How can we do this in XAML or any other alternatives?</p>