When leveraging the MVVM pattern there's always the discussion regarding whether or not the ViewModel should be aware of the presence of the view, and in that case how much. This comes quite apparent when trying to make the UI/view respond to state-changes in the ViewModel and especially if it's not possible to leverage data binding to do so. Most Windows Phone developers already know that the ApplicationBar is one of the elements that doesn't support data binding directly in the platform. There's a couple of additional framework and/or technologies that you can leverage to actually get this support but I have myself found it to be much easier to just write some code.

Read more...