[Example 2 - Binding that responds to value changes - raising notify property changed]

MainViewModel.cs:
 * Note how we no longer implement the "Output" property as an auto implemented property.  Instead, we use the "Set" method provided by the MvvmLightToolkit "ObservableObject" class.  This method raises the "PropertyChangedEvent" which triggers the view to update with the new value.  This is available because our MainViewModel inherits from ViewModelBase which in turn inherits from ObservableObject.  If we wanted to, we could raise the event ourselves manually - but this is just one of the ways in which the MVVM Light Toolkit makes our lives easier.

Run the app, and notice how the message changes after a 3 second delay.  (Note that because this is a windows store app, the change may not occur if the app is suspended).