inotifypropertychanged

Most MVVM examples I have worked through have had the Model implement INotifyPropertyChanged, but in Josh Smith's CommandSink example the ViewModel implements INotifyPropertyChanged . I'm still cognitively putting together the MVVM concepts, so I don't know if: you have to put the INotifyPropertyChanged in the ViewModel to get CommandSink to work this is just an aberration of the norm and it doesn't really matter you should always have the Model implement INotifyPropertyChanged and this is just a mistake which would be corrected if this were developed from a code example to an application What