title: MVVM: Communication between the Model and ViewModels
body: <p>I'm developing a WPF application using the MVVM pattern. I'm using MVVM Light library and I'm also trying to use a dependency injector (I'm considering Ninject and Unity).</p>
  
  <p>I've read a lot of blog articles and I'm quite confused about the "proper" way of making my classes communicate with each other. In particular, I don't know when to use Dependency Injection and when to rely on the mediator pattern.</p>
  
  <p>Lets consider an example. I have a ViewModel, lets call it DataViewModel, and the Data class that provides some kind of data.
  How is it better to communicate between them:</p>
  
  <p>A. Inject a dependency to DataViewModel with an interface of IData? This way Data will not have to rely on Messenger, but it will have to provide an event if the Data changes, and the ViewModel will have to subscribe to it.</p>
  
  <p>B. Rely on the mediator pattern (implemented in MVVM Light as Messenger) and send messages between Model and ViewModel? This way it will not be necessary to use Dependency Injection at all, because whole communication will be based on messages.</p>
  
  <p>Moreover, should my ViewModels have injected dependencies on other ViewModels, or it would be better just to rely on the Messenger? If the first, would it be necessary to define a separate interface for each ViewModel? I think that defining an interface for each VM will be an additional work, but maybe it is worth it.</p>