MVVM in WPF - How to alert ViewModel of changes in Model… or should I?

Question

I am going through some MVVM articles, primarily this and this.

My specific question is: How do I communicate Model changes from the Model to the ViewModel?<

Answer 1

Short answer: it depends on the specifics.

In your example the models are being updated "on their own" and these changes of course need to somehow propagate to the views. Since the views can only directly access the viewmodels, it means the model must communicate these changes to the corresponding viewmodel. The established mechanism for doing so is of course INotifyPropertyChanged, which means that you 'll get a workflow like this:

1. Viewmodel is created and wraps model
2. Viewmodel subscribes to model's PropertyChanged event
3. Viewmodel is set as view's DataContext, properties are bound etc
4. View triggers action on viewmodel
5. Viewmodel calls method on model
6. Model updates itself
7. Viewmodel handles model's PropertyChanged and raises its own PropertyChanged in response
8. View reflects the changes in its bindings, closing the feedback loop

On the other hand if your models contained little (or no) business logic, or if for some other reason (such as gaining transactional capability) you decided to let each viewmodel "own" its wrapped model then all modifications to the model would pass through the viewmodel so such an arrangement would not be necessary.

I describe such a design in another MVVM question here.