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

You can raise events from the model, which the viewmodel would need to subscribe to.

For example, I recently worked on a project for which I had to generate a treeview (naturally, the model had a hierarchical nature to it). In the model I had an observablecollection called ChildElements.

In the viewmodel, I had stored a reference to the object in the model, and subscribed to the CollectionChanged event of the observablecollection, like so: ModelObject.ChildElements.CollectionChanged += new CollectionChangedEventHandler(insert function reference here)...

Then your viewmodel gets automatically notified once a change happens in the model. You can follow the same concept using PropertyChanged, but you will need to explicitly raise property change events from your model for that to work.