How do you handle change tracking in MVVM?

Question

I have written a small helper for doing client-side change tracking objects/dtos to use in my ViewModels (see http://viss.be/2009/04/02/modelviewcontext-client-side-change-tracking/)

I didn't find a lot of resources about the subject. So I wonder; how do you typically handle it?

Thanks, Marc

Answer 1

A ViewModel is all about binding the View (UI) to the Model (data). So it should not be responsible for tracking changes, that is something you should be doing inside of you Model classes as they contain the data.

To notify the UI both Model and ViewModel implement INotyfyPorpertyChanged. The ViewModel catches the Model events and propagates them to the View as needed.

See http://msmvps.com/blogs/theproblemsolver/archive/2009/04/07/using-model-view-viewmodel-with-silverlight.aspx for a more complete example.

Answer 2

I may need to make it more clear what the ViewModelContext is.

The ViewModelContext is a gateway to the model which is exposed by the ViewModel to the view (instead of exposing the model directly).

All the context does is monitor the model, and when it changes states (via code inside commands or databinding) it updates some change tracking information on the model.

The ViewModel itself can react on that when it happens, or could query that change tracking information much later (for instance on a save command). And also; the view can directly databind to this information (eg. a view could filter on items that have IsDeleted on true).

So, I'm actually taking change tracking out of the ViewModel whereas in most implementation of ViewModel that I have seen, some form of change track is always present inside the ViewModels (eg. ListOfRemovedItems, etc...)

Any thoughts?

Answer 3

I didn't find a lot of resources about the subject.

I too want to see more discussion on this topic

So I wonder; how do you typically handle it?

1. Just use DataSet.
2. Keep looking for solutions.