MVVM pass values between view models

Question

I try to deal with problem of passing value from one ViewModel to another. Here is an example.

We have Parent View and its corresponding ViewModel, in that View we s

Answer 1

What if any framework are you using? By that, I mean MvvmLight, Caliburn Micro, or Prism. Each framework has a messaging infrastructure. You can harness them to pass state back and forth using a publish/subscribe methodology. For example, take a look at Prism. There are several Quickstarts that show the eventing model. You can also maintain a view controller to orchestrate communication between views.

Take a look at Ward Bell's Prism Explorer sample app. This is an article from '09 however it's still relevant today. Especially see how he passes an entity object from a list view to a child detail view.

Answer 2

One of the main tenants of the MVVM pattern is that you should be able to execute your ViewModel code without a View, in order to unit test your View logic. In othe words, ideally you should be able to execute your application in a 'headless' mode.

In your example you state that the ParentView creates a ChildView which in turn creates a ChildViewModel (which you are struggling to connect up). Can this work in headless mode? It seems to me that you are relying on your View to perform this Parent-Child navigation.

If you flip it the other way, have ParentViewModel create ChildViewModel, you no longer have a problem with communication between ViewModels. The ParentView needs to 'watch' (i.e. property change) for the new ChildViewModel being creates, and constructs the ChildView accordingly.

In more detail:

1. ParentView instantiates ParentVM
2. User interacts in such a way that the child is required
3. ParentVM creates a ChildVM, exposing it via a ChildVM property
4. ParentView handles the resultant PropertyChanged event, creating a ChildView, setting its DataContext to ChildVM.