What is the good practise to prepare lousily coupled ViewModels which has to interact together somehow

Question

Use case:

• Assume that there is a tool build in WPF plus MVVM manner.
• Saying about it's appearance it could be compared to Visual Studio IDE or another IDE.
• It contain a couple of dockable tools.
• It contains menu actions and some additional footer infos which can changes according to different actions, e.g. when project is succesfully loaded.
• Given use loads another project then some data shall be reloaded on different views.
• I would like to know how to accomplish good separation of concerns and at the same time interaction between modules, viewmodels.

Proposed, known solutions:

• Everything can be combined in one core MainViewModel - unfortunately it does not give separation of concrns and code is not clean, could be compared to God object antipattern.
• Another options would be to use a Messenger or EventAggregator e.g something like Messenger embedded in MVVM light toolkit but from my experience it is not best solution.
• I thought about Prism framework but maybe it is overkill.
• Another options it is to divide modules into functional ViewModels and some of them should be injected to another one but it is not easy to accomplish.

Do you know any good practise advise?

Goal:

• Separation of concerns
• Single functinal responsibility per ViewModel
• Testability

Answer 1

I have used the mediator pattern in the past for interactions between view models. It basically allows you to have a number of "colleagues" (the view models) that are interested in information and changes and can alert each other through the mediator.