Relationship between two presentation models in WPF MVVM

Question

I am developing a WPF application and I have some problems for linking one view model with another.

I have:

• MainViewModel
• ChildViewModel1
• Childviewmodel2

Each time a property changes in MainViewModel, ChildViewModel1 and ChildViewModel2 should be notified.

Can anyone suggest a workaround?

EDIT: I think in the solution descrided MVVM Light ( http://simplemvvmtoolkit.codeplex.com/SourceControl/changeset/view/23821#313594. ) That implements the message bus. The right approach?

Answer 1

A common way to view models among themselves is to implicate a Mediator design pattern.

here is how it is done in MVVMLight http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/27aaefff-e463-451c-87d9-37367a343e0e

in Prism: http://blogs.u2u.be/diederik/post/2011/01/15/Using-the-Prism-40-Event-Aggregator.aspx

in Calibern: http://www.mindscapehq.com/blog/index.php/2012/02/01/caliburn-micro-part-4-the-event-aggregator/

Answer 2

In most cases, I would not suggest using a centralized place to exchange “events” / “notifications”, such as EventAggregator, etc. This leads to later issues related to obscure relationships between ViewModels. Such notifications make sense in very specific cases where the relationship between the listener / publisher is unknown even at the design stage. I would suggest drawing a simple diagram with the relationships between ViewModels and finding a way to use standard .NET events, so when you have clear relationships between ViewModels, for example ViewModel1, has a link to ViewModel2, so you can subscribe to the event or provide your own callback, therefore It will be easy to create such event notifications.

Answer 3

I would use an IService that is implemented by each view model. Then, in view models, you can pass service properties to view model properties that implement INotifypropertychanged. For example, I have a service called INavigationService, which is implemented by my view models, and it has properties like CanNavigate, currentView, etc., which I bind in my view models. Changes to these properties can cause navigation or change properties that other view models are bound to.