How to have multiple pairs “View-ViewModel”?

Question

I am building an application that is based on MVVM-Light. I am in the need of creating multiple instances of the same View, and each one should bind to its own ViewModel.

Answer 1

There is no obligation to store the ViewModels as singletons in the ViewModelLocator, but it certainly makes them easier to find if the view is a singleton too. Obviously, if you have multiple instances of the same View class, you will have multiple instances of the same ViewModel class, and it cannot be a singleton anymore.

To keep track of the multiple instances of the ViewModel, you can implement a dictionary in the ViewModelLocator that looks up for a ViewModel according to a key. The key can be a unique ID for the view, for example. Once you get hold of the view, retrieve its key and then retrieve the viewmodel from the locator.

Update: Often you don't even need to track multiple viewmodels. For instance, you can have the Messenger class send a message to all instances of a given viewmodel class using the Send overload. So before implementing a dictionary to keep track of the VMs, ask yourself if you really need it! ;)

Hope that helps, Laurent