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.

The default ViewModelLocator implements ViewModels as singletons, therefore different instances of the same View will bind to the same ViewModel.

I could create the ViewModel in the VMLocator as a non-static object (as simple as returning new VM()...), but that would only partially help me. In fact, I still need to keep track of the opened windows. Nevertheless, each window might open several other windows (of a different kind, though). In this situation I might need to execute some operation on the parent View and all its children. For example before closing the View P, I might want to close all its children (view C1, view C2, etc.).

Hence, is there any simple and easy way to achieve this? Or is there any best practice you would advice me to follow?

Thanks in advance for your precious help.

Cheers,
Gianluca.

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

Answer 2

I used the naming system of the unity container.

See "How to distinguish multiple view/view model pairs using unity container".

Answer 3

I had a problem posted and solved in this SO question. It turned out to be very much related to Mr Bugnion's answer here (which helped me tremendously, thank you!)

What I've found is you don't need to store the view-model property in ViewModelLocator at all. Just use ServiceLocator to create an instance with a key, and in your "Dialog Service", pass the key to ShowDialog<T>(string key = null).

Also, as mentioned in this thread already, use method Messenger.Default.Send and remember to call viewModel.Cleanup() afterwards to unregister the view-model from Messenger, thereby preventing phantom view-models from trying to process future messages sent to all instances of the view-model class type.