WPF - Should a user control have it's own ViewModel?

Question

I have a window made up of several user controls and was wondering should each user control have it's own ViewModel or should the window as a whole have only one ViewModel?

Cheers

AWC

Answer 1

This is not a yes or no question. It depends on whether having extra view models affords you better maintainability or testability. There's no point adding view models if it doesn't gain you anything. You'll need to gauge whether the overhead is worth it to your particular use case.

HTH,
Kent

Answer 2

I would say that each user control should have its own ViewModel, because that would allow you to reuse the ViewModel/UserControl pair in new constellations in the future.

As I understand it, your window is a Composite of user controls, so you can always create a ViewModel that composes all the separate ViewModels for each of the user controls. This will give you the best of both worlds.

Answer 3

I guess your application is doing some sort of view composition, so if you make your user controls to have its own view model, you'll have more freedom to embed them in other host windows without changing the window global view model.

As an added bonus, your application will be more suited to evolve to a more architecturally-sound composition model as that provided by Prism or Caliburn frameworks, if the application requirements arise.