Should I register ViewModels in Container?

Question

Should I register ViewModels in Container and resolve from there?

Benefits:

1. I can perform some actions when view model is activated
2. Container will inject dependencies for me
3. ???

Drawbacks:

1. ViewModel lifetime management can be tricky:
   • if I make ViewModel singleton then I can't instantiate several controls of the same type
   • if I make ViewModel transient then I can easily end up in a situation of having several different instances when I actually expect same instance injected
2. ???

What's the right answer? I'd prefer to register if I could mitigate lifetime drawback.

I'm using Caliburn and Autofac if it matters.

Answer 1

The benefits of 2) is enough for me to let the container handle viewmodels. We're using our own MVVM framework where there is a strict one-to-one relationship between view instances and viewmodel instances. Thus, the 1) drawbacks are non-existent.

In cases where we need to share data between views, we accomplish this by injecting the viewmodels with a shared service instance.

Except from that, are there other circumstances where you expect one viewmodel instance to be shared among several views?

Answer 2

Not sure about autofac or Caliburn (might still apply) but when it comers to the Unity Container I will only register the ViewModel if...

I need to have it disposed of when the container is disposed of. You can create a lifetime manager that will store the newly created (non singleton) view model instance.

container.RegisterType<MyViewModel>(new DisposeableInstanceLifetimeManager());
...
container.Resolve<MyViewModel>();  // here all dependencies will get injected
...
container.Dispose(); 

If you want to share data I tend to create a sub (child) container and register the model as a singleton and have multiple view models share the same model.

var child = container.CreateChildContainer();
child.RegisterInstance(model, new ContainerControlledLifetimeManager());
child.Resolve<MyViewModel1>();
child.Resolve<MyViewModel2>(); // both can share the model instance

(Note: with Unity all dependencies are injected when using Resolve on the ViewModel even if it is not registered with the container).

Otherwise unless you need a singleton ViewModel (although I cannot think of when that would be useful) I feel adding the ViewModel to the container just adds more code for no benefit.

Answer 3

A container is an ecosystem inhabited by the objects it creates. View models interact with those inhabitants and thus are also part of the ecosystem. To accurately reflect that relationship, you should register view models in the container.

You should always use InstancePerDependency with view models. A view model represents the state and behavior of a particular piece of UI - it is the non-framework-specific analogue of a control. Just as you can't generally place the same control instance in two locations in a UI tree, you can't also reuse the same view model instance.

If you could, we'd call it a ViewsModel :-)