What is the suggested pattern for subscribing to messages using IEventAggregator inside view models connected to UserControl views?

I have many UserControl views that are connected to view models that inherit from Screen. I’d read elsewhere that the recommended pattern for subscribing and unsubscribing to messages using IEventAggregator is this code:

protected override void OnActivate()
{
    EventAggregator.Subscribe(this);
}

protected override void OnDeactivate(bool close)
{
    EventAggregator.Unsubscribe(this);
}

Unfortunately, it does not seem that OnActivate and OnDeactivate are called for view models connected to UserControl views. I have been doing the following in the meantime, but I’d really like to call Unsubscribe explicitly:

protected override void OnViewLoaded(object view)
{
    EventAggregator.Subscribe(this);
}

What I want to do is subscribe to messages when the UserControl loads and unsubscribe when it unloads. Do you have any recommendations?

[nigel-sampson]

Excellent, so the lifecycle here for the child view models isn’t being handled by the root view model.

If you’re just instantiating it then it won’t have lifecycle beyond the view being attached / loaded.

If you don’t want your root view model to be a conductor of some sort you can use.

MyUserControlProperty = new MyUserControlViewModel();
MyUserControlProperty.ConductWith(this);

What ConductWith does is wire a one time activation / deactivation from the root view model to the child view model.

[nathan-alden-sr]

Hmm, yeah, I can see how separation of concerns would be an issue. It does not make sense for the view to be orchestrating this kind of configuration.

I will stick with calling ConductWith(this) for now. Thanks for the advice!