MVVM pattern and nested view models - communication and lookup lists

Question

I am using Prism for a new application that I am creating. There are several lookup lists that will be used in several places in the application. Therefore it makes sense to define it once and use that everywhere I need that functionality. My current solution is to use typed data templates to render the controls inside a content control.

   <DataTemplate DataType={x:Type ListOfCountriesViewModel}>
       <ComboBox ItemsSource={Binding Countries} SelectedItem="{Binding SelectedCountry"/>      </DataTemplate>
   <DataTemplate DataType={x:Type ListOfRegionsViewModel}>
       <ComboBox ItemsSource={Binding Countries} SelectedItem={Binding SelectedRegion} />   </DataTemplate>

    public class ParentViewModel
   {
        SelectedCountry get; set;
        SelectedRegion get; set;
        ListOfCountriesViewModel CountriesVM;
        ListOfRegionsViewModel RgnsVM;
   }

Then in my window I have 2 content controls and the rest of the controls

<ContentControl Content="{Binding CountriesVM}"></ContentControl>
<ContentControl Content="{Binding RgnsVM}"></ContentControl>
<Rest of controls on view>

At the moment I have this working and the SelectedItems for the combo boxes are publising events via EventAggregator from the child view models which are then subscribed to in the parent view model.

I am not sure that this is the best way to go as I can imagine I would end up with a lot of events very quickly and it will become unwieldy. Also if I was to use the same view model on another window it will publish the event and this parent viewmodel is subscribed to it which could have unintended consequences.

My questions are :-

1. Is this the best way to put lookup lists in a view which can be re-used across screens?
2. How do I make it so that the combobox which is bound to the child viewmodel sets the relevant property on the parent viewmodel without using events / mediator. e.g in this case SelectedCountry for example?
3. Any alternative implementation proposals for what I am trying to do?

I have a feeling I am missing something obvious and there is so much info it is hard to know what is right so any help would be most gratefully received.

Answer 1

In your scenario, it seems that the parent is aware of the type of child. Can't you just use properties in this situation? :

ListOfCountriesVM exposes a property for SelectedCountry.

In the parent VM, the implementation of SelectedCountry simply returns ListOfCountriesVM.SelectedCountry.

You could do this for any Parent View Model that is aware that it owns a ListOfCountriesVM.

In my experience, messaging/event aggregation should really only be used in scenarios where the publisher doesn't care who's listening or how many listeners there are. For example:

1. Sending notifications to other modules in the application
2. Sending notifications to view models in the same module that you can't guarantee exist.

In your situation, the subscriber, (the parent) already knows who the publisher (the child) is, so a simple solution using properties seems to be the most appropriate. I don't think this particular scenario warrants a messaging or event based solution.