Question

I’m still starting out with MVVM and read through all the MSDN Prism examples but I’d nonetheless like some guidance with the following design problem which I feel will help me latch onto the Prism concepts and reinforce what I just learnt

I have a multi-window application – each window has a filter drop down control. The value of this filter control will affect the display of other controls within that same window, eg grids, charts etc.

Certain interactions (eg double clicking a row on a grid) will spawn another window which will have its own separate filter control, which will similarly affect the display of other controls only within that window

How do I implement this behavior of the filter driving the display of other user controls it has no idea of and have it restrict its behavior to only the window hosting it? How do I keep these interactions loosely coupled?

I’m thinking I need to use the EventAggregator and for the filter control to publish an update event when the selection changes? Am I thinking about this the right way? Would each window need a separate EventAggregator?

Answer 1

Yes, you are thinking about this the right way. The EventAggregator is a good tool for what you’re doing. You’ll need to have the EventAggregator on every window you plan to raise an event from. You can inject the EA into your constructor or use the ServiceLocator. Here are 2 examples:

// Ctor injection
private IEventAggregator _eventAggregator;
public ViewModelBase(IEventAggregator eventAggregator)
{
_eventAggregator = eventAggregator;
}

// Service Locator
 ServiceLocator.Current.GetInstance<IEventAggregator>().GetEvent<YourEvent>().Publish();

Now, you’ll need to create a CompositePresentationEvent for the EA to publish. You can minimize the number of these that get created by including a payload in the CPE. Something like this:

public class NavigationSelectedEvent : CompositePresentationEvent<NavigationEnum.Destination>{}

So now you’re ready to publish the event:

 _eventAggregator.GetEvent<NavigationSelectedEvent>().Publish(NavigationEnum.Destination.Home);

And then subscribe to it – using an optional filter on the payload so you’re not wasting resources:

this.EventAggregator.GetEvent<NavigationSelectedEvent>().Subscribe(HandleNavigationEvent, ThreadOption.UIThread, false, i => i == NavigationEnum.Destination.Home);