MVVM: Provide a service to work with a specific part of the UI?

Question

let's say that in my application, there is an user interface for representing a (geographic) map. It is integrated into the application as a UserControl and has its view model behind it.

Now, suppose I want to provide other parts of my application with a generic service interface to perform common tasks on the map (zoom, pan etc.) and not worry about the UI specifics. I could give away direct reference to the viewmodel, but I am pretty sure I would violate separation of concerns principle, not to mention it would be less testable.

So there are few questions:

1. Does it make sense and is it good practice to implement such services (which act as an intermediate link to the UI) in the first place?
2. Since the service operates directly on the map's viewmodel, should it be the viewmodel itself which implements the service interface?
3. Is it appropriate for the service interface to provide events (e.g. besides providing a method to change the map scale, provide an event that the map scale was changed as well)? Or is it preferable to employ some kind of event broadcaster (aggregator) mechanism to push such notifications out of service interfaces?

Thanks in advance for your help.

Answer 1

Consider using the Messenger in the MVVM Light toolkit. See more in another SO answer:

https://stackoverflow.com/a/2700324/117625

Answer 2

EventAggregator is the another mechanism which establish communication between disconnected view models. I believe all the other parts of your application will be using the same MVVm and will have viewmodel to do the operations. Publish event say Zoom with required arguments from other parts of the app and catch it in the map using the subscribe mechanism.

http://msdn.microsoft.com/en-us/magazine/dd943055.aspx#id0420209

Prism has a good implementation of Event Aggregator. you can use that part.