MVVM- View Model-View Model Communications

Question

How do I go about having two view models communicate with one another using MVVM Light. I know how to use the messenger class and register etc.. Here is my Scenario

A Settings View ---> a Settings View Model 
                                  .
                                  .
                                  .

A MainPage View ---> A MainPage ViewModel

If something changes in the Settings View it will Message back to the Settings View Model. So then I want the Settings View Model to communicate to the MainPage View Model about what changed. THe MainPage ViewModel will then tell the View.

score 17 · Accepted Answer · edited Jul 11 '14 at 13:30

17

A common pattern for this style of problem is Mediator (a class that both view models reference and can be used to pass messages between the two).

The Mediator class has since been moved to the Cinch WPF/SL MVVM Framework, which appears to still be actively developed/supported.

The pattern I prefer is the Event Aggregator, an example can be found in the Prism framework. In this pattern different view models subscribe to events from the aggregator and others publish events.

Hope this helps

edited Jul 11 '14 at 13:30

Forest Kunecke

2,160
15
32

answered Mar 19 '10 at 03:30

Nigel Sampson

10,549
1
28
31

This. The Event Aggregator is a nice and clean way of doing decoupled communication. The only thing I dislike about it is the need to place the event definitions in a common location, but that's just a minor quibble. – JerKimball Mar 19 '10 at 14:07

score 3 · Answer 2 · answered Mar 19 '10 at 09:03

I second Nigel's suggestion of using the Mediator, take a look at Josh Smith's blog and his implementation of this:

http://joshsmithonwpf.wordpress.com/?s=mediator

At the bottom you can download the Mediator Prototype and Demo, just remember to rename it from .doc to a .zip.

Hope this helps...

score 1 · Answer 3 · answered Mar 19 '10 at 09:29

1

One thing you can try out is try to implement Dependency Inversion. Define an interface with some actions/contracts. Implement that interface on MainviewModel. Pass that interface as member variable to SettingsViewModel. So whenever settings view model has to notify something to main, it will use that interface. And additionally, other view models can use same strategy.

public interface IMessenger
    {
      void NotifyAction();
    }

public class MainViewModel:InotifyProprtyChanged,IMessenger
{
 public void NotifyAction()
{
}
}

public class SettingsViewModel:INotifyPropertyChanged
{
  public IMessenger Messenger{get;set;}

  public void SomeCommandExecutor()
  {
    if(Messenger!=null)
     {
       Messenger.NotifyAction();    
     }

  }
}

answered Mar 19 '10 at 09:29

RockWorld

1,278
2
11
24

Rakesh, I like where your going with this can you exand a little? In your example can you include the part where you "Pass that interface as member variable to SettingsViewModel." – May 10 '10 at 23:19
If you are going to construct SettingsViewModel in MainViewModel methods, you just have to do SettingsViewModel svm = new SettingsViewModel(){Messenger = this}; Or you can map MainViewModel and SettingsViewModel at place where SettingsViewModel is constructed. This scenario will occur when SettingsViewModel is constructed outside MainViewModel In that case you will be having acces to both SettingViewModel instance and MainViewModel instance at som other place, and it will do the mapping In design principle world, they call it as Mediator pattern... Wht do u say? – RockWorld May 19 '10 at 13:50

score 0 · Answer 4 · answered Mar 27 '11 at 09:28

0

May be you can use sur Mediator pattern V2 Made by Josh Smith & Marlon Grech.

Check out the Messenger class V2 in MVVM Foundation library, or directly on Marlon Grech blog

answered Mar 27 '11 at 09:28

scrat789

2,961
3
30
26

MVVM- View Model-View Model Communications

4 Answers4

Linked