Communication between ViewModels with MVVM

June 5, 2011

Communication between ViewModels can be tricky at first glance, but there are some easy frameworks and patterns that can help you out.

For example, lets say you had a ShellViewModel that controlled your entire application, and it has an Exit command. Now lets say one of the sub-view-models in the application would like to call this exit command.

The easiest way is through a Relative Binding to the Window’s DataContext.ExitCommand.

<Button Command="{Binding 
    RelativeSource={RelativeSource AncestorType={x:Type Window},
    Path=DataContext.ExitCommand}}

Of course, this only works well if you can find the control containing the command you want through a binding.

Enter the Event System

Another way is through a loosely coupled event system. I would recommend either MVVM Light’s Messenger or Microsoft Prism’s EventAggregator. You could always build your own event system as well if you really wanted to.

Both event systems makes me think of a paging system. Any ViewModel can broadcast a message, and any ViewModel can subscribe to receive broadcasted messages. Messages are often packaged into different types, so a ViewModel can subscribe to only listen for specific message types such as CloseApplicationMessages.

This kind of loosely coupled event system is ideal for MVVM because the ViewModels don’t need to know about each other to be able to do their job.

Using either of these systems, your ViewModel that handles the event would subscribe to receive a specific message type. It would tell the event system that if a message of type X is broadcasted, send it to a specified delegate method. The ViewModel that wishes to raise the event simply has to broadcast the message and include any parameters needed in the body of the message.

Example using Prism’s EventAggregator

// Subscribe
eventAggregator.GetEvent<CloseAppliactionMessage>().Subscribe(ExitMethod);

// Broadcast
eventAggregator.GetEvent<CloseAppliactionMessage>().Publish();

(I also have some code posted here which simplifies PRISM’s EventAggregator for smaller applications)

Example using MVVM Light’s Messenger

//Subscribe
Messenger.Default.Register<CloseAppliactionMessage>(ExitMethod);

// Broadcast
Messenger.Default.Send<CloseAppliactionMessage>()