title: How to modules in Prism (CAL) communicate with each other?
body: <p>I've got a WPF application which uses the MVVM pattern throughout, no code-behind, the ViewModels communicate with each other through the MainViewModel which gets injected into each of them.</p>
  
  <p>Eventually, this application needs to be incorporated into an application which uses Composite Application Library, Unity, etc. Looking through the code and documentation of CAL, I can see how I can <strong>register my <em>whole application</em> as a module in the CAL application</strong>, but how is my application-as-module going to communicate with the other modules that are also dynamically loaded? I'm expecting, e.g. that each module <strong>gets the CAL application somehow injected</strong>, or that there is some kind of <strong>Event Controller</strong> or <strong>Messenger</strong> with which I can loosely communicate with the other modules, i.e. can send a message and respond to events but not worry if those modules are actually there or not.</p>
  
  <p><strong>How do Composite Application modules communicate with each other?</strong></p>