Event aggregator

An event aggregator (also known as publisher/subscriber or Pub-Sub) is a messaging pattern where senders of messages/events, called publishers, do not program the messages to be sent directly to a single/specific receiver, called subscribers, but, instead, categorize published messages into classes without knowledge of which subscribers, if any, there may be. Messages are then funneled through a so-called aggregator and delivered to the subscribers. This approach provides a complete decoupling between the publishers and subscribers while maintaining an effective messaging channel.

Event aggregator implementation within the Xamarin.Forms framework is done via MessagingCenter. MessagingCenter exposes a simple API that is composed of two methods for subscribers (that is, subscribe and unsubscribe) and one method for publishers (send).

We can demonstrate the application of the event aggregator pattern by creating a simple event sink for service communication or authentication issues in our application. In this event sink (which will be our subscriber), we can subscribe to error messages that are received from various view-models (given that they all implement the same base type) and alert the user with a friendly dialog:

MessagingCenter.Subscribe<BaseViewModel> (this, "ServiceError", (sender, arg) =>
    {
        // TODO: Handler the error
    });

We would have the following in the event of an unhandled exception on our view-model:

 public void Login()
 {
  try
  {
  //TODO: Login
  Result = "Successfully Logged In!";
  }
  catch(Exception ex)
  {
  MessagingCenter.Send(this, "ServiceError", ex.Message);
  }
 }