Event Aggregator
Motivation
Channel events from multiple objects into a single object to simplify registration for clients.
Summary
An Event Aggregator acts as a single source of events for many objects.
It registers for all the events of the many objects allowing clients to register with just the aggregator.
Routed Events in WPF and Silverlight are very good examples of it.
You can think of an Event Aggregator as a particular form of Facade that focuses only on observer relationships.
When to Use
it is mostly used with Facade pattern.
References