## Mediator

Mediator pattern has behavioral purpose and applies on objects.
The pattern defines an object that encapsulates how a set of objects interact.
It promotes loose coupling by keeping objects from referring to each
other explicitly, and it lets you vary their interaction independently. 

### When to use

* a set of objects communicate in well-defined but complex ways
* reusing an object is difficult because it refers to and communicates with many other objects
* a behavior that's distributed between several classes should be customizable without a lot of subclassing