**Intent:** Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
* a set of objects communicate in well-defined but complex ways. The resulting interdependencies are unstructured and difficult to understand
* 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
##Memento
**Intent:** Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
