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.

Applicability: Use the Mediator pattern when

• 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

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software