1. A Behavior Object Pattern
The Mediator Pattern
A Behavior Object Pattern

2. Battling Class Complexity
Consider an air traffic controller
Many planes circle an airport
If they communicated with each other the skies above an airport would be a chaos
Accepted solution: have the planes communicate directly with an air traffic controller tower for permission to land
The planes do not even have to know about each other
This information is kept with the tower

3. Battling Class Complexity
Let’s abstract this to classes and their objects in a program
When class objects are allowed to communicate directly with each other then they become too tightly coupled
When one object wishes to send a message to another then we need the equivalent of an air traffic controller to forward the message to the recipient
Keep the dispatching information inside the new controller
Call this coordinating object a mediator

4. AirController
Airplane
+mediator
Boeing 747
Yerevan Int’l
Cessna

5. Remarks Airplane is an abstract class for the concrete planes that fly
AirController is an interface to concrete mediators
Cessna and Boeing747 are the objects controlled by the mediator, called colleagues
Yerevan International is the concrete mediator that coordinates the colleague objects

6. Most Importantly The Colleagues do not send messages to each other
They send messages to the mediator
The mediator sends messages to the recipient
Each plane knows about the concrete controller
Whenever something is to be communicated it goes to the AirController

7. The Mediator Pattern Mediator Colleague Concrete Coll 2 Concrete Med

8. Object View (run-time)
aConcreteMediator
aColleague
aColleague
mediator
mediator
aColleague
aColleague
mediator
mediator

9. Mediator Encapsulates interconnects between objects
Is the communications hub
Is responsible for coordinating and conrolling colleague interaction
Promotes loose coupling between classes
By preventing from referring to each other explicitly
Arbitrates the message traffic

10. How to Use Mediator
Identify a collection of interacting objects whose interaction needs simplification
Get a new abstract class that encapsulates that interaction
Create a instance of that class and redo the interaction with that class alone
But, don’t play God!

11. When to Use Mediator
When a set of objects communicates in a well-defined, but complex way
When reusing an object is difficult because it refers to and communicates with many other objects (tight coupling)
When a behavior that is distributed between several classes should be customizable without a lot of subclassing

12. Related Patterns
Observer--communication distributed by using observer and subject objects
Mediator encapsulates the communication
Facade--Create a class interface for an entire subsystem
The subsystem doesn’t know about the Facade
Facade doesn’t add functionality, Mediator does
Mediator’s colleagues are aware of Mediator

13. The Facade Pattern
Purpose: Create a class that is the interface to an entire subsystem whose diverse methods should remain within the system
Consequences
Hide the implementation of the subsystem from clients
Promote weak coupling between the subsystem and the clients
Does not prevent clients from using subsystem classes--should it?

14. Facade Pattern
client
client
Facade
Subsystem

15. Project Use Add a GUI to POST Decouple the GUI from the business logic
Make the business logic totally unaware of the graphical interface--make it get called by the mediator, never directly from the GUI component

16. Example Business Logic Mediator GUI Payment Panel MakePayment()
payment = new...
makePayment()
Complete this diagram for all GUI components!