Mediator pattern

In software engineering, the mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior.

In object-oriented programming, programs often consist of many classes. Business logic and computation are distributed among these classes. However, as more classes are added to a program, especially during maintenance and/or refactoring, the problem of communication between these classes may become more complex. This makes the program harder to read and maintain. Furthermore, it can become difficult to change the program, since any change may affect code in several other classes.

With the mediator pattern, communication between objects is encapsulated within a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator. This reduces the dependencies between communicating objects, thereby reducing coupling.

Overview
The mediator design pattern is one of the twenty-three well-known design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

===Problems that the mediator design pattern can solve ===
 * Tight coupling between a set of interacting objects should be avoided.
 * It should be possible to change the interaction between a set of objects independently.

Defining a set of interacting objects by accessing and updating each other directly is inflexible because it tightly couples the objects to each other and makes it impossible to change the interaction independently from (without having to change) the objects. And it stops the objects from being reusable and makes them hard to test.

Tightly coupled objects are hard to implement, change, test, and reuse because they refer to and know about many different objects.

Solutions described by the mediator design pattern

 * Define a separate (mediator) object that encapsulates the interaction between a set of objects.
 * Objects delegate their interaction to a mediator object instead of interacting with each other directly.

The objects interact with each other indirectly through a mediator object that controls and coordinates the interaction.

This makes the objects loosely coupled. They only refer to and know about their mediator object and have no explicit knowledge of each other.

See also the UML class and sequence diagram below.

Definition
The essence of the mediator pattern is to "define 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 allows their interaction to be varied independently. Client classes can use the mediator to send messages to other clients, and can receive messages from other clients via an event on the mediator class.

UML class and sequence diagram


In the above UML class diagram, the  and   classes do not refer to (and update) each other directly. Instead, they refer to the common  interface for controlling and coordinating interaction, which makes them independent from one another with respect to how the interaction is carried out. The  class implements the interaction between   and.

The UML sequence diagram shows the run-time interactions. In this example, a  object mediates (controls and coordinates) the interaction between  and   objects.

Assuming that  wants to interact with   (to update/synchronize its state, for example),   calls   on the   object, which gets the changed data from   and performs an   on.

Thereafter, calls  on the   object, which gets the changed data from   and performs an   on.

Class diagram


Mediator - defines the interface for communication between Colleague objects
 * Participants

ConcreteMediator - implements the mediator interface and coordinates communication between Colleague objects. It is aware of all of the Colleagues and their purposes with regards to inter-communication.

Colleague - defines the interface for communication with other Colleagues through its Mediator

ConcreteColleague - implements the Colleague interface and communicates with other Colleagues through its Mediator

C#
The mediator pattern ensures that components are loosely coupled, such that they do not call each other explicitly, but instead do so through calls to a mediator. In the following example, the mediator registers all Components and then calls their SetState methods.

A chat room could use the mediator pattern, or a system where many ‘clients’ each receive a message each time one of the other clients performs an action (for chat rooms, this would be when each person sends a message). In reality using the mediator pattern for a chat room would only be practical when used with remoting. Using raw sockets would not allow for the delegate callbacks (people subscribed to the Mediator class’ MessageReceived event).

Java
In the following example, a  object controls the values of several   objects, forcing the user code to access the stored values through the mediator. When a storage object wants to emit an event indicating that its value has changed, it also goes back to the mediator object (via the method ) that controls the list of the observers (implemented using the observer pattern).