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.
Contents
Usually a program is made up of a large number of classes. Logic and computation is 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.
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.
Participants
Mediator - defines the interface for communication between Colleague objects
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
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 don't 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 wouldn’t allow for the delegate callbacks (people subscribed to the Mediator class’ MessageReceived event).
Java
In the following example a mediator object controls the status of three collaborating buttons: for this it contains three methods (book(), view() and search()) that set the status of the buttons. The methods are called by each button upon activation (via the execute() method in each of them).
Hence here the collaboration pattern is that each participant (here the buttons) communicates to the mediator its activity and the mediator dispatches the expected behavior to the other participants.