Design Patterns

Last updated Mar 5, 2002

E. Gamma, et al., Design Pattern : Elements of Reusable Object-Oriented Software, Addison-Wesley, 1995, Chapter 5, pp. 273-282

1. Intent
2. Also Known As
3. Motivation
4. Applicability
5. Structure
6. Participants
7. Collaborations
8. Consequences
9. Implementation
10. Sample Code
11. Known Uses
12. Related Patterns

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.

Motivation

Applicability

Use the Mediator pattern when

• a set of objects communicate in well-defined but complex ways.
• 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 with a lot of subclassing.

Structure

Participants

• Mediator
  • defines an interface for communicating with Colleague objects.
• ConcreteMediator
  • implements cooperative behavior by coordinating Colleague objects.
  • knows and maintains its colleagues.
• Colleague classes
  • each Colleague class knows its Mediator object.
  • each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague.

Collaborations

• Colleagues send and receive requests from a Mediator object.
• The mediator implements the cooperative behavior by routing requests between the appropriate colleague(s).

Consequences

1. It limits subclassing.
   • The Mediator pattern requires subclassing Mediator only to change cooperative behavior among several objects.
   • Colleague classes can be reused as is.
2. It decouples colleagues.
   • Loose coupling between colleagues lets you can vary and reuse Colleague and Mediator classes independently.
3. It simplifies object protocols.
   • A mediator replaces many-to-many interactions with one-to-many interactions between the mediator and its colleagues.
4. It abstracts how objects cooperate.
   • The Mediator pattern makes coordination an independent concept and encapsulates it in an object.
   • Abstracting object-cooperation this way lets you focus on how objects interact apart from their individual behavior.
5. It centralizes control.
   • Because a mediator encapsulates protocols, it can become more complex than any individual colleague and hard to maintain.

Implementation

1. Omitting the abstract Mediator class.
   • The abstract coupling that the Mediator class provides lets colleagues work with different Mediator subclasses, and vice versa.
   • When colleagues work with only one type of Mediator, you don't need to define an abstract Mediator class.
2. Colleague-Mediator communication.
   • Colleagues communicate with their mediator when an event of interest occurs.
   • One approach is to implement the Mediator as an Observer using the Observer pattern.
     • Colleagues act as subjects notifying the mediator whenever they change state.
     • The mediator responds by propagating the effects of the change to other colleagues.

Sample Code

Known Uses

Related Patterns

• Facade abstracts a subsystem of objects to provide a more convenient interface.
• Colleagues can communicate with the mediator using the Observer pattern.