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Design Pattern: Mediator

Design Pattern: Mediator. Modified from Kyle Kimport’s: Design Patterns: Mediator Ref: http://www.stevenblack.com. Outline:. What is a Mediator Why a Mediator? Anatomy of Mediator Consequences of Using a Mediator Mediator and Rest of the World

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Design Pattern: Mediator

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  1. Design Pattern: Mediator Modified from Kyle Kimport’s: Design Patterns: Mediator Ref: http://www.stevenblack.com

  2. Outline: • What is a Mediator • Why a Mediator? • Anatomy of Mediator • Consequences of Using a Mediator • Mediator and Rest of the World • Implementing a Mediator • Conclusions

  3. What is a mediator • An object that abstracts inter-workings of two or more objects. • Acts as communication hub that serve to decouple objects — no need to know about each other, just need to know their Mediator. • Promotes loose coupling by keeping objects from referencing to each other explicitly

  4. Mediator:a system of at least 3 objects messaging in a star topology Colleague OurMediator Mediator Colleague OurMediator Colleague OurMediator Provides a common connection point, centralized (subclassable) behavior and behavior mgmt, all with a common interface

  5. Classification and Intent • Classification: Object Behavior • Encapsulate object-to-object communication • Keeps objects from knowing about each other directly; this allows us easily change an object’s behavior

  6. Motivation • OO-design allows for more reusable and more elegant programs, when objects need to refer to each other, this elegance is often lost. • By consolidating all interaction in a single class, we can regain elegance and reusability

  7. When to use a mediator? • When one or more objects must interact with several different objects. • When centralized control is desired • When simple object need to communicate in complex ways. • When you want to reuse an object that frequently interacts with other objects

  8. Structure abstract Mediator abstract Collegue concrete Mediator concrete Collegue 1 concrete Collegue 2

  9. Participant--Abstract Mediator Define the Collegue-to-Mediator interface Participant--Concrete Mediator • Derivated from Abstract Mediator • Aware of and knows the purpose of all concrete Collegues • Receives messages from a colleague & sends necessary commands to other colleagues

  10. Participant--Abstract Colleague • Define the Mediator-to- Colleague interface • Knows about the mediator, but none of its colleagues Participant--Concrete Colleagues • Derived from abstract Colleague • Each Concrete Colleague knows its own behavior on a small scale, but NOT on a large scale.

  11. Mediator: Pros • Limits subclassing & specialization, allowing Colleague classes to be reused w/o any changes • Decouples colleagues, which facilitates independent variations of the colleague and mediator classes. • Simplifies protocol by replacing many-to-many interaction with one-to-one interaction • Abstracts object behaviors & cooperation. This allows you to deal w/ an object’s small-scale behavior separately form its interaction with other objects

  12. Mediator: Cons Reducing the complexity of Colleaguesincreases the complexity of the Mediator itself. In some situations, maintaining a large Mediator may become as daunting a task as operating w/o a one.

  13. Implementing a Mediator:Design Consideration • When colleagues are general and reusable, we should use an abstract Mediator • When colleagues will not be used elsewhere, you may forego the Abstract Mediator Do we need an abstract Mediator?

  14. How Colleagues Interact w/ the Mediator? • Implement the mediator itself as an Observer • Colleague pass themselves as arguments when communicating with the mediator

  15. Mediator & the Rest of the World Related Design Pattern • Observer: the Mediator class may be implemented using an Observer • Façade: is similar to a Mediator, but with one-way communication from the Façade to its subsystem classes.

  16. Mediator: Real World Example

  17. Sample Java Code abstract class Mediator{ public abstract void colleagueChanged(Colleague c); } abstract class Colleague{ private Mediator myMediator; public void changed(){ myMediator.colleagueChanged(this); } } class ListBox extends Colleague{ public void display(){ //..drawing routines } public void mouseClick(int x, int y){ //ListBox obj does sth… } }

  18. Mediator Object Sample Java Code class FontDialogBox extends Mediator{ private Button ok; private List fontList; private TextField fontName; private CheckBox latino; private CheckBOx latin2; //...more public FontDialogBOx(){ fontList = new ListBox(this); fontName = new TextField(this); //... } public void display(){ fontList.display(); fontName.display(); //... } public void colleagueChanged(Colleague c){ if( c==fontList) //font_list does sth… else if(c==fontName) //fontName does sth. else if…… } } } Declare Colleague Objects Manipulate colleague Objects

  19. Conclusion • A mediator is an objcet-behavior design pattern. • Use a Mediator when simple objects interact in complex ways • The Mediator pattern consists of two types of objects: the Mediator and its Colleagues • All object-to-object communication is encapsulated in the Mediator • Mediator allows for greater reusability, and generally more elegant, readable code.

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