Æliens

A catalogue of design patterns

• a common design vocabulary
• documentation and learning aid
• an adjunct to existing methods
• a target for redesign

 [GOF94]

pattern schema

• increases design vocabulary

• explains the problem and the conflict

• design, responsibilities, collaborations

• to understand the costs and benefits

design for change

• creating an object by specifying a class explicitly -- Abstract Factory, Factory Method, Prototype
• dependence on specific operations -- Chain of Responsibilty, Command
• dependence on hardware & software platforms -- Abstract Factory, Bridge
• dependence on object implementation or representation --Abstract Factory, Bridge, Memento, Proxy
• algorithm dependence -- Iterator, Strategy, Template Method, Visitor
• extending functionality by subclassing -- Bridge, Composite, Decorator, Observer
• tight coupling -- Abstract Factory, Bridge, Chain of Responsibilities, Command, Facade, Mediator, Observer
• inability to alter classes conveniently -- Adaptor, Decorator, Visitor

Creational patterns

• Factory -- hide concrete classes
• Factory Method -- virtual constructors
• Prototype -- dynamic creation by cloning
• Singleton -- one instance only

See Pattern Lectures

Encapsulating behavior

• Command -- action + undo
• Strategy -- choice of algorithms
• Visitor -- decouple traversal and operations
• Iterator -- access and traversal
• State -- object state $->$ behavioral change

Observer

• one-to-many dependencies and notification

• abstract coupling between subject and observer
• constraint propagation
• deals with unexpected updates