decorator

Decorator

General Notes

This is to add functionality to a specific object, not an entire class.

"Change the skin - not the guts"

Intent

Attach additional responsibilities to an object dynamically. Decorators provice a flexible alternative to subclassing for extending functionality

When to Use

• To add responsibilities to individual ojects dynamically and transperently (without affecting other objects)
• For responsibilities that can be withdrawn
• When extension by subclassing is impractical

Participants

• Components
  • Defines the interface for objects that can have responsibilities added to them dynamically
• ConcreteComponent
  • Defines an object to which additional responsibilities can be attached
• Decorator
  • Maintains a reference to a Component object and defines an interface that conforms to Components interface
• ConcreteDecorator (BorderDecorator, ScrollDecorator)
  • Adds responsibilities to the component

Collaborations

• Decorator forwards requests to its Component object. It may perform additional operations before and after forwarding the request

Consequences

• More flexibility than static inheritance
  • Added and removed at runtime
• Offer a pay as you go approach
  • No need for feature-laden classes high in the hierarchy
• Typcially, decorator dense systems have lots of little and similar objects

Implementation

• A decorator objects interface must conform to the interface of the component it decorates
  • (inherit from common classes)
• Dont ened to use an abstract Decorator - just roll it out raw
• Need everything to inherit from a common Component class
  • Helps to keep this Component as simple as possible
  • (dont put lots of functionality in it)

Related Patterns

• Adapter: A decorator is different in that it changes the objects responsibilites, not the interface

• Composite: A decorator can be viewed as a degenerate composite with only one component

• Strategy: A decorator lets you change the skin, a strategy lets you change the guts