Decorator

Static diagram :

Description :

Type :

St (Structural)

Intent :

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

Applicability :

Use Decorator :
- to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects.
- for responsibilities that can be withdrawn.
- when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing.

Strong points :

1. Extensibility
1. 1.1 Addition or removal of a decorator does not need code modification
2. 1.2 Addition or removal of an object to decorate does not need code modification
2. Decoupling between decorator objects and objects to decorate
1. 2.1 Minimal number of «Decorator» classes
2. 2.2 Maximal factorization between decorators and object to decorate
3. Decorators managing on program execution
1. 3.1 Objects to decorate have any knowledge on decorators
2. 3.2 A decorator may be decorated by another decorator

Pattern's participants :

Component
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 Component's interface.
ConcreteDecorator
Adds responsibilities to the component.

Spoiled patterns :

Problems :

1. Recursive objects decoration

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