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  • 1. Decorator
  • 2.  
  • 3. Decorator
    • Attach additional responsib
    • ilities to an object dynamically.
    • Decorators provide a
    • flexible alternative to
    • subclassing for extending
    • functionality.
    • Aka: Wrapper
  • 4. Decorator Intent
    • Attach additional responsib ilities to an object dynamically.
    • Decorators provide a flexible alternative to
    • subclassing for extending functionality.
    • Aka: Wrapper
  • 5. Motivation
    • Adding behavior to individual object not entire class
      • scrollbar
      • border
  • 6. Inheritance?
    • Does not work well
    • Too much combination border, color, scrollbars, bounds, translation...
    • To static: Clients cannot control when to put a border or not...
  • 7. Decorator
    • Enclose the component into another one that adds border...in another one that adds scrollbar...
    • The decorator conforms to the interface of the component it decorates so that its presence is transparent to the component's clients.
    • The decorator forwards requests to the component and may perform additional actions (such as drawing a border) before or after forwarding
  • 8. Decorator Solution
  • 9. Applicability
    • When responsibilities can be withdrawn
    • When responsibilities can be added transparently
    • When subclassing is not possible (combination explosion)
  • 10. Possible Decorator Structure
  • 11. Participants
    • Component (VisualComponent)
      • defines the interface for objects that can have responsibilities added to them dynamically.
    • ConcreteComponent (TextView)
      • 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 (BorderDecorator, ScrollDecorator)
      • adds responsibilities to the component.
  • 12. Collaborations
    • Decorator forwards requests to its Component object. It may optionally perform additional operations before and after forwarding the request.
  • 13. About Identity
    • A decorator and its component aren't identical.
    • A decorator acts as a transparent enclosure. But from an object identity point of view, a decorated component is not identical to the component itself.
    • If the decorator is wrapping, then identity of the object may change.
    • Good at construction time, but else should “adapt” the references from the decorated to the decorator.
  • 14. Consequences
    • More flexibility than static inheritance. Dynamic addition of properties
    • Avoids feature-laden classes high up in the hierarchy.
    • Lots of little objects.
  • 15. Implementation
    • Interface conformance. A decorator object's interface must conform to the interface of the component it decorates. ConcreteDecorator classes must therefore inherit from a common class (at least in C++).
    • Omitting the abstract Decorator class. There's no need to define an abstract Decorator class when you only need to add one responsibility.
    • Keeping Component classes lightweight. Component should specify an interface, decorators are then easier to define
  • 16. Wrapping or not? Conforming or not
    • With decorator
    • With strategies
  • 17. Strategies?
    • Strategies are a better choice when the Component class is heavyweight, thereby making the Decorator pattern too costly to apply.
    • The Strategy-based approach might require modifying the component to accommodate new extensions.
      • a strategy can have its own specialized interface,
      • a decorator's interface must conform to the component's.
    • A strategy needs only define the interface for rendering a border, which means that the strategy can be lightweight even if the Component class is heavyweight.
  • 18. Known Uses
    • VisualWorks Wrapper hierarchy
    • Stream Decorators in VisualWorks:
      • BOSSTransporter is a stream decorator
      • FormattedStream is a stream decorator