stoop 440-adaptor

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  • 1. Adapter

2. 3. Adapter

  • Convert the interface of a class into another interface clients expect.
  • Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
  • Aka: Wrapper

4. Motivation

  • Sometimes a toolkit class designed for reuse isn't reusable only because its interface doesn't match the domain-specific interface an application requires.

5. Adapter Solution 6. Possible Adapter Structures 7. Participants

  • Target (Shape)
    • defines the domain-specific interface that Client uses.
  • Client (DrawingEditor)
    • collaborates with objects conforming to the Target interface.
  • Adaptee (TextView)
    • defines an existing interface that needs adapting.
  • Adapter (TextShape)
    • adapts the interface of Adaptee to the Target interface.

8. Collaborations

  • Clients call operations on an Adapter instance. In turn, the adapter calls Adaptee operations that carry out the request.

9. 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.

10. Consequences

  • More flexibility than static inheritance. Dynamic addition of properties
  • Avoids feature-laden classes high up in the hierarchy.
  • Lots of little objects.

11. 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

12. Wrapping or not? Conforming or not

  • With decorator
  • With strategies

13. 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 interfacemustconform 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.

14. Known Uses

  • VisualWorks Wrapper hierarchy
  • Stream Decorators in VisualWorks:
    • BOSSTransporter is a stream decorator
    • FormattedStream is a stream decorator