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Stéphane DucasseStephane.[email protected]://www.listic.univ-savoie.fr/~ducasse/
License: CC-Attribution-ShareAlike 2.0http://creativecommons.org/licenses/by-sa/2.0/
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.
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.
Possible Adapter Structures
Participants• Target (Shape)
• defines the domain-specific interface that Client uses.
•Client (DrawingEditor) • collaborates with objects conforming to the Target
•Adaptee (TextView) • defines an existing interface that needs adapting.
•Adapter (TextShape)• adapts the interface of Adaptee to the Target
Collaborations•Clients call operations on an Adapter
instance. In turn, the adapter calls Adaptee operations that carry out the request.
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.
Consequences•More flexibility than static inheritance.
Dynamic addition of properties
•Avoids feature-laden classes high up in the hierarchy.
• Lots of little objects.
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
Wrapping or not? Conforming or not•With decorator
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
•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.
Known Uses•VisualWorks Wrapper hierarchy
•Stream Decorators in VisualWorks: • BOSSTransporter is a stream decorator• FormattedStream is a stream decorator