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Stéphane DucasseStephane.Ducasse@univ-savoie.frhttp://www.listic.univ-savoie.fr/~ducasse/

Decorator

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License: CC-Attribution-ShareAlike 2.0http://creativecommons.org/licenses/by-sa/2.0/

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Decorator•Attach additional responsib• ilities to an object dynamically. •Decorators provide a •flexible alternative to • subclassing for extending • functionality.•Aka: Wrapper

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Decorator Intent•Attach additional responsib ilities to an

object dynamically. •Decorators provide a flexible alternative to • subclassing for extending functionality.•Aka: Wrapper

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Motivation•Adding behavior to individual object not

entire class• scrollbar• border

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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...

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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

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Decorator Solution

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Applicability•When responsibilities can be withdrawn•When responsibilities can be added

transparently•When subclassing is not possible

(combination explosion)

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Possible Decorator Structure

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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.

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Collaborations•Decorator forwards requests to its

Component object. It may optionally perform additional operations before and after forwarding the request.

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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.

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Consequences•More flexibility than static inheritance.

Dynamic addition of properties

•Avoids feature-laden classes high up in the hierarchy.

• Lots of little objects.

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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

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Wrapping or not? Conforming or not

•With decorator

•With strategies

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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.

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Known Uses•VisualWorks Wrapper hierarchy

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