Should Decorators Preserve the Component Interface?

TL;DR

This paper introduces variants of the Decorator pattern, such as MixDecorator, D2Decorator, and HybridDecorator, to overcome limitations related to interface constraints and linear composition, enabling more flexible and dynamic object extension.

Contribution

The paper proposes new Decorator variants that allow direct access to added responsibilities and support multiple interfaces, enhancing flexibility over classical Decorator pattern.

Findings

MixDecorator eliminates interface constraints and linear composition issues.

D2Decorator uses double-dispatch for more flexible decoration.

HybridDecorator combines inheritance and composition for dynamic object typing.

Abstract

Decorator design pattern is a well known pattern that allows dynamical attachment of additional functionality to an object. Decorators have been proposed as flexible alternative to subclassing for extending functionality. Still, the Decorator pattern has certain limitations, especially related to the fact that in its classical form it is constrained to a single interface, which is implicitly defined by the type of the concrete components that we intend to decorate. Another problem associated to the Decorator pattern is related to the linear composition of the decorations, which could lead to problems in accessing the newly added responsibilities. In this context, the paper presents variants of the Decorator pattern: MixDecorator and D2Decorator, and a variant specific only to C++ language based on templates - HybridDecorator. MixDecorator could be considered a new enhanced version of the Decorator pattern that eliminates some constraints of the Decorator pattern, but also it could be used as a base of a general extension mechanism. The main advantage of using MixDecorator is that it allows direct access to all newly added responsibilities, and so, we may combine different interface-responsibilities (newly added public methods) and operate with them directly and in any order, hiding the linear composition of the decorations. D2Decorator is a variant based on a double-dispatch mechanism. The C++ metaprogramming mechanism based on templates allows an interesting hybrid variant of the Decorator - HybridDecorator, which mixes on-demand defined inheritance with composition. Using these variants of the Decorator pattern we are not longer limited to one single interface; the set of the messages that could be sent to an object could be enlarged, and so, we may consider that using them, we can dynamically change the type of objects.