Development on <<Decorator>> without uniform protocol

Static diagram :

Strong points :

1. 1. Extensibility
   1. 1.1 Addition or removal of a decorator does not need code modification
   2. 1.2 Addition or removal of an object to decorate does not need code modification
2. 2. Decoupling between decorator objects and objects to decorate
   1. 2.1 Minimal number of «Decorator» classes
   2. 2.2 Maximal factorization between decorators and object to decorate
3. 3. Decorators managing on program execution
   1. 3.1 Objects to decorate have any knowledge on decorators
   2. 3.2 A decorator may be decorated by another decorator

Spoiled pattern instantiations on different problems

Problem 1 :

Name :

Recursive objects decoration

Problem wording :

Design a system enabling to display visual objects on a screen. A visual object can be composed with one or more texts or images. If needed, the system must allow to add to this object a vertical scrollbar, an horizontal scrollbar, an edge and a menu (these additions may be cumulated).

Optimal solution :

Static diagram :

In this solution, if we add a new decoration, there is no code modification. Moreover, the decorators are able to decorate themselves.

Alternative solution :

Static diagram :

In this solution, there is no extensibility possibilities, no maximal factorisation between decorators and objects to decorate, and a decorator can't be decorated by another decorator