4.2. Recreating Decorator

So far in this chapter, you’ve seen how very small concepts and design issues can be composed in well-formed and specific ways to form larger concepts. You’ve also seen how these concepts are an encapsulation of the implementation, so the design discussion can be simplified through abstraction. We’re going to combine these elements to build up a much more satisfying and robust definition of Decorator than we started with in Chapter 2.

We start by revisiting our initial UML diagram for Decorator in Figure 4.10 and our UML version of Fulfill Method in Figure 4.11.

Figure 4.10. Decorator’s usual example UML.

Figure 4.11. Fulfill Method definition as annotated UML.

Looking again at Decorator (Figure 4.10), we can see where the structure from Fulfill Method is found, in two places, with the Component class acting as the Abstractor role in both. On second thought, this is looking quite like our Objectifier pattern from Figure 2.2, isn’t it? Good eye. This is exactly what Objectifier is—the expansion of a single Fulfill Method across several concrete subclasses, as in Figure 4.12. Again, we’re using the PIN for Fulfill Method, shown here in its stacked form (see Section 3.2.4) to indicate how the underlying concepts fit together. We just defined a design pattern that already existed in the literature, but from first principles, by building it out of smaller, well-understood pieces. In Figures 4.2 through 4.5, we wrapped Fulfill Method in a PINbox. We do the same wrapping of Objectifier using Figure 4.12 as the internals but omitting the diagrams in between. This gives us a PINbox for Objectifier.