A decorator pattern is one of the best ways to add features to an object without changing its interface. I use composable decorators quite often and always question myself as to how to design them right when the list of features must be configurable. I'm not sure I have the right answer, but here is some food for thought.
Let's say I have a list of numbers:
Now I want to create a list that will only have odd, unique, positive, and sorted numbers. The first approach is vertical (I just made this name up):
The second approach is horizontal (again, a name I made up):
See the difference? The first approach decorates
ArrayNumbers "vertically," adding functionality through the composable decorators
The second approach introduces the new interface
Diff, which implements the core functionality of iterating numbers through instances of
For the user of
numbers, both approaches are the same. The difference is only in the design. Which one is better and when? It seems that vertical decorating is easier to implement and is more suitable for smaller objects that expose just a few methods.
As for my experience, I always tend to start with vertical decorating since it's easier to implement but eventually migrate to a horizontal one when the number of decorators starts to grow.