Design Patterns Explained

Chapter 12 • Solving the CAD/CAM Problem with Pattern* 215
In the original solution, I had a model that contains Features.
Features are either slot features, hole features, cutout features,
irregular features, or special features. Slot features are either V1
slots or V2 slots. V1 slots use the V1 system while the V2 slots use
the OOGSlot. Hole features are either V1 hole features or V2 hole
features. V1 hole features use the V1 system while the V2 hole features
use the OOGHole. Getting tired of this already, aren't you?
Now read the latest solution. I have a model that contains Features.
Features are either slot features, hole features, cutout features,
irregular features, or special features. All features contain an implementation
which is either a V1 implementation or a V2 implementation.
V1 implementations use a V1 Facade to access the V1 system
while V2 implementations adapt an OOGFeature. That's it. It
sounds much better than just a portion of the other solution.
Summary
In this chapter, I showed how the standard way of doing designs
can often lock us into systems that are hard to maintain. Often, it
can be difficult to see the forest for the trees because I become
overly focused on the details of the system— the classes.
Christopher Alexander gives us a better way. By using patterns in
the problem domain, I can look at the problem in a different way. I
start with the big picture and add distinctions as I go. Each pattern
gives me more information than what I had before I used it.
By selecting the pattern that creates the biggest picture— the context
for the system— and then inserting the next significant pattern,
I developed an application architecture that I could not have seen by
looking at the classes alone. Thus, I begin to learn to design by
context instead of by putting together pieces that were identified
locally.
In this chapter