Design Patterns Explained

Chapter 12 • Solving the CAD/CAM Problem with Patterns 2O7
Remember, I am not concerned about the design of the expert sys- Expanding the
tem. While interesting (and, in many ways more challenging), that design design
had already been worked out. My focus is on the design of the Mode l. I know that
the Model consists of Features, as shown in Figure 12-5. l
Figure 12-5 The Model design.
Now, I am ready for the Bridge pattern. It is apparent that I have
multiple Features (the abstraction) with multiple CAD/CAM systems
(the implementations). These are the objects that set the context
for the Bridge pattern.
The Bridge pattern relates the Features to the different CAD/CAM
system implementations. The Feature class is the Abstraction in
the Bridge pattern while the V1 and V2 systems are the Implementations.
But what about the Model? Is there a Bridge pattern
present here as well? Not really. I can build the Model using inheritance
because the only thing about the Model that varies is the
implementation that is being used. In this case, I could make derivations
of the Model for each CAD/CAM system as in Figure 12-6.
If I tried using a Bridge pattern for the Model, I'd get the design
shown in Figure 12-7.
1. The differences between V1Model and V2Model present little difficulty. Therefore,
I will only discuss Model in general.
Ready for the Bridge
How do we handle
the Model?