Design Patterns Explained

Chapter 12 • Solving the CAD/CAM Problem with Patterns 213
Thinking in Patterns: Step 2d
(Abstract Factory)
All that is left is the Abstract Factory. As it turns out, this pattern is
not needed. The rationale for using an Abstract Factory was to
ensure all of the implementation objects were of type V1 if I had a
V1 system or of type V2 if I had a V2 system. However, the Model
object itself will know this. There is no point implementing a pattern
if some other object can easily encapsulate the rules of creation. I
left the Abstract Factory in the set of patterns because while I was
first solving this problem I did think the Abstract Factory was present.
It also illustrates how thinking that a pattern is present when it is
not is not necessarily counterproductive.
Thinking in Patterns: Step 3
The details of the design may still take some work. However, I
would continue with the design by following Alexander's mandate
of designing by context. For example, when I see how I need to
implement a SlotFeature class or the V1Imp class, I should
remember how the patterns involved are used. In this case, I note
that in the Bridge pattern, the methods involving the abstractions
are independent of implementation. This means that the
Abstraction class (Feature) and all of its derivations
(Slot-Feature, HoleFeature, and so forth) contain no
implementation information. Implementation information is left to
the Implementation classes.
This means the Feature derivations will have methods such as
getLocation and getLength, while the Implementations will
contain a way to access this required information. A Vllmp object,
for example, would need to know the ID of the Feature in the V1
system. Since each Feature has a unique ID, this means there will
be one Implementation object for each Feature object. The
methods in the Vllmp object will use this ID to ask the VlFacade
for information about the object.
Finally, do Abstract
Factory
Finishing the rest
Assigning
responsibilities