Friday, April 29, 2011

Chapter 52: Design Pattern Elements

Before we dig into the specifics of the Design Patterns that are part of the SCWCD Exam, we need to know one important thing. “The Elements of a Design Pattern”. This is what you are going to learn in this chapter.

So, lets get started!!!

Elements of a Design Pattern:

There are many ways to define a pattern, but the classical way is to describe its elements, or the aspects of a pattern.

The 3 main elements in any Design pattern are:
• Context
• Problem and
• Solution

The following are their definitions:
• Context is the recurring situation in which a problem to be solved is found.
• Problems are the so-called forces, such as marketing and technological forces, that occur in this context.
• Solution is the defined design that reorganizes or manipulates, some say resolves, the problem forces into a desired outcome within that context.
The design pattern is not only these three, but the relationship between the three and the formal language that describes the whole business.

For the sake of easy understanding & explanation, the following are the elements that would be used while explaining each of the design patterns that are covered in this series:
• Is — A direct explanation of what the pattern is.
• Is Not — An attempt at contrasting the concept because it is often helpful to understand what something is by looking at what it isn't.
• Analogy — This provides a comparison based on general aspects. Analogies, hopefully, give you a way to connect what you already know to these patterns which may be new to you.
• Problem — A statement of the problem that describes the patterns reason/purpose and intent.
• Responsibility — This describes what the pattern is accountable for; the primary things it accomplishes.
• Aim — These are the goals and objectives the pattern should reach within the given context.
• Primary Activity — What the main activities done by the pattern are.
• Context — The conditions and environment in which the problem and its solution recur. In other words, where should you consider and apply this pattern?
• Benefits — Why use a particular pattern, its benefits or advantages.
• Usage — Which kinds of situations are good candidates for this pattern.
• Consequences — This describes the result of using the pattern, the final state of the system. There are good and bad consequences of applying the pattern. It may solve one problem, but give rise to a new one.
• Uses — This tells you one or more examples of how this pattern is being used.
• Other Related Patterns — This names other patterns that are related by context, solution, or consequences. The related pattern may solve the same problem in another way or share the same context, but resolve different forces.

Patterns on the Exam

From the SCWCD Exam perspective, you need to worry about only 5 patters and we will be covering all of them, one by one. As a precursor, the 5 patters are:
1. Value Object
2. Data Access Object
3. Business Delegate
4. Front Controller &
5. Model View Controller (MVC)

Exam Trivia:
If you see any other pattern name, ignore it unless the question is one of those about which one is not a pattern.

Previous Chapter: Chapter 51 - Introduction to Design Patterns

Next chapter: Chapter 53 - Value Object Pattern

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