Designing with the GRASP Patterns

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Designing with the GRASP Patterns
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Overview

• Assigning responsibilities to classes is a
  critical aspect of object-oriented design.
• There are well-known best principles for
  assigning responsibilities.
• Codified as the GRASP patterns.
• We explore
• Ideas.
• Examples.
• Practice

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Objectives

• Apply the GRASP patterns to assign
  responsibilities and design object interactions.
• This is the most important objective of the
  course.
• An inevitable activity, no matter what else is
  skipped.
• Greatest influence on the quality of the software.

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A Sample Partial Process
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The Design of Behavior

• What methods in what classes? How should objects
  interact?
• These are critical questions in the design of
  behavior.
• Poor answers lead to abysmal, fragile systems
  with low reuse and high maintenance.
• Collaboration diagrams illustrate the design of
  behavior.
• What is a good design?
• What are the principles of a good design?

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Assignment of Responsibilities

• Design of behavior implies assigning
  responsibilities to software classes.
• Responsibilities.
• Knowing.
• Doing.
• Calculating, coordinating, creating,
• A message in a collaboration diagram suggests a
  related responsibility.

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Assignment of Responsibilities

• A responsibility is
• Related to the methods and data of classes.
• Fulfilled with one or more methods.
• Possibly spread across classes.

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Responsibility Assignment is the Key

• Appropriate assignment of responsibilities to
  classes is the key to successful design.
• There are fundamental principles in assigning
  responsibilities that experienced designers
  apply.
• These principles are summarized in the GRASP
  patterns.

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The GRASP Patterns

• GRASP
• Acronym for General Responsibility Assignment
  Software Patterns.
• Has nine core principles that object-oriented
  designers apply when assigning responsibilities
  to classes and designing message interactions.
• Can be applied during the creation of
  collaboration diagrams.

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The GRASP Patterns

• Memorization and application of these patterns
  are critical for successful object-oriented
  designs.
• 1. Expert.
• 2. Creator.
• 3. Controller.
• 4. Low Coupling.
• 5. High Cohesion.
• 6. Polymorphism.
• 7. Pure Fabrication.
• 8. Indirection.
• 9. Dont Talk to Strangers.

Memorizing and applying these nine principles
during every design exercise is the primary
course objective.
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Expert

• The most general purpose responsibility
  assignment principle?
• Assign a responsibility to the information
  expert the class with the information necessary
  to fulfill the responsibility.

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Expert

• Related to another fundamental responsibility
  assignment strategy central to object-oriented
  design
• Do It Myself. (Coad)
• A Book calculates its own due date.
• Closely related to and also known as
• Put services with attributes. (Coad)
• That which knows, does it. (Seive)

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Creator

• Who should create an instance of a particular
  class?
• Consider assigning Class B the responsibility to
  create an instance of class A if one of the
  following is true
• B contains A.
• B aggregates A.
• B records A.
• B closely uses A.
• B is the creator of A instances.
• This pattern supports low coupling.

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Creator
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Controller

• A controller is a class that handles a system
  event message.
• What class should handle a system event message?

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Controller (continued)

• Assign the responsibility for handling a system
  operation message to one of these options
• The business or overall organization (a façade
  controller).
• The overall system (a façade controller).
• An animate thing in the domain that would perform
  the work (a role controller).
• An artificial class representing the use case (a
  use-case controller).

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Controller - Facades

• Facades are covers.
• Intent - A class that (in some way) represents an
  overall cover.
• Many possibilities.
• Example The entire organization.

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Controller - Facades

• Other facades? A class representing the system.
  Examples
• The software information system.
• The device that includes a computer and
  software such as an ATM.
• Others.

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Controller
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• Before applying the GRASP patterns, lets
  step back and review the big-picture relationship
  between the artifacts we have developed...

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Artifact Relationships

• Every system operation initiates a
  collaboration diagram.

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Optional Exercise

• As far as possible, name and draw influence
  lines between artifacts.

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Exercises

• From memory, individually write down the
  definitions for
• Expert.
• Creator.
• Controller.
• Write the definitions on a large poster.
• Create collaboration diagrams for the system
  operations indicated by the instructor.
• Apply the GRASP patterns.
• Annotate each message in your collaboration
  diagram with the chosen GRASP pattern.

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What We Learned Reflection

• Skillful assignment of responsibilities is
  critical to object-oriented design.
• The GRASP patterns codify the fundamental
  principles.
• Reflection.
• Value?
• When would you use this?
• Use at work?

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