1. Apply Expert, Creator, Controller, Low Coupling, High Cohesion
Overview
A critical skill is designing or thinking in objects. This can be practiced based on explainable principles. We will start applying these.
You will be able to:
Apply Expert, Creator, Controller, Low Coupling, High Cohesion
Design for low representational gap
Design for visibility
Define use case realizations
Relate the UP artifacts
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2. MOTIVATION: Introduction
Now what happens?
What object should receive this message?
What objects should interact to fulfill the request, and how?
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3. DEFINITION: Responsibility-Driven Design (RDD)
Detailed object design is usually done from the point of view of the metaphor of:
Objects have responsibilities
Objects collaborate
In RDD we do object design such that we will ask questions such as:
What are the responsibilities of this object?
Who does it collaborate with?
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4. DEFINITION: Responsibilities
Responsibilities are an abstraction.
The responsibility for persistence.
Large-grained responsibility.
The responsibility for the sales tax calculation.
More fine-grained responsibility.
They are implemented with methods in objects.
1 method in 1 object
5 methods in 1 object
50 methods across 10 objects
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5. “Façade” “Information Expert” … They provide a vocabulary of design.
DEFINITION: Patterns
Patterns are named problem-solution pairs to common problems, typically showing a popular, robust solution.
“Façade” “Information Expert” …
They provide a vocabulary of design.
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6. DEFINITION: GRASP Principles
What guiding principles to help us assign responsibilities?
These principles are captured in the GRASP patterns.
General Responsibility Assignment Software Patterns.
Very fundamental, basic principles of object design.
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7. The 9 GRASP Principles Expert Creator Controller Low Coupling
High Cohesion
Polymorphism
Pure Fabrication
Indirection
Protected Variations
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8. PATTERN: Information Expert
What is most basic, general principle of responsibility assignment?
Assign a responsibility to the object that has the information necessary to fulfill it.
“That which has the information, does the work.”
E.g., What software object calculates sales tax?
What information is needed to do this?
What object or objects has the majority of this information.
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9. Designing for Low Representational Gap
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10. Designing for Low Representational Gap
Contraindication:
We do not design for low representational gap regarding actors.
E.g., we don’t make a software class Clerk do all the work in the software system.
Why??
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11. Designing for Visibility
Fact: To send a message to B, A must have visibility to B. It doesn’t happen by “magic.”
Kinds of visibility:
Attribute
Parameter
Local
Global
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12. DEFINITION: Attribute Visibility
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13. DEFINITION: Parameter Visibility
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14. DEFINITION: Local Visibility
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15. What object creates an X?
PATTERN: Creator
What object creates an X?
Ignores special-case patterns such as Factory.
Choose an object C, such that:
C contains or aggregates X
C closely uses X
C has the initializing data for X
The more, the better.
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16. PATTERN: Controller
What object in the domain (or application coordination layer) receives requests for work from the UI layer?
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17. Solution: Choose a class whose name suggests:
PATTERN: Controller
Solution: Choose a class whose name suggests:
The overall “system,” device, or subsystem
A kind of Façade class
Or, represents the use case scenario or session
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18. Cohesion Coupling PATTERN: Low Coupling
Assign responsibilities so that coupling remains low.
What does low coupling mean?
Coupling
Cohesion
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19. PATTERN: High Cohesion
Assign responsibilities so that cohesion remains high?
What does high cohesion mean?
Coupling
Cohesion
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20. DEFINITION: Use Case Realizations
In the vocabulary of the UP and use case modeling, when we design the objects to handle the systems operation for a scenario, we are designing a use case realization.
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