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IntroductionInformation ExpertCreatorLow CouplingHigh CohesionController
IntroductionIntroduction
Design processWhat’s GRASP patternWhat’s responsibilityResponsibilities and methodsResponsibilities and Interaction Diagrams
Design ProcessDesign Process
After identifying your requirements and creating a domain model, then add methods to the software classes, and define the messaging between the objects to fulfill the requirements.
What’s GRASP patternWhat’s GRASP pattern
This approach to understanding and using design principles is based on patterns of assigning responsibilities.
The GRASP patterns are a learning aid to help one understand essential object design, and apply design reasoning in a methodical, rational, explainable way.
What’s responsibilityWhat’s responsibility
Doing:– Doing something itself, such as creating an object or
doing a calculation– Initiating action in other objects– Controlling and coordinating activities in other objects.
Knowing:– Knowing about private encapsulated data– Knowing about related objects– Knowing about things it can derive or calculate
Responsibilities and methodsResponsibilities and methods
:Sale
:Payment
makePaymentcreate
makePayment implies Sale object has a responsibility to create a Payment object
Responsibilities and Responsibilities and Interaction DiagramsInteraction Diagrams
Interaction diagrams show choices in assigning responsibilities to objects.
GRASP patterns guide choices in where to assign responsibilities.
GRASP patterns are a codification of widely used basic principles.
Information ExpertInformation Expert
Assign a responsibility to the information expert – the class that has the information necessary to fulfill the responsibility.
What is a general principle of assigning responsibilities to objects?– Who should be responsible for knowing/doing …?– Domain model (domain expert, domain analysis) to
design model (software classes).– Any existing to any representative.
Example: What information is Example: What information is needed to determine the grand total?needed to determine the grand total?
Sale
date
Product
descriptionpriceitemID
Sale
date
Product
descriptionpriceitemID
SalesLineItem
quantity
Who is information expert?
Sale
date
Product
descriptionpriceitemID
SalesLineItem
quantity
getTotal
getSubTotal
getPrice
Time sequence?
CreatorCreator Who should be responsible for creating a new instance
of some class? Creational Design Pattern Assign class B the responsibility to create an instance of
class A if one or more of the following is true:– B aggregates A– B contains A– B records instance of A objects– B closely use A objects– B passes initializing data to A
Low CouplingLow Coupling
Coupling is a measure of how strongly one element is connected to, has knowledge of, or relies on other elements.
Payment, register, and sale.– Time sequence?– Classes?
:Sale
:Payment
:Register
create
addPayment
makePayment
:Payment
:Sale
:Register
makePayment
create
makePayment
High CohesionHigh Cohesion How to keep complexity manageable? Time sequence of the above example. More tasks? Key role, key object, key class, key relations Modular Design (indirect control)
Yin and YangYin and Yang
Yin and Yang – The light color area which indicates more
sunlight is called Yang (Sun). The dark color area has less sunlight (more moonlight) and is called Yin (Moon). Yang is like man. Yin is like woman. Yang wouldn't grow without Yin. Yin couldn't give birth without Yang.
Interdependent influence Trade-off
ControllerController Assign the responsibility for receiving or handling a
system event message to a class representing one of the following choices:– Represents the overall system, device, or subsystem– Represents a use case within which the system event occurs.
Who should be the controller for system events such as enterItem and endSale?
Which class should be responsible for receiving system event message?
Interface layer does not handle system events. The controller receives the service request from the UI
layer and coordinates their fulfillment, ie., Command, Façade, and etc.
