Design Patterns

1. Submitted To:
Submitted by:
Fizza Durrani
11073
7th Semester UET(A)
October 15, 2014
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2. “The critical design tool for software
development is a mind well educated in
design principles. It is not the UML or any
other technology.“
Craig Larman
Thus, GRASP is really a mental toolset, a learning
aid to help in the design of object-oriented
software.
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3. It stands for:
 General
 Responsibilities
 Assignment
 Software
 Patterns (Principles)
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4. Responsibility
A contract / obligation that a class / module /
component must accomplish
 Knowledge
 Private state
 Computed state


 Behavior
 Send messages itself and modify its private
state
 Instantiate another objects
 Send messages to another objects
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5.  “doing” responsibilities
 Doing something itself, such as creation an
object or doing a calculation.
 Initiating action in other objects
 Controlling and coordinating activities in other
objects.
 “knowing” responsibilities
 Knowing about encapsulated data.
 Knowing about related objects.
 Knowing about things it can derive or
calculate.
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7.  Informational Expert
 Creator
 Low Coupling
 High Cohesion
 Controller
 Polymorphism
 Pure Fabrication
 Indirection
 Controlled Variation
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9. Problem: Which class possesses information
about object A?
More common question: What is a general
principle of assigning responsibilities to
objects?
Assign the responsibility to the class that
knows the necessary information for
performing required action and fulfill the
responsibility.
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10. A B C D
DO
GetDataX()
GetDataY()
GetDataZ()
return Z
return Y
return X
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11. A B C D
DO
DoAction()
DoAction(X)
DoAction(Y)
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13. Problem: Who should be responsible for creating
object
A?
 Class B must have that responsibility if:
 B is composed by A (composition)
 B knows the necessary information in order to
instantiate A objects
 B depends heavily on A
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14. • The goal is to define creator-object, which will be
related to all created objects.
64
Board Square
Square
Board
create
create
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16. Problem: How to minimize dependencies between
classes ?
Coupling is the degree, defines how tightly one
component linked to other components, or how
much
information it knows about other components.
Examples
 Inheritance
 Composition / aggregation / association
 A send messages to B
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17.  Assign a responsibility so that coupling remains
low.
Low Coupling is an evaluative pattern, which
dictates how to assign responsibilities to support:
 lower dependency between the classes,
 change in one class having lower impact on
other classes,
 higher reuse potential.
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19. Problem: How to keep objects focused,
understandable, manageable, and support low
coupling?
Cohesion is a measure of how strongly related
or focused the responsibilities of a single module
are.
High Cohesion is an evaluative pattern that
attempts to keep objects appropriately focused,
manageable and understandable.
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20.  Alternatively, low cohesion is a situation in
which a given element has too many unrelated
responsibilities (“God Object”)
Low Cohesion implies:
 Hard to reuse
 Hard to maintain
 Constantly changing
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21. A A ?? ??
DoA()
DoB()
DoC()
DoA()
DoB()
DoC()
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23. Problem: Who should be responsible for
handling events and messages from external
actors (UI, …)?
 Assign the responsibility to a class, such as:
A class that represents the overall system,
device, or subsystem.
Façade Controller Pattern
A class that represent a use case, whereby
performs handling particular system operation.
Use Case Controller Pattern
 Generally does not perform operation by itself, but delegate
responsibility to component objects.
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25. Problem: How to act different depending in
object’s
class, or how to design pluggable components?
 In case of class behavior might changes,
responsibilities segregates to different behavior
specific classes, using polymorphic operations
for this class.
 Advise: Do not use type checking, but
conditional logic for implementation different
variations based on object type.
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26. Player
 There are (roughly) 3 types a polymorphism:
 Ad hoc Polymorphism
 Parametric Polymorphism
 Subtype Polymorphism
Square
landedOn
RegularSquare
landedOn
GoSquare
landedOn
OtherSquare
landedOn
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28.  Problem: How to assign responsibilities if applying the
Informational Expert principle decreases cohesion and
increases coupling?
 Assign the responsibility to an artificial class that does
not belongs to the domain model.
 Pure Fabrication is a class that does not reflect any
business domain object, but required only for increase
cohesion and decrease coupling.
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30.  Problem: How to assign responsibilities in order to avoid
direct coupling between two components, and keep
ability for reuse.
 Assign responsibility to intermediate class for providing
linking between objects not linking directly.
 Related design patterns: Adapter, Bridge, Mediator.
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32.  Problem: How to design system and subsystems,
that changes in these components does not affects
on other components.
 Identify points of possible variations and instability;
create stable interfaces upon instable components.
Open-Closed Principle almost equivalent to CV
pattern.
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33.  There are 2 types of points:
 Variation point – branching point on existing system or
in requirements. For example we need to support
several types of interfaces for tax payment system
 Evolution point – supposed branching point, which
might occur in future, but does not declared by existing
requirements.
 Protected variation pattern applying for both variation
and evolution points.
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34. Informational Expert Assign a responsibility to the class that has the
information needed to fulfill it.
Creator Assign class B the responsibility to create an instance
of class A if one of these is true (the more the better):
• B "contains" or compositely aggregates A.
• B records A.
• B closely uses A.
• B has the initializing data for A that will be passed
to A when it is crated. Thus B is an Expert with
respect to creating A.
Controller Assign the responsibility to a class representing one
of the following choices:
• Major subsystem classes
• A use case scenario classes within which the
system event occurs
Low Coupling Assign a responsibility so that coupling remains low.
High Cohesion Lahore GarArissons Uingivners iaty responsibility so that coh3e4 sion remains high.

35. Polymorphism The same name operations (methods) in the
difference classes is defined. And assign a
responsibility to the class the class that the behavior
is changed.
Pure Fabrication Define a class for convenience' sake that doesn't
express the concept of the problem area at all.
Indirection Assign the responsibility to an intermediate object to
mediate between other components or services, so
that they are not directly coupled.
Protected Variations Assign responsibility to create a stable interface
around an unstable or predictably variable subsystem
or component.
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36.  http://en.wikipedia.org/wiki/GRASP_%28object-oriented_
design%29
 http://www.slideshare.net/snmgian/grasp-principles
 http://www.slideshare.net/eduardo_diederichsen/d
esign-talk-grasp-patterns?related=1
 http://www.slideshare.net/eduardo_diederichsen/d
esign-talk-grasp-patterns?related=1
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37. Any Question
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