Dept. of Computer Engineering, Amirkabir University of Tech.
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Design PatternsDr. Noorhosseini
Introduction
Dept. of Computer Engineering, Amirkabir University of Tech.
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Introduction Designing OO software is not easy. Designing Reusable OO
software is really hard. Why? Reusable OO software is not generated in the first attempt.
Experienced designers however make good design. It takes a long time for a novice designer to learn what a good OO design is. So there is something that an experienced designer must know that a novice one doesn’t. What’s that?
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Introduction (cont) Experienced designers know NOT to solve every
problem from scratch. Rather they use (reuse) solutions that have worked for them in the past. ( e.g. setting up a socket connection using c lib)
In other words they have developed a pattern in their mind. A good designer will also polish these patterns on every use and even a better designer documents these patterns for future use by himself or others.
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Introduction (cont..)
This is the subject of this course, patterns that get used repeatedly in many software projects.
We want to prevent re-invention of the wheels A designer who is familiar with these patterns can
apply them immediately to design problems without having to re-discover them.
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The benefits of knowing these patterns More elegant design More readable design. Easier communication with
co-designer (better understanding among team members)
Easier understanding of existing code (over %80 of an engineer’s job)
Faster and more reliable design etc..
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What is a design pattern?
Design patterns are not about designs such as linked lists and hash tables that can be coded in classes and reused as is.
Nor are they complex, domain-specific designs for an entire application
Design patterns are description of communicating objects and classes that are customized to solve a general design problem in a particular context.
They are flexible, elegant, proven and reusable solutions to design problems.
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MVC (quick review)
MVC Model: application object View: screen presentation Controller: defines the way the user interface
reacts to user input
Diagram on board
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An example: MVC model Model/View/Controller (MVC) triad of classes is an
example of a structural pattern used to build user interfaces
Three kind of classes: The model (application objects), the view (its screen presentation), the controller (how view reacts to user input)
Before, UI design tend to lump these together Decoupling of MVC, many advantages, e.g. one model
and several views, adding new views without changing model, changing model without affecting views, …
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MVC (cont..) The ideas behind MVC:
- decoupling objects so that changes to one can affect many others without the need for changed object to know details of the others. The more general design is described by Observer pattern.
- MVC let you change responses to input without affecting the views. Depending on the context we can have different reaction to the same input. Having a controller object let us encapsulate the response. The view-controller relationship is an example of the Strategy design pattern.
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MVC Design Pattern Examples Observer Pattern:
decoupling of model from view changes to one object can affect multiple views, without
requiring object to know details of view Composite Pattern:
nesting of views class hierarchy in which some subclasses define primitive
objects and other classes define composite objects that assemble primitives into more complex objects
Strategy Pattern: view-controller relationship allows controller to replaced
statically or dynamically
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Main elements of a design pattern
Name: Facilitates communication Problem: context of application, when to apply Solution: Elements that makeup the Design Consequences: Results and trade offs of
applying the pattern
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Design Pattern Template Classification Intent Other names Motivation Applicability Structure Participants Collaborations Implementation Sample code Known uses Related Patterns
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Template cont.. Pattern Name and Classification: Succinct,
captures essences of pattern, reflects scheme Intent: Short statement that answers the
following: What does it do? What is its rationale and intent What particular design issue or problem does it
address?
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Template cont.. Also Know As: aliases Motivation: Scenario which illustrates the design
problem and how class and object structures in the pattern solve the problem.
Applicability: When design pattern can be applied Examples of poor design the pattern can
address How to recognize design patterns
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Template cont.. Structure: Graphical representation of classes in
the design pattern using OMT (Object Modeling Technique). Also utilizes interaction diagrams.
Participants: Classes and/or objects participating in the design pattern and their responsibilities.
Collaborations: How participants carry out their responsibilities.
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Template cont.. Consequences:
How a design pattern supports its objectives Trade-offs, results of using design pattern Aspects of system which can vary independently
Implementation: pitfalls, hints or techniques language-specific issues
Sample Code: Code fragments in C++ or Java
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Template cont.. Known Uses: At least two examples of the
pattern found in real systems. Related Patterns:
What patterns are closely related? Important differences between closely related
patterns. Which patterns is this design pattern used with?
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Organization of Catalog Design patterns vary in granularity and level of
abstraction. Families of related patterns help to programmer to
learn patterns faster and can direct efforts to find new patterns.
23 design patterns total
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Two classifications of a design pattern Purpose: What a design pattern does Scope: Specifies whether a design pattern
applies primarily to classes or to objects
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Design Pattern Purpose
Creational (5): Concerns object creation Structural (7): Deals with composition of
classes or objects Behavioral (11): Characterizes ways in which
classes or objects interact and distribute responsibility.
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The Catalog of Design PatternsCreational Structural Behavioral
Factory Method Adapter Interpreter
Template Method
Abstract Factory
Builder
Prototype
Singleton
Adapter (object)
Bridge
Composite
Decorator
Façade
Flyweight
Proxy
Chain of Responsibility
Command
Iterator
Mediator
Observer
State
Strategy
Visitor
Scope
Class
Object
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Design Pattern Scope
Class Patterns (4) Deal with relationships between classes and their
subclasses Relationships established through inheritance, so
they are fixed at compile time (static)
Object patterns (20) Deal with object relationships Relationships can be changed at runtime (dynamic)
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Six types of design patterns1. Creational class patterns defer some part of object
creation to subclasses2. Creational object patterns defer some part of object
creation to another object3. Structural class patterns use inheritance to compose
classes4. Structural object patterns describe ways to assemble
objects5. Behavioral class patterns use inheritance to describe
algorithms and flow of control6. Behavioral object patterns describe how a group of
objects cooperate to perform a task that no single object can carry out alone
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Additional ways to organize design patterns Patterns which are used together Some patterns are alternatives for one another Some patterns result in similar design although
they have different intents Patterns which reference one another (see
Figure 1.1, p. 12) Additional ways to organize design patterns
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What don’t these design patterns apply to? Concurrency Distributed programming Real-time programming Domain-specific patterns User interface design Device drivers Object-oriented DB
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Course Outline• Introduction to the course• Review of some OO design problems and some OO design concepts.
How design patterns can solve design problems• A case Study. Showing some design problems and solutions leading to
some patterns.• Reviewing pattern structure and other attributes and some sample code
related to the use case.• Review of the rest of patterns, finding use cases (collaboratively with
class)• Your presentation of use cases in your assignments • Midterm & Final Exam
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What you need to know to excel in this course Good knowledge and experience with OO
design Knowledge of Java or C++ Design and coding experience. You must have
experienced solving a problem over and over to appreciate the patterns presented
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Course requirements
Group assignments %30 Student presentations of the assignment solutions Class work %15 Midterm & Final Exams %25 &%25 Class participation %10 Students will be penalized for being absent from
lectures and for late delivery of assignments