Patterns 1

Patterns in OO - history

77 - Work of the architect Christopher Alexander

80s - Kent Beck playing with Smalltalk

91 - Gamma’s PhD thesis on ET++

92 - Coad’s patterns paper

92 - Johnson’s OOPSLA paper

93 - OOPSLA workshop Coad & Anderson

93 - Patterns discussion group on WWW

94 - First PLoP conference

95 - Gang of four book (first draft at OOPSLA 93)

Patterns 2

Christopher Alexander

Alexander,C., Ishikawa, S. & Silverstein, M., A Pattern Language, Oxford University Press 1977

This text is handbook of patterns to be used in the construction of built environments.

“Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice.”

Alexander presented 253 patterns in 35 groups at different levels of abstraction or resolution.

Patterns 3

The structure of a pattern

Each pattern is presented in the same format for clarity, ease of navigation etc.

Name. A Short descriptive phrase.

Picture. Followed by an introductory paragraph to outline the context.

Problem. An outline of the problem that the pattern aims to solve.

Solution. The heart of the pattern, a description of how to solve the problem.

Interconnections. The other patterns which are related to this one.

Patterns 4

Some typical examples

Intimacy Gradient

Common Areas at the Heart

Eating Atmosphere

Roof Layout

Pools of Light

Patterns 5

Peter Coad’s patterns

Coad, P.,Object-Oriented Patterns, Communications of the ACM Vol 35 No. 9 September 1992

"An object-oriented pattern is an abstraction of a doublet, triplet, or other small grouping of classes that is likely to be helpful again and again in object-oriented development."

Coad’s patterns originally had a focus on OOA.

Patterns 6

Patterns and layers

Service Layer

Item DescriptionTime Association

Event Logging

Attribute Layer

State across a collection

Behaviour across a collection Roles Played

Broadcast pattern

Patterns 7

Event logging

10,m ThresholdViolation

dateTimevalue

TemperatureSensor state

monitor

10,m EventRemembered

dateTimevalue

Device state

monitor

Use whenever an event is detected and you need to log its occurence

Patterns 8

Larman’s GRASP patterns

Larman proposes a small set of (what he calls) patterns for assigning responsibilities to classes.

General Responsibility Assignment Software Patterns.

Expert, Creator, Low Coupling, High Cohesion, Polymorphism, Pure Fabrication, Indirection and Don’t Talk to Strangers.

Patterns 9

Grasp 1Expert - assign the responsibility to the class that has the information necessary to fulfill the responsibility.

Creator - Assign B the responsibility to create an A if ..• B aggregates or contains A objects

• B records instances of A objects

• B closely uses A objects

• B has the initialising data that A needs (expert)

Low coupling - Assign a responsibility so that coupling remains low.

High cohesion - Assign a responsibility so that cohesion remains high.

Patterns 10

Grasp 2

Polymorphism - When behaviours vary by type, assign responsibilities (using polymorphism) to the type with the variation.

Pure Fabrication - Invent a cohesive class which is not in the problem domain to further low coupling.

Indirection - Assign the responsibility to an intermediate object to mediate between components so that they are not directly coupled.

Don’t talk to strangers - The Law of Demeter

Patterns 11

Gang of Four

Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns: Elements of Reusable Object-Oriented Software Addison-Wesley 1995.

Work started with Gamma’s doctoral thesis and continued for a couple of years.

The final published book shows signs of evolution from the earliest drafts. You should try to read chapter 2.

There are 25 patterns arranged under 3 headings

Patterns 12

Gang of four - format

Pattern Name and Classification

Intent

Also known as

Motivation

Applicability

Structure

Participants

Collaborations

Consequences

Implementation

Sample code

Known uses

Related Patterns

Patterns 13

Design Pattern Space

Factory Method

Abstract FactoryBuilderPrototypeSingleton

Adapter (class)

Adapter (object)BridgeCompositeDecoratorFacadeFlyweightProxy

InterpreterTemplate MethodChain of ResponsibilityCommandIteratorMediatorMementoObserverStateStrategyVisitor

Creational Structural Behavioural

Purpose

Scope Class

Object

Patterns 14

Strategy patternIntent.

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Context Strategy

ContextInterface()

ConcreteStrategyA

AlgorithmInterface()

ConcreteStrategyB ConcreteStrategyC

AlgorithmInterface()AlgorithmInterface()

AlgorithmInterface()

MotivationA text compositor with a number of line breaking algorithms

Structure

Patterns 15

Observer pattern outline

a b cx 60 30 10y 50 30 20z 80 10 10 a b c

abc

a = 50%b = 30%c = 20%

Change notification

requests, modifications

Patterns 16

Observer pattern

ConcreteSubject

subjectState

getState( )

ConcreteObserver

observerState

update( )

Subject

attach( )detach( )notify( )

Observer

update( )

Patterns 17

Observer details

Defines a one to many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Used when you need to maintain consistency between objects.

Particularly used in GUI development, it is the basis for Smalltalk’s Model-View-Controller.

Also known as Dependents or Publish-Subscribe.

Patterns 18

Abstract Factory

Used to provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Java uses this pattern in its AWT for creating platform specific objects with the correct look and feel. (Peers)

WidgetFactory

createScrollBar( )createWindow( )

MotifWidgetFactory PMWidgetFactory

Patterns 19

Abstract factory

MotifWidgetFactory PMWidgetFactory

PMWindow MotifWindow

PMScrollBar MotifScrollBar

Window

ScrollBar

WidgetFactory

createScrollBar( )createWindow( )

Client

Patterns 20

Patterns and design skills

If patterns capture design expertise, then learning patterns makes me an expert designer.

Teaching patterns to beginners can help them become better designers.

How do I know when to apply a pattern, Riel argues that design heuristics may tell me?

Are there such things as anti-patterns? Examples of bad practice to be avoided.

A. J. Riel Object-Oriented Design Heuristics Addison-Wesley 1996

Patterns 21

Patterns and reuse

The focus was never on component reuse but on the reuse of design ideas and insights.

Patterns are increasingly becoming specialised to problem domains, apparently losing generality.

Might patterns provide accelerated familiarity with domains for designers?

Documenting patterns is hard work, are they worth building?

Patterns 22

The scope and granularity of patterns

ScopeCoad’s analysis patterns versusGamma’s design patterns

Granularity

Coplien’s idioms for C++

Gamma’s patterns

Frameworks - implemented patterns

Patterns 23

References

Lea, D., Christopher Alexander: An Introduction for Object-Oriented Designers SIGSOFT Jan 1994 39-46 (lots of references)

Beck, K., et al Industrial Experience with Design Patterns proceedings of ICSE 18 1996 103-114

Larman, C. Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design Prentice Hall 1998

http://st-www.cs.uiuc.edu/users/patterns/patterns.html