Design Patterns

1
Design Patterns

• William A. Hoffman
• NYU OOP Class

2
Design Patterns Overview

• Why Design Patterns?
• History
• Definitions / Terminology
• Pattern Examples - from DP book
• Summary

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Why OO Design Patterns

• Effective for teaching OOD
• OOP is a new Art form
• People have only had 20 years experience with
  OOP
• Architecture and painting have been around for
  thousands of years.
• Effective method of transferring skill and
  experience
• new OO programmers can be overwhelmed by the
  options

4
History of Design Patterns

• Christopher Alexander - an architect
• The Timeless Way of Building, 1979
• A Pattern Language, 1977
• Pattern - A solution to a problem in a context
• Purposes
• effective reuse
• dissemination of solutions

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History in OOP

• 1987 workshop at OOPSLA Beck and Ward
• 1993 The Hillside Group - Beck, Ward, Coplien,
  Booch, Kerth, Johnson
• 1994 Pattern Languages of Programming (PLoP)
  Conference
• 1995 Design Patterns Elements of Reusable OO
  software - Gamma, Helm, Johnson, Vlissides (Gang
  of Four)

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Definitions / Terminology

• Pattern Language - a term from Christopher
  Alexander, not a software language but a group of
  patterns used to construct a whole
• Pattern - many definitions see FAQ, lets try this
  one Patterns represent distilled experience
  which, through their assimilation, convey expert
  insight and knowledge to inexpert developers.
  - Brad Appleton

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Types of Software

• Application Programs - internal reuse, extension.
  Excell, PowerPoint
• Toolkits - set of related, reusable general
  purpose classes. (Fresco)
• Frameworks- reusable set of cooperating classes
  for a specific class of software (TargetJr)

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How a Pattern is Defined(GoF form)

• Name - good name
• Intent- what does it do
• Also Known As
• Motivation - a scenario
• Applicability - when to use
• Structure- UML

• Participants - classes
• Collaborations - how they work together
• Consequences - trade offs
• Implementation- hints on implementation
• Sample Code
• Known Uses
• Related Patterns

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New Patterns

• Rule of 3, should be used in 3 places, it should
  be a reoccurring pattern, not an algorithm.
• Each Domain has patterns

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Purpose of Design Patterns

• Creational - Abstact Factory
• Structural - Adapter, Bridge
• Behavioral - Iterator, State

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Singleton (creational)

• Intent - ensure a class has only one instance and
  provide a global point of access
• Motivation - Some classes must only have one
  instance, (one file system, one window mgr),
  allow class to ensure only one instance.
• Applicability
• Must be only one instance
• The one instance should be extensible by
  subclassing, and clients should be able to use
  extended version without modification of client

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Singleton Cont.

• Structure

Singleton
return uniqueInstance
static Instance() SingleOperation() GetSingleton
Date()
static uniqueInstance singletonData
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Singleton Cont

• Participants
• Singleton
• defines an Instance operation that lets clients
  access its unique instance, a static or class
  method is used
• Collaborations
• clients access only from the unique instance

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Singleton Cont

• Consequences
• controlled access to sole instance
• reduced name space (no globals)
• permits refinement via sub-classing
• permits a variable number of instnaces
• more flexible than class operations, specifically
  in c/java static is not virtual
• clients access only from the unique instance

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class Singleton public static Singleton
Instance() protected Singleton() private
static Singleton _instance Singleton
Singleton_instance 0 Singleton
SingletonInstance() if(!_instance)
_instance new Singleton() return _instance
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Singleton Cont

• Sample Code - mazefactory class
• Known Uses
• meta class
• InterViews WidgetKit
• Related Patterns
• many patterns use singleton See Abstarct
  Factory, Builder and Prototype

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Proxy

• A surrogate or placeholder for another object to
  control access to

Client1 Image_proxy
TCIP
ORB Image (instance)
Client2 Image_proxy
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CacheProxy
Client1 Image_proxy_wcache
ORB Image (instance)
Client2 Image_proxy_wcache
Can use Observer Pattern to Notify clients of
changes in the source image class
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Interaction of Patterns

• Patterns are often interelated
• AbstractFactory is often done as a Singleton
• Observer and Proxy in CORBA

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Template Method

• Intent - Define a skeleton of an algorithm in an
  operation, deferring some steps to subclasses.
  Sub-classes can redefine steps in an algorithm
  without changing its structure
• Motivation- Document example
• Applicability
• implement invariant parts of an algorithm
• avoid code duplications by noticing common
  behavior in sub-classes
• hook extenstions

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AbstractClass
TemplateMethod() PrimativeOp1() PrimativeOp2()
PrimativeOp1() PrimativeOp2() ...
ConcreateClass
PrimativeOp1() PrimativeOp2()
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// From NeXT AppKit void ViewDisplay()
SetFocus() DoDisplay() ResetFocus() void
ViewDoDisplay() void MyViewDoDisplay()
// do my render
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Overview of Patterns in GoF Design Patterns

• Creational
• Factory Method
• Abstract Factory
• Builder
• Prototype
• Singleton

• Structural
• Adapter
• Bridge
• Composite
• Decorator
• Façade
• Flyweight
• Proxy

• Behavioral
• Interpreter
• Template Method
• Chain of Responsibility
• Command
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Visitor

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Design Pattern Benefits

• Adds to the language of OOP
• much information is expressed in a single term
• Documentation
• Reuse of ideas not just code
• CORBA problem - found a pattern on the web
• Anti-Patterns

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Problems

• Classification
• Tools
• Formalization

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Patterns on the web

• Patterns Home Page http//hillside.net/patterns
• http//st-www.cs.uiuc.edu/users/patterns/patterns.
  html
• http//c2.com/ppr/index.html - Portland Pattern
  Repository
• http//www.entract.com/bradapp/docs/patterns-intr
  o.html

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Parting Quote

• It is possible to make buildings by stringing
  together patterns, in a rather loose way. A
  building make like this, is an assembly of
  patterns. It is not dense. It is not profound.
  But I is also possible to put patterns together
  in such a way hat many patterns overlap in the
  same physical space the building is very dense
  it has many meanings captured in a small space
  and through this density, it becomes profound.
• Christopher Alexander