Design Patterns

William A. Hoffman

NYU OOP Class

Design Patterns Overview

• Why Design Patterns?

• History

• Definitions / Terminology

• Pattern Examples - from DP book

• Summary

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

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

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)

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

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)

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

New Patterns

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

• Each Domain has patterns

Purpose of Design Patterns

• Creational - Abstact Factory

• Structural - Adapter, Bridge

• Behavioral - Iterator, State

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

Singleton Cont.

• Structure

static Instance();SingleOperation();

GetSingletonDate();

Singletonreturn uniqueInstance

static uniqueInstancesingletonData

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

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

class Singleton { public: static Singleton* Instance();protected: Singleton();private: static Singleton* _instance;};

Singleton* Singleton::_instance = 0;

Singleton* Singleton::Instance(){ if(!_instance) _instance = new Singleton(); return _instance;}

Singleton Cont

• Sample Code - mazefactory class

• Known Uses– meta class– InterViews WidgetKit

• Related Patterns– many patterns use singleton See: Abstarct

Factory, Builder and Prototype

Proxy

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

ORBImage (instance)

Client1Image_proxy

Client2Image_proxy

TCIP

CacheProxy

ORBImage (instance)

Client1Image_proxy_wcache

Client2Image_proxy_wcache

Can use Observer Pattern to Notify clients of changes in the source

image class

Interaction of Patterns

• Patterns are often interelated– AbstractFactory is often done as a Singleton

• Observer and Proxy in CORBA

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

AbstractClass

TemplateMethod()PrimativeOp1();PrimativeOp2();

ConcreateClass

PrimativeOp1();PrimativeOp2();

…PrimativeOp1();PrimativeOp2();

...

// From NeXT AppKit

void View::Display(){ SetFocus(); DoDisplay(); ResetFocus();}

void View::DoDisplay() { }

void MyView::DoDisplay() { // do my render }

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

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

Problems

• Classification

• Tools

• Formalization

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-intro.html

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