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Design PatternsDesign Patterns What is a Design PatternWhat is a Design Pattern

-- 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(by Christopher Alexander)

Essential elements of a Design PatternEssential elements of a Design Pattern

- pattern name - problem - solution - consequence

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Classification of design patternClassification of design pattern

According to its purposeAccording to its purpose

-- 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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Classification of design Classification of design pattern(continued)pattern(continued)

According to its scopeAccording to its scope

-- classclass

Factory Method, Adapter(class), Interpreter, Template Method

-- ObjectObject

The rest of the patterns

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Patterns with possible use in Patterns with possible use in frameworkframework

Creational Patterns includingCreational Patterns including

Abstract Factory, Builder, Factory Method and Prototype

Behavioral PatternBehavioral Pattern

Command, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor

Notes: Factory Method and Template Method are class patterns

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Abstract Factory Abstract Factory Object CreationalObject Creational

IntentIntent

Provide an interface for creating families of related or dependent objects without specifying their concrete class

MotivationMotivation

Consider a user interface toolkit that supports multiple look-and- feel standards, such as Motif and Presentation Manager. Different look-and-feels define different appearances and behaviors for user interface “widgets” like scroll bars, windows, and buttons. To be portable across look-and-feel standards, an application should not hard-code its widgets for a particular look-and-feel. Instantiating look-and-feel specific classes of widgets throughout the application makes it hard to change the look-and- feel later

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Window

Client

MOtifWindowPMWindow

ScrollBar

MotifScorllBarPMScrollBar

WidgetFactory

createScrollBar()CreateWindow()

MotifWidgetFactory PMWidgetFactory

createScrollBar()CreateWindow()

createScrollBar()CreateWindow()

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StructureStructure

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CollaborationsCollaborations

-- -- Normally a single instance of a ConcreteFactory class is Normally a single instance of a ConcreteFactory class is created at run-time. This concrete factory creates product created at run-time. This concrete factory creates product objects having a particular implementation. To create objects having a particular implementation. To create different product objects, clients should use a different different product objects, clients should use a different concrete factoryconcrete factory

-- -- AbstractFactory defers creation of product objects to its AbstractFactory defers creation of product objects to its

ConcreteFactory subclassConcreteFactory subclass

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Builder Builder -- Object Creational-- Object Creational

IntentIntent -- Separate the construction of a complex object from its

representation so that the same construction process can create different representation

MotivationMotivation -- A reader for the RTF(Rich Text Format) document exchange

format should be able to convert RTF to many text formats.

The reader might convert RTF documents into plain ASCII text or into a text widget that can be edited interactively. The problem, however, is that the number of possible conversions is open-ended. So it should be easy to add a new conversion without modifying the reader.

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ConvertCccharacter(char)GetASCIITEXT()

ASCIIConverter TeXConverter

ConvertCccharacter(char)convertFontChange(Font)convertParagraph()GetTeXtext()

TeXTextASCIIText TexWidget

While(t=get the next token{Switch t.Type{CHAR:Builder->ConvertCharacter(t.Char)FONT:Builder->ConvertFontChange(t.Font)PARA:Biolder->convertParagraph()}}

TexWidgetConverter

ConvertCccharacter(char)convertFontChange(Font)convertParagraph()GetTextWidget()

ConvertCccharacter(char)convertFontChange(Font)convertParagraph()

TextConverterbuilders

RTFReader

ParseRTF()

builder

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StructureStructure

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CollaborationsCollaborations

-- The client creates the Director object and configures it with

the desired Builder object -- Director notifies the builder whenever a part of the product

should be built -- The client retrieves the product from the builder

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Factory Method Factory Method Class CreationalClass Creational

IntentIntent Define an interface for creating an object, but let subclass

decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses

MotivationMotivation Frameworks use abstract classes to define and maintain

relationships between objects. A framework is often responsible for creating these objects as well.

Consider a framework for applications that can present multiple documents to the user. Tow key abstractions in this framework are the classes Application and Document. Both classes are abstract,and clients have to subclass them to realize their application-specific implementations. To create a drawing application,for example,we define the class DrawingApplciation and DrawingDocument. The Application class is responsible for managing Documents and will create them as required-when when the user selects Open or New from a menu, for example.

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MyDocument

Document

Open()Close(0Save()Revert()

createDocument()NewDocument()openDocument()

Application

createDocument()

MyApplication

Document* doc= createDocument();Docs.Add(doc);Doc->open()

Return new MyDocument

docs

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Structure

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CollaborationsCollaborations

-- Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate ConcreteProduct

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Prototype Prototype Object CreationalObject Creational

IntentIntent

Specify the kinds of Objects to create using a prototypical instance,and create new objects by copying this prototype

MotivationMotivation

You could build an editor for music scores by customizing a general framework for graphical editors and adding new objects that represent notes,rests, and staves. The editor framework may have a palette of tools for adding these music objects to the score. The palette would also include tools for selecting, moving, and otherwise manipulation music objects. Users will click on the quarter-note tool and use it to add quarter notes to the score. Or they can use the move tool to move a note up or down on the staff, thereby changing its pitch

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Let’s assume the framework provides an abstract Graphics class for graphical components, like notes and staves. Moreover, it’ll provide an abstract Tool class for defining tools like those in the palette. The framework also predefines a GraphicTool subclass for tools that create instances of graphical objects and add them to the document

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structurestructure

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CollaborationsCollaborations

A client asks a prototype to clone itself

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Command Command Object BehavioralObject Behavioral

IntentIntent

Encapsulate a request as an object,thereby letting you

parameterize clients with different requests, queue or log requests, and support undoable operations

MotivationMotivation Sometimes it’s necessary to issueSometimes it’s necessary to issue

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StructureStructure

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CollaborationsCollaborations-- The clients creates a ConcreteCommand ojbect and specifies -- The clients creates a ConcreteCommand ojbect and specifies

its recieverits reciever

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Iterator Iterator Object CreationalObject Creational

IntentIntent

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation

MotivationMotivation

List

Count()Append(Element)Remove(Element)…

ListIteratorFirst()Next()IsDone()CurrentItem()

index

list

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StructureStructure

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CollaborationsCollaborations

-- A ConcreteIterator keeps track of the current object in the aggregate and can compute the succeeding object in the traversal

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MediatorMediator Object Behavioral

IntentIntent

Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

What is a MediatorWhat is a Mediator -- A mediator is responsible for controlling and coordinating

the interactions of a group of objects. The mediator serves as an intermediary that keeps objects in the group form referring to each other explicitly. The objects only know the mediator, thereby reducing the number of interconnections

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Example

aClient director

aListBox

director

aFontDialogDirector

aButton

director

anEntryField

director

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StructureStructure

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CollaborationsCollaborations

Colleagues send and receive requests from a Mediator object. The mediator implements the cooperative behavior by routing requests between the appropriate colleague(s)

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Observer Observer Object BehavioralObject Behavioral

IntentIntent

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

ExampleExample

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

1 2 3 4

x

a

x

1

2

3

4

A = 50%B= 30%C= 20%

observers

subjectChange notificationChange notification

Requests, modificationsRequests, modifications

x

y

z

a b c

60

50

80

30

30

10

10

20

10

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StructureStructure

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CollaborationCollaboration - ConcreteSubject notifies its observers whenever a change occurs that could make its observers’ state inconsistent with its own. - After being informed of a change in the concrete subject, a ConcreteObserver object may query the subject for information. ConcreteObserver uses this information to reconcile its state with that of the subject.

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State State Object BehavioralObject Behavioral

IntentIntent

Allow an object to alter its behavior when its internal state changes. The Object will appear to change its class

ExampleExample

State->Open

TCPConnectionOpen()Close()Acknowledge()

TcpState

Open()Close()Acknowledge()

TCPEstablished

TCPListen TCPClosedOpen()Close()Acknowledge()

Open()Close()Acknowledge()

Open()Close()Acknowledge()

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StructureStructure

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CollabortionsCollabortions

- Context delegates state-specific requests to the

current ConcreteState object. - Context is the primary interface for clients. Clients can configure a context with State objects. Once a context is configured, its clients don’t have to deal with the State objects directly. - Either Context or the ConcreteState subclasses can decide which state succeeds another and under what circumstances

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IntentIntent

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

ExampleExample

Compositor->compose()

CompositionTraverse()Repair()

Compositor

compose()

compose()Compose()

compose()

SimpleComposer ArrayCompositor

StrategyStrategy object Behavioralobject Behavioral

TexCompositor

compose()

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StructureStructure

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CollaborationsCollaborations - Strategy and Context interact to implement the chosen

algorithm. A context may pass all data required by the algorithm to the strategy when the algorithm is called . Alternatively, the context can pass itself as an argument to Strategy operations. That lets the strategy call back on the context as required.

- A context forwards requests from its clients to its strategy. Clients usually create and pass a ConcreteStrategy object to the context; thereafter, clients interact with the context exclusively. There is often a family of ConcreteStrategy classes for a client to choose from

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Template Method Template Method Class BehavioralClass Behavioral

IntentIntent Define the skeleton of an algorithm in an operation, deferring

some steps to subclasses. Template method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.

ExampleExample

ApplicationApplication

AddDocument()openDocument()DoCreateDocument()canOpenDocument()AboutToOpenDocument

MyApplication

DoCreateDocument()canOpenDocument()AboutToOpenDocument

Document

Save()Open()Close()doRead()

MyDocument

DoRead() Return new MyDocument

docs

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StructureStructure

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CollaborationCollaboration

ConcreteClass relies on AbstractClass to implement the invariant steps of the algorithm

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Visitor Visitor Object BehavioralObject Behavioral

IntentIntent

Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates

ExampleExample

VariableRefNode AssignmentNode

TypeCheck()GenerateCode()PrettyPrint()

TypeCheck()GenerateCode()PrettyPrint()

TypeCheck()GenerateCode()PrettyPrint()

Node

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StructureStructure

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CollaborationCollaboration - A client that uses the Visitor pattern must create a - A client that uses the Visitor pattern must create a

ConcreteVisitor object and then traverse the object ConcreteVisitor object and then traverse the object structure, visiting each element with the visitorstructure, visiting each element with the visitor

- When an element is visited, it calls the visitor operation - When an element is visited, it calls the visitor operation that corresponds to its class. The element supplies itself as that corresponds to its class. The element supplies itself as an argument to this operation to let the visitor access its an argument to this operation to let the visitor access its state, if necessary.state, if necessary.