designpatterns-10.pdf

7/29/2019 designPatterns-10.pdf

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Chain of Responsibility

andComposite

CSCI 3132 Summer 2011 1


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Mo#va#on

Consideracontext-sensi#vehelpsystemforaGUIDE Theobjectthatul#matelyprovidesthehelpisn'tknownexplicitlytotheobject(e.g.,abuHon)thatini#atesthehelprequest.

Souseachainofobjectstodecouplethesendersfromthereceivers.

Therequestgetspassedalongthechainun#loneoftheobjectshandlesit.

Eachobjectonthechainsharesacommoninterfaceforhandlingrequestsandforaccessingitssuccessoronthechain

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ChainofResponsibilityPaHern

IntentAvoidcouplingsenderofrequesttoitsreceiverby

givingmorethanoneobjectchancetohandle

request.Chainreceivingobjectsandpassrequest

alongun#lanobjecthandlesit.

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Example2

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Applicability

UseChainofresponsibilitypaHernwhenmorethanoneobjectmayhandlearequest,and

thehandlerisntknownapriori.

youwanttoissuearequesttooneofseveralobjectswithoutspecifyingthereceiverexplicitly

thesetofobjectsthatcanhandlearequestshouldbespecifieddynamically

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ChainofResponsibilityStructure

Avoid coupling the sender of a request to its receiver by giving more

than one object a chance to handle the request 6


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Chainofresponsibility-Par#cipants

andler(elpandler)definesaninterfaceforhandlingrequests (op#onal)implementsthesuccessorlink

Concreteandler(PrintBuHon,PrintDialog)handlesrequestsitisresponsibleforcanaccessitssuccessor iftheConcreteHandlercanhandletherequest,is

doesso;otherwiseitforwardstherequesttoits

successor Client ini#atestherequesttoaConcreteHandlerobjecton

thechain

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ChainofresponsibilityConsequences

Reducedcoupling ThepaHernfreesanobjectfromknowingotherobject

handlesarequest

Addedflexibilityinassigningresponsibili#estoobjects changingthechaininrun-#me usesub-classingtospecifyhandlerssta#cally

Receiptisntguaranteed

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ChainofresponsibilityImplementa#on

Implemen#ngthesuccessorchaindefineanewlinks(inandlerusually),oruseexistentlinks(forexample,child-parentlinks).

Connec#ngsuccessorsclass HelpHandler {

public:

HelpHandler(HelpHandler* s) : _successor(s) {}

virtualvoid HandleHelp();

private:

HelpHandler* _successor;};

void HelpHandler::HandleHelp() {

if (_successor)

_successor->HandleHelp();

}

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ChainofresponsibilityImplementa#on

Represen#ngrequestsThesimplestform:therequestisahard-coded

opera#oninvoca#on

youcanforwardonlythefixedsetofrequestsAnalterna#ve:useasinglehandlerfunc#on

whichtakesarequestobjectasparameter

void Handler::HandleRequest(Request* theRequest) {

switch (theRequest->getKind()) {case Help:

HandleHelp((HelpRequest*)theRequest);

break;

}

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BehavioralPaHernSummary

BehavioralpaHernsareconcernedwiththeassignmentofresponsibili#esbetweenobjects,or,encapsula#ng

behaviorinanobjectanddelega#ngrequeststoit.

Chainofresponsibility,Command,andObserver,addresshowyoucandecouplesendersandreceivers,butwithdifferenttrade-offs.

Chainofresponsibilitypassesasenderrequestalongachainofpoten#alreceivers.

Commandnormallyspecifiesasender-receiverconnec#onwithasubclass.

Observerdefinesaverydecoupledinterfacethatallowsformul#plereceiverstobeconfiguredatrun-#me.

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StructuralPaHerns

Concernedwithhowclassesandobjectsarecombinedtocreatelargerstructures.

Concernedwithcomposingobjectstoestablishnewfunc#onality.

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CompositePaHern

A composite is a group of objects in which some objectscontain others; one object may represent groups, andanother may represent an individual item, a leaf.

Facilitates the composition of objects into a stree structure,a hierarchy, that represent part-whole hierarchies.

These hierarchies consist of both primitive and compositeobjects.

The operations are appropriate for processing andtraversing trees.

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Example

Example:complexgraphicapplica#on,withhierarchicallynestedobjects.

Mainidea:Containersandleafobjectsappearthesametoclients. Upanddowndelega6on:

Acontainermaydelegatetoallcontainedobjects. Aleaf/containermaydelegatetocontainingobjects.

aLeaf aLeaf aLeaf

aComposite

aLeaf aLeaf

aComposite

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Line

Draw()

Rectangle

Draw()

Text

Draw()forallgingraphics

g.Draw()

addgtolistofgraphics

graphics

Picture

Draw()

Add(Graphicg)

Remove(Graphic)

GetChild(int)

Graphic

Draw()

Add(Graphic)

Remove(Graphic)

GetChild(int)

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Par#cipants

Component(Graphic) declaresinterfaceforobjectsinthecomposi#on implementsdefaultbehavior declaresaninterfaceforaccessingandmanagingitschildcomponents (op#onal)declaresaninterfaceforaccessingacomponentsparent

Leaf(Rectangle,Line,Textetc.) definesbehaviorforprimi#veobjectsinthecomposi#on

Composite(Picture) definesbehaviorforcomponentshavingchildren

storeschildcomponents

implementschild-relatedopera#onsintheComponentinterface Client

manipulatesobjectsinthecomposi#onthroughtheComponentinterface

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CompositepaHern-implementa#on

ExplicitparentreferencesdefinetheparentreferenceintheComponentclassessen#altomaintaintheinvariantthatallchildrenof

acompositehaveastheirparentthecompositethat

inturnhasthemaschildreneasiestwaytoensure-changethecomponents

parentonlywhenitsbeingaddedorremovedfromcomposite

Sharingcomponents

mul#pleparentsambigui#esasarequestpropagatesupthestructure

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CompositePaHernClassDiagram

Component

operation()

Leaf

operation()

Composite

operation()

other()

*Client

Each node of the Component structure can respond to some commonoperation(s). I.e. the client can send the common operation to Componentand the structure responds appropriately.

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Example

MenuComponent

add()

remove()getChild()

print()

MenuItem

print()

Menu

add()remove()getChild()

print()

*Waitress

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CompositePaHern

Menu

menuComponents: ArrayList

add()

remove()getChild()

print()

Note the association from Menu to MenuComponentswith an array list data type.

How to implementation of print() in Menu and inMenuItem

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CompositePaHern

In the Menu class we have a print() that is appropriate foran internal node.

public void print() {System.out.print("\n" + getName());System.out.println(", " + getDescription());

System.out.println("---------------------");

Iterator iterator = menuComponents.iterator();while (iterator.hasNext()) {

MenuComponent menuComponent =(MenuComponent)iterator.next();

menuComponent.print();}

}

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CompositePaHern

In the MenuItem class we have a print() that is appropriatefor a leaf:

public void print() {

System.out.print(" " + getName());

if (isVegetarian()) {System.out.print("(v)");}System.out.println(", " + getPrice());

System.out.println(" -- " + getDescription());}

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Summary

TheCompositePaHernisusedtorepresentpart-wholeobjecthierarchies.

Clientsinteractwithobjectsthroughthecomponentclass.

Itenablesclientstotoignorethespecificsofwhichleaforcompositeclasstheyuse.

Canbeusedrecursively,sothatDisplaycanshowbothflaresandstars.

Newcomponentscaneasilybeaddedtoadesign.

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