designpatterns-05.pdf

7/29/2019 designPatterns-05.pdf

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The Observer Pattern

CSCI 3132 Summer 20111


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SevenSinsofDesign

Rigiditymakeithardtochange FragilitymakeiteasytobreakImmobilitymakeithardtoreuse

Viscositymakeithardtodotherightthing NeedlessComplexityoverdesignNeedlessRepeAAonerrorprone

Notdoingany2


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TheRules

pplyCommonSense DontgettoodogmaAc/religious Everydecisionisatradeoff llotherprinciplesarejustthat

GuidelinesbestpracAcesConsidercarefullyifyoushouldviolatethem-but,

knowyoucan.

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OpenClosedPrinciple

SowareenAAes(Classes,Modules,Methods,etc.)shouldbeopenforextension,butclosed

formodificaAon.

lsoKnownsProtectedVariaAon QuesAon:ThisprincipleisappliedtowhichofthedesignpaZernswehavelearnedsofar?

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SingleResponsibilityPrinciple

ClassshouldhaveonereasontochangeResponsibilityisareasonstochange

Canbetrickytogetgranularityright SingleResponsibility=increasedcohesion Notfollowingresultsinneedlessdependencies

Morereasonstochange.Rigidity,Immobility

QuesAon:ThisprincipleisappliedtowhichofthedesignpaZernswehavelearnedsofar?

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ConsidertheFollowingpplicaAon

Humidity

Temperature

Pressure

Weather

StaAon

Weather

Data

Object

Three

Displays

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WhatsGiven

TheWeatherDataclasshasgeZermethodsthatobtainmeasurementvaluesfromtemperature,humidityandpressure.

TheclasshasameasurementsChanged()methodthatupdatesthethreevalues. Threedisplaysmustbeimplemented:current

condiAons,staAsAcsandforecastdisplay.

Systemmustbeexpandableotherdisplayelementsmaybeaddedorremoved.7


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FirstZempt

class WeatherData{!public: ! !void measurementChanged(){!

! ! float temp = getTemperature();!! ! float humidity = getHumidity();!! ! float pressure = getPressure();!! ! currentConditionsDisplay.update(temp,humidity,!! ! ! ! ! pressure)!! !! statisticsDisplay.update(temp,humidity,pressure)!! !! forecastDisplay.update(temp,humidity,pressure)!}!! //other WeatherData methods here!

}; !

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TheObserverPaZernObserved

SubjectObject

int2

Dog Object Registers

Observers

Dog

Object

Cat

Object

Duck

Object

SubjectBroadcasts

2

CatObjectRegistersDuckObjectRegistersSubjectisupdated

int8

Catobjectde-registers

Cat

Object

SubjectBroadcasts

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Other examples: publish and

subscription services; eBay

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HandlingtheaddiAonofnewcustomers:

SendingawelcomeleZer Verifythecustomersaddresswiththepostoffice

notherExample

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Customer:Whenacustomerisadded,thisobjectwillmakethe

callstotheotherobjectstohavethecorresponding

acAonstakeplace

WelcomeLeZer: CreateswelcomeleZersforcustomersthatlet

themknowtheywereaddedtothesystem.

ddrVerificaAon: Thisobjectwillverifytheaddressofany

customerthatasksitto

FirstTry

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WedonotwanttochangethebroadcasAngobjectevery

Amethereisachangetothesetofobjectslisteningto

thebroadcast.

WewanttodecouplethenoAfy-ers(subject)andthenoAfy-

ees(observer).

Twothingsvary DifferentkindsofobjectsonesthatneedtobenoAfied

ofachangeinstate.

DifferentinterfacesEachobjectthatrequiresnoAficaAonmayhaveadifferentinterface.

Decoupling

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llobserversmusthavethesameinterface.llobserversmustregisterthemselves.Thismakesthem

responsibleforknowingwhattheyarewatchingfor.

Wemustaddtwomethodstothesubject.

attach(Observer) addsthegivenobservertoitslistofobservers.

detatch(Observer) removesthegivenobserverfromitslistofobservers.

Theobservermustimplementamethodcalledupdate.Thesubjectimplementsthenotifymethodthatgoes

throughitslistofObserversandcallsthisupdatemethod

foreachofthem.

BeZerSoluAon

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TheClassDiagram

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class Customer{!public:!

void attach( Observer *myObserver);!void detach( Observer *myObserver);!void notify();!

private:!vector myObs;!

};!void Customer::attach( Observer *myObserver)!{!

myObs.push_back( myObserver);!}!

Example(Contd)

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void Customer::detach( Observer *myObserver){!for (int i= 0; i< myObs.size(); i++){!

if (myObs[i]== myObserver){!myObs.erase(myObs.begin()+i);!return;!}!

}!}!void Customer::notify(){!

for (int i= 0; i< myObs.size(); i++){!myObs[i]->update(this);!}!

}!

Example(Contd)

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DefiniAon

TheobserverpaZernimplementsaone-to-manyrelaAonshipbetweenasetofobjects.

singleobjectchangesstateandupdatestheobjects(dependants)thatareaffectedbythechange.

Theobjectthatchangesstateiscalledthe subjectandtheotherobjectsaretheobservers.

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LooseCoupling

Subjectsandobserversarelooselycoupled. Thesubjectonlyknowstheobserverinterfaceand

notitsimplementaAon.

ObserverscanbeaddedandremovedatanyAme. Inaddingnewobserversthesubjectdoesnotneed

tobemodified.

Subjectsandobserverscanbereusedindependently.

Changestothesubjectorobserverwillnotaffecttheother.

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DesignPrinciple

Striveforlooselycoupleddesignsbetweenobjectsthatinteract.

Looselycoupleddesignsallowustobuildflexibleobject-orientedsystems.

Thesesystemscanhandlechangebecausetheyminimizetheinterdependencybetweenobjects.

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Observerpplicability

UsetheObserverpaZerninanyofthefollowingsituaAons

WhenanabstracAonhastwoaspects,onedependentontheother.EncapsulaAngtheseaspectsinseparateobjectsletsyouvaryandreusethemindependtly.

Whenachangetooneobjectrequireschangingothers,andyoudonotknowhow

manyobjectsneedtobechanged.

WhenanobjectshouldbeabletonoAfyotherobjectswithoutmakingassumpAonsaboutwhotheseobjectsare.

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ObserverParAcipants

SubjectKnowsitsobservers.nynumberofObserver

objectsmayobserveasubject.

ProvidesaninterfaceforaZachinganddetachingObserverObjects.

ObserverDefinesanupdaAnginterfaceforobjectsthat

shouldbenoAfiedofchangesinasubject

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ObserverParAcipants

ConcreteSubjectStoresastateofinteresttoConcreteObserver

objects.

SendsanoAficaAontoitsobserverswhenitsstatechanges.

ConcreteObserverMaintainsareferencetoaConcreteSubjectobjectStoresstatethatshouldstayconsistentwiththe

subjectstate.

ImplementstheObserverupdaAnginterfacetokeepitsstateconsistentwiththesubjectstate. 22


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WeatherStaAonClassDiagram

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TheObserverPaZern:KeyFeatures

Intent:Defineaone-to-manydependencybetweenobjectssothatwhenoneobjectchangesstate,allitsdependentsarenoAfiedandupdatedautomaAcally.

Problem:YouneedtonoAfyavaryinglistofobjectsthatan

eventhasoccurred.

Solu.on:Observersdelegatetheresponsibilityformonitoringforaneventtoacentralobject:Thesubject.

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TheObserverPaZern:KeyFeatures

Implementa.on:Haveobjects(Observers)thatwanttoknowwhenaneventhappenaZachthemselvestoanotherobject(Subject)thatiswatchingfortheeventtooccurorthattriggerstheeventitself.

Whentheeventoccurs,theSubjecttellstheObserversthatithasoccurred.

ThedapterpaZernissomeAmesneededtobeable

toimplementtheObserverinterfaceforalltheObserver-typeobjects.

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ObserverPaZern:ClassDiagram

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ObserverSequenceDiagram

ConcreteSubjectObject

ConcreteObserverObjectA

ConcreteObserverObjectB

Notify()

SetState()

Update()

GetState()

Update()

GetState()

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ObserverConsequences

bstractcouplingbetweenSubjectandObjectllasubjectknowsisthatithasalistof

observers,eachconformingtothesimple

interfaceoftheabstractObserverclass.Thesubjectdoesnotknowtheconcreteclassofanyobserver.

SupportforbroadcastcommunicaAonUnlikeandordinaryrequest,thenoAficaAonthat

asubjectsendsneednotspecifyitsreceiver.ThenoAficaAonisbroadcastautomaAcallytoallinterestedobjectsthatsubscribedtoit.

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ObserverImplementaAon

HowtonoAfy-pproach1void ConcreteSubject::setstate(int newstate)

{

state_=newstate;

notify(); // notify all observers that // statehas changed

}

Subject s1;

subject.setstate(6);// automatic call to notify

Subject.setstate(5);

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HowtonoAfy-pproach2void ConcreteSubject::setstate(int newstate)

{

state_=newstate;}

Subject s1;

subject.setstate(6);subject.setstate(5);

Subject.notify(); // explicit call to notify

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thePushmodelandthePullmodelImplementaAonsoftheObserverpaZernoen

havethesubjectbroadcastaddiAonal

informaAonaboutthechange.ThesubjectpassesthisinformaAonasanargumentto

Update.

Pushmodel:thesubjectsendsoberversdetailedinformaAonaboutthechange

Pullmodel:thesubjectsendsonlyaminimalnoAficaAonandobserversaskfordetails

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Whathappenswhenweaddnewobservers?

ImagineaddingtheabilitytosendaleZerwithcouponstocustomerslocatedwithin20milesofoneofthecompanysBrickandMortarstores.

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Iftheclasswewanttoaddalreadyexistsandwecannotmodifyit,thenwecanadaptitasshownbelow:

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