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Issues

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Protocols

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Aprotocol,inObjectiveC,isalistofmethoddeclarationsthatanyclassthatwishestoadopttheprotocolmust

implement.

Wewriteaprotocolinasimilarwaytowritingan@interfacedeclaration.Hereisonetypicalprotocol.

@protocolDocument(int)wordCount;(id)title:(char*)new_title;

(char*)title;@end

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The protocol specification is quite simple. it is basically@protocol ProtocolName(methods you must implement)@end.To conform to a protocol, you put the protocols you're conforming toin 's, and comma separate them.Example: @interface SomeClass

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CodeTime!

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OneuniqueaspectofObjectiveC'sinterfacesystemishowyouspecifytypes.RatherthanspecifyingitlikeJavaorC++as:Printing*someVar=(Printing*)frac;forexample,youusetheidtypewitharestrictedprotocol:idvar=frac;Thisallowsyoutodynamicallyspecifyatypethatrequiresmultipleprotocols,allwithonevariable.

Suchas:idvar=frac;

Muchlikeusing@selectorfortestinganobject'sinheritance,youcanuse@protocoltotestforconformanceofinterfaces.[objectconformsToProtocol:@protocol(SomeProtocol)]returnsaBOOLiftheobjectconformstothatprotocol.

Thisworksthesameforclassesaswell:[SomeClassconformsToProtocol:@protocol(SomeProtocol)].

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Categories

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Whenyouwanttoaddmethodstoaclass,

youtypicallyextendit.Howeverthis

solutionisn'talwaysperfect,especiallyifyouwanttorewritethefunctionalityofa

classthatyoudon'thavethesourcecode

to.Categoriesallowyoutoadd

functionalitytoalreadyexistingclasses

withoutextendingthem.

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Code Time!

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Themagichereisthetwo@implementationand@interfacelines:@interfaceFraction(Math)and@implementationFraction(Math).

Therecanonlybeonecategorywiththesamename.Additionalcateogiesmaybeaddedonwithdifferentbutunqiuenames.

Categoriescan'taddinstancevariables.

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Categoriesallowustosplitupalargeclassintoseveralpieces.Eachcanbedevelopedandcreatedindependently,andthenbroughttogetherattheendtoformtheclass.

Asaconsequencehowever,thisallowsussomeniceadvantages.Wecan,inessence,extendthefunctionalityofanyclassthatmayfallintoourhands,whetheritisinaclassyoucreateyourself,orinabinaryform,suchasintheCocoaframeworks,wecanaddacategorytoNSStringtoprovidefunctionalitythattheoriginalNSStringdidnotprovide.Categoriescanalsobeusedtooverridepreviousfunctionality,whichallowsusthento"patch"thesepreexistingclassesalso,whichmightbeprovidedwithabug.

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