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swift,NSUserDefaults的swift化封装

2016-03-09 02:00 375 查看


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第一部分:SwiftyAPIs:NSUserDefaults01传统弊端(方法太长)Verbosity02使用下边改进:Thesubscript03类型的问题Types04可能为空Optionals05Existence:key是否存在?06Optionalassignment可选赋值07算法方面Arithmetic08简单的封装WrappingupResult最后的效果下一步Nextsteps第二部分:Statically-typedNSUserDefaults01不足之处Shortcomings02静态类型Thepowerofstatictyping03下标访问的进阶Subscriptsareawesome04走捷径Takingshortcuts05Theoptionalityconundrum缺省值的问题06更多的类型Moretypes07序列化的问题Archiving08结论Resultandconclusions

第一部分:SwiftyAPIs:NSUserDefaults

http://radex.io/swift/nsuserdefaults/
https://github.com/radex/SwiftyUserDefaultshttps://github.com/radex/SwiftyUserDefaults
InSwiftymethods,IwasarguingthatweshouldresistthetemptationtowriteSwiftthewayObjective-Cusedtobewritten.Iarguedthatweshoulddevelopnewapproachesandconventionstodesigninginterfaces,andthatweshould,onceandforall,abolishObjective-C’sexcessiveverbosity.

Althoughitwasverywellreceived,theissueI’vehadwiththeessayisthattheargumentsIwasmakingwereallintheabstract.Igavesomeexampleshereandthere,buttheywerelow-level,andmostlysilly.

AndsoIdecidedtostartaseriesofarticlescalledSwiftyAPIs,whereI’mgoingtotakesomeclassorcomponentfromanAppleframeworkandshowhowIwoulddesignitwithSwiftinmind.

Today,we’lldissectNSUserDefaultsandgiveitalittlemake-over.We’regoingtomakeitlessverbose,abitcleaner,andmoreconsistentwithotherclasses.We’llpolishitsroughedgesandwe’llmakeuseofSwift’scoolfeaturesalongtheway.

01传统弊端(方法太长)Verbosity

ItwouldbeexaggeratedtosaythatNSUserDefaults,asawhole,isparticularlyverbose.Butthere’sonebit,repeatedoverandover,that’sabitexcessive:NSUserDefaults.standardUserDefaults().Somanywordsandcharacters,beforeweevengettosaywhatwewanttofetchorsavethere.

NSUserDefaults.standardUserDefaults().stringForKey("color")
[code]​
NSUserDefaults.standardUserDefaults().setObject(NSDate(),forKey:"updatedAt")

WecouldaddanaliastostandardUserDefaults()—forexample,apropertynamedstandard,sharedorevenjusts.Theresultwouldbeabitbetter,butlet’strysomethingdifferent.

letDefaults=NSUserDefaults.standardUserDefaults()
[code]​
//Usage:
​
Defaults.integerForKey("launchCount")
​

Iadmit,it’salittleunconventionaltomakeaglobalvariablewithanupper-casename,butIthinkitlooksprettyneat.Itlooksasifitwasaclassname,notavariable,andwewereusingclassmethods.AndIthinkthat’sgreat,because99%ofthetime,wejustwanttousethestandarduserdefaults,andsowethinkofitasasingleglobalthing.(Anotherreasontomakethenameupper-caseisthatthelower-case“defaults”couldeasilyconflictwithlocalvariables.)

02使用下边改进:Thesubscript

Ithinkof NSUserDefaultssimilarlytoNSDictionary.It’ssomesortofakey-valuestructurethatservesasadatastore.Thereareobviousdifferences,likethefactthatyougenerallyuseonlyoneNSUserDefaultsobject,butstill,theprimarytwointeractionsarethesame:puttingstuffinunderakey;andfetchingstuffout.Andifthat’sthecase,itwouldmakesensetomodelthesyntaxsimilarly,makeitmoreconsistent.

Compare:

letdictionary=["launchCount":10]
[code]​
dictionary["launchCount"]
​
dictionary["color"]="red"
​
Defaults.integerForKey("launchCount")
​
Defaults.setObject("red",forKey:"color")

Wouldn’titbeniceifwecouldusethesamesquarebracketsubscriptingsyntaxinbothplaces?

Here’smyfirsttake:

extensionNSUserDefaults{
[code]subscript(key:String)->NSObject?{
get{
returnobjectForKey(key)
}
set{
setObject(newValue,forKey:key)
}
}
}

Theresult,atfirstglance,isquitenice:

Defaults["color"]
[code]​
Defaults["color"]="red"

(IhopeyoucanseethethirdreasonwhyImadeDefaultsaglobalvariable—it’snotpossibletodefinea“classsubscript”inSwift)

这里说不能定义
classsubscript
 ,应该是可以的(swift2.1)

03类型的问题Types

Thereisaseriousproblemwiththisimplementation,though. ThegetteralwaysreturnsNSObject.Thatwon’tflyinastaticallytypedlanguage. Wereallyneedamechanismtotellcompilerthetypeofdatawewanttofetch. That’sthenecessaryevilwheninterfacingoutsideofthetype-inferredworldofSwift.(我们需要让编译器知道返回的数据的类型)

Wecould,ofcourse,justcastthevaluetodesiredtype(e.g.as?NSString),butthenagain,weweresupposedtomaketheAPInicerandlessverbose.Here’sanalternativeIcameupwith:

extensionNSUserDefaults{
[code]classProxy{
privateletdefaults:NSUserDefaults
privateletkey:String
privateinit(_defaults:NSUserDefaults,_key:String){
self.defaults=defaults
self.key=key
}
varstring:String?{
returndefaults.stringForKey(key)
}
varint:Int{
returndefaults.integerForKey(key)
}
}
subscript(key:String)->Proxy{
returnProxy(self,key)
}
}
​
//Usage:
Defaults["color"].string
Defaults["launchCount"].int
Soessentially,sincewecan’tmakethesubscriptconveytypeinformation,Imadeitreturnanobjectthatrepresentstheactualvalueinuserdefaults.Andthenyouusethepropertiesofthatobjecttodotheactualfetching.

Notethatwedon’thavetodothiswhengoingtheotherwayaround. Whenwewanttoputstuffin,objectsalreadycarrythetypeinformationweneed(存储数据的时候,就知道数据是什么类型了).Wedon’tneedtobeexplicitaboutit.

extensionNSUserDefaults{
[code]subscript(key:String)->Any?{
get{
returnself[key]
}
set{
ifletv=newValueas?String{
setObject(v,forKey:key)
}elseifletv=newValueas?Int{
setInteger(v,forKey:key)
}
}
}
}

YoumightwonderwhyImadeaString->Any?subscriptinsteadofaddingseparatesubscriptsforeachacceptedtype.Well,Itrieddoingitthatwayanditdidn’twork.I’mnotsureifit’sabugoraconsequenceofhowtypesystemworksinSwift.Eitherway,thisimplementationworksjustfine.Youcanwrite

Defaults["color"]="red"

AndtheString->Any?setterwillbeexecuted.Butyoucanstillwrite:

[code]Defaults["color"].string
Andthecompilerwilldotherightthing—runString->Proxygetter.So,whilewehavetodefinesomegetterthatwillreturnAny?(youcan’tdefineasetter-onlysubscript),itwon’tactuallybeusedinpractice.

04可能为空Optionals
MakingtheAPIconciseandniceisjustonepartoftheequation.Butifthereareinconsistenciesorotherissueswiththeactualbehavior,you’vegotarealproblem.

Considerwhathappensifyoutrytofetchavaluefromuserdefaultsthatdoesn’texist:

[code]Defaults.stringForKey("color")//=>nil[code]​
Defaults.arrayForKey("lastPaths")//=>nil
​
Defaults.integerForKey("launchCount")//=>0
​
Defaults.boolForKey("loggingEnabled")//=>false

Huh?You’llgetnilinsomecases,butnotifwhatyouwanttofetchisanumberoraboolean—thenyou’llget0andfalse,respectively.It’sunderstandablewhytheydidthat—inObjective-C,thosetypesaredumb“primitivetypes”,andnilonlymakessenseforpointers. ButinSwift,anythingcanbeoptional.Solet’sbringconsistency!(swift的思想是返回的一个optionalNil)

extensionNSUserDefaults{
[code]classProxy{
varobject:NSObject?{
returndefaults.objectForKey(key)as?NSObject
}
varnumber:NSNumber?{
returnobjectas?NSNumber
}
varint:Int?{
returnnumber?.integerValue
}
varbool:Bool?{
returnnumber?.boolValue
}
}
}

Ifkeydoesn’texist,objectForKey()willreturnnil.Andifitdoesexist,butisn’tanumber,theoptionalcasttoNSNumberwillfail,andyou’llalsogetnil.

Andincaseswhenyoudowantthestandardbehavior,nilcoalescingcomestorescue:

Defaults["launchCount"].int??0
[code]​
Defaults["loggingEnabled"].bool??false
​
Defaults["color"].string??""


05Existence:key是否存在?
Believeitornot,NSUserDefaultsdoesn’thaveamethodforcheckingifakeyexists.ItonlytakesaquickGooglesearchtofigureoutthatobjectForKey()willreturnnilifavaluedoesn’texist.Still,thisshouldbejustanimplementationdetail,andthereshouldbeaproperinterfaceforit.

extensionNSUserDefaults{
[code]funchasKey(key:String)->Bool{
returnobjectForKey(key)!=nil
}
}

Andwhilewe’reatit,let’smentionremovingthings.ThereisaremoveObjectForKey()methodforit,butIdecidedtoshortenittoremove().Also,it’spossibletoremoveobjectsbysettingkey’svaluetonil,soIalsoaddedthatfeaturetoourString->Any?subscriptincasesomeonetrieddoingitthisway.

IwasalsoplayingwiththeideaofaddingthosetwofeaturestoourNSUserDefaults.Proxyobject.

extensionNSUserDefaults{
[code]classProxy{
varexists:Bool{
returndefaults.hasKey(key)
}
​
funcremove(){
defaults.removeObjectForKey(key)
}
}
}
​
//Usage:
Defaults["color"].exists
Defaults["color"].remove()

Iwasquitetornonthis.Ononehand,itplayswellwiththeideathatNSUserDefaults’ssubscriptreturnsanobjectrepresentingavalue.Ontheotherhand,theProxyclasswasjustanecessaryevil;checkingforexistenceandremovingobjectsisanoperationontheentiredatastructure,notanelementofit.Intheend,Isidedwiththelatterargument.

06Optionalassignment可选赋值
是什么意思呢?

就是这个符号:??

也就是如果前面有值,返回就是前面的值. 如果前面是值,就返回后面的值.

(当然后面的值被当成了一个闭包.)

InRuby,there’sausefuloperator,||=,forconditionalassignment.It’susedlikethis:

a=nil#=>nil
[code]​
a||="foo"#=>"foo"
​
a||="bar"#=>"foo"

Essentially,theright-handvalueisassignedtotheleft-handvariable,butonlyifit’sundefined,nilorfalse.

IthinkSwiftshouldalsohavethisoperator,onlythatI’dcallit?=,theoptionalassignmentoperator.Itwouldsetthevalueofavariable(anoptional)ifit’snil.

ThemagicofSwiftisthatyoucandefineitonyourown:

infixoperator?={
[code]associativityright
precedence90
}
func?=<T>(inoutvariable:T?,@autoclosureexpr:()->T){
ifvariable==nil{
variable=expr()
}
}

WhatdoesithavetodowithNSUserDefaults?Well,ifwecanoptionallyassignvaluestovariables,whynotoptionallyassignvaluestouserdefaultskeys?

func?=(proxy:NSUserDefaults.Proxy,@autoclosureexpr:()->Any){
[code]if!proxy.defaults.hasKey(proxy.key){
proxy.defaults[proxy.key]=expr()
}
}
​
//Usage:
Defaults["color"]//=>nil
Defaults["color"]?="white"//=>"white"
Defaults["color"]?="red"//=>"white"


很明显,这借鉴了自定义运算符.

NotethatthisisdifferentfromusingregisterDefaults.Theoptionalassignmentoperatorchangestheactualuserdefaultsdataandsavesthenewvaluestodisk.registerDefaultshassimilarbehavior,butitonlymodifiesthedefaultsinmemory.

07算法方面Arithmetic
Considerwhatyouhavetodoifyouwanttoincrementthevalueofauserdefault:

Defaults["launchCount"].int=(Defaults["launchCount"]??0)+1

Ifitwasavariable,youcouldmakeitshorterandclearerbyusingthe+=operator.Well,whosayswecan’tdothataswell?

[code]func+=(proxy:NSUserDefaults.Proxy,b:Int){[code]​
leta=proxy.defaults[proxy.key].int??0
proxy.defaults[proxy.key]=a+b
​
}
​
//Usage:
​
Defaults["launchCount"]+=1
​
Nice!Butheck,let’smakeitevenshorter:
​
postfixfunc++(proxy:NSUserDefaults.Proxy){
​
proxy+=1
​
}
​
//Usage:
​
Defaults["launchCount"]++
​
Voilà!

可以看到,值的改变,也是将实现封装到了一个自定义运算符中去了.



08简单的封装Wrappingup
Let’sfillintheblanks.IaddeddoublepropertytoNSUserDefaults.Proxythatworksjustlikeintandbool(usingconversionfromNSNumber,notthedoubleForKeymethod).Ididnotaddafloatproperty,becauseyou’resupposedtojustuseDoubleinSwift.

Iadded array, dictionary and data propertiesthatmirror arrayForKey, dictionaryForKey and dataForKey.Ialsoaddeddateproperty,because NSDate isoneofthetypessupportedbyNSUserDefaults,andyetitdoesn’thaveabuilt-ingetter.

Andfinally,Iupdatedoursetter:

subscript(key:String)->Any?{
[code]​
get{
returnself[key]
}
set{
ifletv=newValueas?Int{
setInteger(v,forKey:key)
}elseifletv=newValueas?Double{
setDouble(v,forKey:key)
}elseifletv=newValueas?Bool{
setBool(v,forKey:key)
}elseifletv=newValueas?NSObject{
setObject(v,forKey:key)
}elseifnewValue==nil{
removeObjectForKey(key)
}else{
assertionFailure("Invalidvaluetype")
}
}
​
}
​

Onmyfirsttry,IdefinedthesubscripttoreturnAnyObject?,butwhenIaddedDoubleandBoolafterInt,Iwassurprisedthatitdidn’tworkproperly.Youcouldsetandfetchbackdoublesandbooleans,buttheywouldbeencodedasintegers.Turnsout,whenyoutrytopassnumbersorbooleansasAnyObject,theygetautomaticallymappedtoNSNumberunderthehood,whichcausedthisoddbehavior.SimplechangetoAnyfixedtheissue.

Result最后的效果
Let’slookatourbrandnewNSUserDefaultsAPIinitsfullglory:

//Fetchingdata
[code]​
Defaults["color"].string
​
Defaults["launchCount"].int??0
​
Defaults["lastPaths"].array?.firstObject
​
//Settingdata
​
Defaults["color"]="red"
​
Defaults["firstLaunchAt"]?=NSDate()
​
Defaults["launchCount"]++
​
Defaults["totalTime"]+=3600
​
//Checking&removing
​
if!Defaults.hasKey("hotkey"){
​
Defaults.remove("hotkeyOptions")
​
}


下一步Nextsteps
Now,thejourneydoesn’tstophere.Althoughwemadegreatprogressonsyntaxandnoisereduction,thisisn’tthebestwecando.CheckoutStatically-typedNSUserDefaultstoseehowwecantakeUserDefaultstothenextlevel,simplifytheiruse,andgetcompilerchecksforfreebyadoptingtyped,statically-definedkeys.
https://github.com/radex/SwiftyUserDefaults
第二部分:Statically-typedNSUserDefaults
Ayearago,notlongafterSwiftbecameathing,Inoticedatendencyamongprogrammerstowriteitthewayyou’dwriteObjective-C.IthoughtthatSwiftwasadifferentlanguage,withdifferentsyntax,philosophyandcapabilities,andsoweshoulddevelopnewconventionsandapproachestoit.IrespondedwithSwiftymethods,whereIarguedforabetter,clearerwayofnamingthings.Then,sometimelater,IstartedtheSwiftyAPIsseriestoputthoseideasinpracticeandexplorehowtodesigneasy-to-useinterfaces.

Inthefirstarticleinsaidseries,wetooktheNSUserDefaultsAPI:

NSUserDefaults.standardUserDefaults().stringForKey("color")
[code]​
NSUserDefaults.standardUserDefaults().setObject("red",forKey:"color")
​

…andwemadeitlooklikethis:

Defaults["color"].string
[code]​
Defaults["color"]="red"

Theresultwasclearer,lessverbose,andnicerlookingthantheoriginal.WefixedsomeconsistencyissuesandmadeitallfitbetterwithSwift.Thisfeltlikeasignificantimprovement.

Andyet,asI’vebeenactuallyusingthenewAPI,andlearningSwiftalongtheway,Irealizedthatitwasn’tactuallyverySwiftyatall. IdrewinspirationfromRuby’sandSwift’ssyntaxindesigningit,andthatmatters,butwedidn’timproveitonasemanticlevelatall. WeonlyputaSwiftycoatofpaintonafundamentallyObjective-C-likemechanism.(在OC的机制上披上了一层swift的外衣)

语法层次上的提示:来自Ruby的灵感

01不足之处Shortcomings
Ofcourse,“notSwifty”isn’tagoodreasontostartfromscratch.FamiliaritymakesanAPIeasiertolearn,butwedon’twanttobedogmaticaboutit.Wedon’tjustwantaSwift-likedesign,wewantwhatworksbestinSwift,period.Sohere’safewissueswithNSUserDefaults:

Supposeyouhaveapreferenceforuser’sfavoritecolor:

Defaults["color"]="red"
[code]​
//elsewhereintheapp:
​
Defaults["colour"].string//=>nil

Ooops,butyou’vemadeatypointhekeyname.Boom,that’sabug.

 将key拼写错误了.

Let’ssayyoukeepadateobjectinthedefaults:

Defaults["deadline"]=NSDate.distantFuture()
[code]​
Defaults["deadline"].data//=>nil
Thistimeyoumistypedthedategetter,andonceagain,youhaveabug.Unlikelytohappen?Probably.Butwhydowehavetospecifythereturntypeeverysingletimewewanttofetch,anyway?It’skindofannoying.

将getter方法拼错了.

Here’sonemore:

Defaults["deadline"]=NSData()
[code]​
Defaults["deadline"].date//=>nil

Clearlywemeant“rightnow”,not“emptydata”.Ohwell.

Whataboutthis:

Defaults["magic"]=3.14
[code]​
Defaults["magic"]+=10
​
Defaults["magic"]//=>13
Theonlyreasonwhy+=worksatallisbecausewedefineditasamagicoperator.Butitcanonlyinfertypes(IntorDouble)fromtheargumentpassed.Soifyoupassaninteger10,theresultwillbestoredasaninteger,cuttingoffthefractionalpart.Andwehaveabug.

Youmightbethinkingthatthesearepurelytheoreticalproblemsthatwouldn’tariseintherealworld.Butthinktwice. Thesearethesamekindsofbugsasmistypingavariableormethodname,orpassingaparameterofthewrongtype. Thesethingshappenallthetime. 

key值写错
方法名写错
传入了错误的参数类型
Ifyouworkwithacompiled,statically-typedlanguage,yougrowtoappreciategettingfeedbackfromthecompilermorequicklythanbytestingit. Moreimportantly,investmentincompile-timecheckspaysdividendsovertime. (在编译期间做更多的事情比在测试时候发现问题更好.)Thisisn’tjustforyouwritingthecodeforthefirsttime,thisisforpreventingbugsasyouchangeandrefactoritlater.Thisisaboutprotectingthefuture-youfrompast-you.

02静态类型Thepowerofstatictyping
Therootofallevilhereisthatthereisnostatically-definedstructureoftheuserdefaults.

Withthepreviousredesign,IrecognizedthisproblemandtriedtoslaptypesonthesubscriptbyhavingitreturnaProxyobjectprovidingtypedaccessors.This,inmyestimation,wasbetterthanmanuallycastingAnyObjectorhavinggettermethodsinsteadofasubscript.

Butitwasahack,notasolution.TomakeameaningfulimprovementtotheAPI, weneedtocentralizeinformationaboutuserdefaultkeysandmakeitavailabletothecompiler.

现在的问题在于怎么把keys集中化,暴露给编译器

How?Considerhowpeoplesometimesdefinestringkeysaheadoftimetoavoidtyposandgetnameauto-completionforfree:

[code]x
letcolorKey="color"
借鉴定义常量字符串:

1.避免了拼写错误

2.编译器自动补全

Whatifwedojustthat,butwithtypeinformation?

[code]x
classDefaultsKey<ValueType>{
​
letkey:String
init(_key:String){
self.key=key
}
​
}
letcolorKey=DefaultsKey<String?>("color")

Wewrappedthekeynameinanobject,andembeddedthevalue’stypeinagenerictypeparameter.

上面的代码做了什么:

将一个key封装到一个类对象中了.
key对应的值呢?被嵌入到了类需要使用的泛型参数.
 NowwecandefineanewsubscriptonNSUserDefaultsthatacceptsthesekeys:

[code]x
extensionNSUserDefaults{
​
subscript(key:DefaultsKey<String?>)->String?{
get{returnstringForKey(key.key)}
set{setObject(newValue,forKey:key.key)}
}
​
}

Andhere’stheresult:

[code]x
letkey=DefaultsKey<String?>("color")
​
Defaults[key]="green"
​
Defaults[key]//=>"green",typedasString?
​

Boom,assimpleasthat.Morerefinementstothesyntaxandfunctionalityarepossible(aswe’llsoonexplore),butwithThisOneSimpleTrick™,wefixedmanyofourproblems.Wecan’teasilymakeakeynametypo,becauseit’sdefinedonlyonce.Wecan’tassignavalueofthewrongtype,becausethecompilerwon’tletus.Andwedon’tneedtowrite.string,becausethecompileralreadyknowswhatwewant.

解决的问题:

1)只定义了一次,减少拼写错误

2)赋值时候编译器做类型检查

Bytheway.WeshouldprobablyjustusegenericstodefinetheNSUserDefaultssubscriptsinsteadoftypingalloftheneededvariantsbyhand.Well,thatwouldbewonderful,butsadly,theSwiftcompilerdoesn’tsupportgenericsubscriptsrightnow.Sigh.

上面的下标访问只支持字符串,如果我们不想全部手动写,使用泛型,但是问题是:

swift编译器不支持下标访问的时候使用泛型.

 Thesquarebracketsmightbenice,butlet’snotbestupidaboutthesyntax,andjustmakegenericmethodsforsettingandgetting…Right?

Ahh,butyouhaven’tseenwhatsubscriptscandoyet!

03下标访问的进阶Subscriptsareawesome
Considerthis:

[code]x
vararray=[1,2,3]
​
array.first!+=10
编译不通过,原因:值类型的数据,传递的只是拷贝,离开表达式后就不存在了==>对比变量

Itwon’tcompile.We’retryingtoincrementanintegerinsideanarray,butthismakesnosenseinSwift.Integershavevaluesemantics.They’reimmutable.Youcan’tjustchangethemwhenthey’rereturnedfromsomeplaceelse,becausetheydon’treallyexistoutsideofthatexpression.They’remerelytransientcopies.

It’svariablesthathavetheconceptofmutability.Whenyoudo:

varnumber=1
[code]​
number+=10
​

You’renotactuallychangingthevalue,you’rechangingthevariable—assigninganewvaluetoit.

Buttakealookatthis:

vararray=[1,2,3]

array[0]+=10

array//=>[11,2,3]

Thisjustworks.Itdoesexactlywhatyou’dexpect,butit’sactuallynotobviouswhy.Huh!

奇怪了,上面的array.first就不能编译通过,但是array[0]却能达到预期的效果,这是为什么?

See,subscriptshavesemanticsthatplayreallywellwithvaluetypesinSwift.Thereasonwhyyoucanchangevaluesinanarraythroughthesubscriptisbecauseitdefinesbothagetterandasetter. Thisallowsthecompilertorewritearray[0]+=10toarray[0]=array[0]+10.Ifyoumakeagetter-onlysubscript,itwon’twork.

原因在于下标访问方法定义了getter和setter两个方法,而不仅仅是一个只读的getter方法.

Again,thisisn’tsomemagiconArray’spart.Thisisaconsequenceofcarefullydesignedsemanticsofsubscripts.Wegettheexactsamebehaviorforfree,anditallowsustodothingslike:

[code]x
Defaults[launchCountKey]++
​
Defaults[volumeKey]-=0.1
​
Defaults[favoriteColorsKey].append("green")
​
Defaults[stringKey]+="…caneasilybeextended!"

Youknow,it’sfunny.In1.0,weusedsubscriptspurelyfortheirsyntacticvalue(becauseitmadeuserdefaultslooklikeadictionary),andmissedoutcompletelyonsemanticbenefits.

语法值

语义

Weaddedafewoperatorslike+=and++,buttheirbehaviorwasdangerousanditreliedonmagicimplementations.Here,byencapsulatingtypeinformationinthekey,andbydefiningboththesubscriptgetterandsetter,allofthisactuallyjustworks.

04走捷径Takingshortcuts
Onenicethingaboutusingplainoldstringkeysisthatyoucouldjustusetheminplace,withouthavingtocreateanyintermediateobjects.

Obviously,creatingthekeyobjecteverytimewewanttouseitdoesn’tmakemuchsense.Thiswouldbeawfullyrepetitiveandwouldeliminatethebenefitsofstatictyping.Solet’sseehowwecanorganizeourdefaultskeysbetter.

Onewayistodefinekeysattheclasslevelwhereweusethem:

[code]x
classFoo{
​
structKeys{
staticletcolor=DefaultsKey<String>("color")
staticletcounter=DefaultsKey<Int>("counter")
}
funcfooify(){
letcolor=Defaults[Keys.color]
letcounter=Defaults[Keys.counter]
}
​
}
​

ThisseemstoalreadybeastandardSwiftpracticewithstringkeys.

AnotherwayistotakeadvantageofSwift’s implicitmemberexpressions.(暗含的成员表达式)Themostcommonuseofthisfeatureiswithenumcases.(最常见的是枚举)

WhencallingamethodthattakesaparameterofenumtypeDirection,youcanpass.Right,andthecompilerwillinferthatyoumeantDirection.Right.Lessknownisthefactthatthisalsoworkswithanystaticmembersofargument’stype.SoyoucancallamethodtakingaCGRectwith.zeroRectinsteadoftypingCGRect.zeroRect.

举个例子,有个方法带一个参数:Direction枚举类型,

如果直接传入
.Righ
,那么编译器认为你传入的是
Direction.Right


其实,这种情况也适用于静态成员变量作为函数参数的情况.

例如,带一个CGRect的参数的时候,可以传入
.zeroRect
,而不用传入
CGRect.zeroRect




enumDirection{
[code]caseLeft,Right,Up,Down
}
​
classPerson{
classfuncwalkTo(direction:Direction){
print("nowwalkto\(direction)")
}
}
​
Person.walkTo(.Right)
//nowwalktoRight


Infact,wecandothesamethingherebydefiningourkeysasstaticconstantsonDefaultsKey.Well,almost.Weneedtodefineitslightlydifferentlytoworkaroundacompilerlimitationatthemoment:

[code]x
classDefaultsKeys{}
​
classDefaultsKey<ValueType>:DefaultsKeys{...}
​
extensionDefaultsKeys{
​
staticletcolor=DefaultsKey<String>("color")
​
}

Andnow,ohwow!

[code]x
Defaults[.color]="red"
Isn’tthiscool?Atcallersite,verbosityisnowlowerthanwiththetraditionalstringly-typedapproach.Wedolessworkwritingandreadingthecode,andgetallthebenefitsalmostforfree.

(Oneshortcomingofthistechniqueisthatthereisnomechanismfornamespacing.Inalargeproject,itmightbebettertogowiththeKeysstructapproach.)

不足之处:工程变大的时候,由于没有命名空间,更好的解决方法是使用结构体

05Theoptionalityconundrum缺省值的问题
IntheoriginalAPIredesign,wemadeallgettersreturnoptionals.IdislikedthelackofconsistencyinhowNSUserDefaultstreatsdifferenttypes.Forstrings,amissingvaluewouldgetyounil,butfornumbersandbooleans,you’dget0andfalse.

Thedownsideofthisapproach,asIquicklyrealized,wasverbosity.Muchofthetime,wedon’treallycareaboutthenilcase—wejustwanttogetsomedefaultvalue.Whensubscriptsreturnanoptional,wehavetocoalesceiteverytimewefetch.

默认值返回的问题:

string类型,返回nil

数值类型,返回0

布尔类型,返回false

AsolutiontothisproblemwasproposedbyOlegKokhtenko.Inadditiontothestandard,Optional-returninggetters,anothersetofgetterswasadded.Thosehavenamesendingwith-Valueandreplacenilwithadefaultvaluethatmakessenseforthetype:

Defaults["color"].stringValue//defaultsto""
[code]​
Defaults["launchCount"].intValue//defaultsto0
​
Defaults["loggingEnabled"].boolValue//defaultstofalse
​
Defaults["lastPaths"].arrayValue//defaultsto[]
​
Defaults["credentials"].dictionaryValue//defaultsto[:]
​
Defaults["hotkey"].dataValue//defaultstoNSData()
​
通过对可能为空的值,再次进行封装,给出一个默认值.

Wecandothesamethingunderthestatictypingregimebyprovidingsubscriptvariantsforoptionalandnon-optionaltypes,likeso:

[code]x
extensionNSUserDefaults{
​
subscript(key:DefaultsKey<NSData?>)->NSData?{
get{returndataForKey(key.key)}
set{setObject(newValue,forKey:key.key)}
}
subscript(key:DefaultsKey<NSData>)->NSData{
get{returndataForKey(key.key)??NSData()}
set{setObject(newValue,forKey:key.key)}
}
​
}
在get方法的时候,给出了默认值.

Ilikeit,becausewe’renotrelyingonconvention(typeandtypeValue)toconveynullability.We’reusingtheactualtypewhendefiningauserdefaultskey,andthenlettingthecompilerhandletherest.

06更多的类型Moretypes
Fillingintheblanks,Iextendedthescopeofsupportedtypesbyaddingsubscriptsforallofthese:String,Int,Double,Bool,NSData,[AnyObject],[String:AnyObject],NSString,NSArray,NSDictionary,andtheiroptionalvariants(plusNSDate?,NSURL?,andAnyObject?,whichdon’thaveanon-optionalcounterpart,becausethereisn’tareasonabledefaultvalue).

Andyes,strings,dictionariesandarrayshavebothSwiftandFoundationvariants.NativeSwifttypesarepreferrable,buttheydon’thaveallofthecapabilitiesoftheirNScounterparts.Ifsomeonewantsthose,Iwanttomakethateasy.

Andspeakingofarrays,whylimitourselvestountypedarraysonly?Inmostcases,arraysstoredinuserdefaultsaregoingtohavehomogenouselementsofasimpletype,likeString,Int,orNSData.

Sincewecan’tdefinegenericsubscripts,we’llcreateapairofgenerichelpermethods:

extensionNSUserDefaults{
[code]​
funcgetArray<T>(key:DefaultsKey<[T]>)->[T]{
returnarrayForKey(key.key)as?[T]??[]
}
funcgetArray<T>(key:DefaultsKey<[T]?>)->[T]?{
returnarrayForKey(key.key)as?[T]
}
​
}
​

…andthencopy&pastethisstubforalltypeswe’reinterestedin:

extensionNSUserDefaults{
[code]​
subscript(key:DefaultsKey<[String]?>)->[String]?{
get{returngetArray(key)}
set{set(key,newValue)}
}
​
}

Andnowwecandothis:

letkey=DefaultsKey<[String]>("colors")
[code]Defaults[key].append("red")
letred=Defaults[key][0]

SubscriptingthearrayreturnsString,andappendingtoitverifiesatcompiletimethatyouarepassingastring.Winforconvenience,winforsafety.

07序列化的问题Archiving
AlimitationofNSUserDefaultsisthatitonlysupportsahandfuloftypes.AcommonworkaroundforstoringcustomtypesistoserializethemwithNSKeyedArchiver.

Let’smakethiseasytodo.SimilarlytothegetArrayhelper,Idefinedgenericarchive()andunarchive()methods.Withthose,youcaneasilydefineastubsubscriptforwhateverNSCoder-complianttypeyouwant:

extensionNSUserDefaults{
[code]​
subscript(key:DefaultsKey<NSColor?>)->NSColor?{
get{returnunarchive(key)}
set{archive(key,newValue)}
}
​
}
​
extensionDefaultsKeys{
​
staticletcolor=DefaultsKey<NSColor?>("color")
​
}
​
Defaults[.color]//=>nil
​
Defaults[.color]=NSColor.whiteColor()
​
Defaults[.color]//=>w1.0,a1.0
​
Defaults[.color]?.whiteComponent//=>1.0
​

Clearlynotperfect,butit’snicethatwithjustafewlinesofboilerplate,wecanmakeanycustomtypeworknaturallywithNSUserDefaults.

08结论Resultandconclusions
Andthat’sit!Here’showournewAPIpresentsitself:

//Definekeysaheadoftime
[code]​
extensionDefaultsKeys{
​
staticletusername=DefaultsKey<String?>("username")
staticletlaunchCount=DefaultsKey<Int>("launchCount")
staticletlibraries=DefaultsKey<[String]>("libraries")
staticletcolor=DefaultsKey<NSColor?>("color")
​
}
​
//Usethedotsyntaxtoaccessuserdefaults
​
Defaults[.username]
​
//Definekey’stypewithout?tousedefaultvaluesinsteadofoptionals
​
Defaults[.launchCount]//Int,defaultsto0
​
//Modifyvaluetypesinplace
​
Defaults[.launchCount]++
​
Defaults[.volume]+=0.1
​
Defaults[.strings]+="…caneasilybeextended!"
​
//Useandmodifytypedarrays
​
Defaults[.libraries].append("SwiftyUserDefaults")
​
Defaults.libraries+="2.0"
​
//Easilyworkwithcustomserializedtypes
​
Defaults[.color]=NSColor.whiteColor()
​
Defaults[.color]?.whiteComponent//=>1.0
​
Statictypingdoesn’thurtinSwift
​


Ihopeyoucanseethesignificantbenefitsofthisstatically-typedapproach.Yes,there’sasmallpricetooptin.WeneedtopayourrespectstothetypesystemanddefineaDefaultsKeyaheadoftime.Butinreturn,thecompilerbringsusasackfullofpresents:

compile-timechecks(keyname,read/writetypesafety)编译器类型检测
autocompletionofkeynames自动补全
inferredtypes—noneedtotype.stringormanuallycastAnyObject.类型推断
wecaneasilymanipulatevaluetypesstoredinuserdefaultswithoutintermediatestepsormagicoperators
consistency—weirdkeysaside,Defaultslooksandactslikeatypeddictionary一致性
AndmorebenefitsmightbepossibleasSwiftadvances.

TrulySwiftyAPIstakeadvantageofstatictyping.Idon’tmeantobedogmaticaboutit—therearesurelyproblemsthatarebestsolvedindifferentways.Butdoconsiderthebenefitsbeforeyoufallbacktothewayyou’ddothingsinObjective-CorJavaScript.Remember,thisisn’tyourgrandfather’sstatictypingwe’retalkingabout.ArichtypesystemSwifthasusuallyallowsforreallyexpressiveandnice-to-useAPIswithlittleoverhead.
                                            
                                        

                        
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