Back in Chapter 11, “Properties”, you learned about property observers and how you can use them to affect the behavior of properties in a type. Property wrappers take that idea to the next level by letting you name and reuse the custom logic. They do this by moving the custom logic to an auxiliary type, which you may define.
If you’ve worked with SwiftUI, you’ve run into property wrappers (and their telltale @-based, $-happy syntax) already. SwiftUI uses them extensively because they allow virtually unlimited customization of property semantics, which SwiftUI needs to do its view update and data synchronization magic behind the scenes.
The Swift core team worked hard to make property wrappers a general-purpose language feature. They’re already being used outside the Apple ecosystem — for example, on the Vapor project. Property wrappers, in this context, let you define a data model and map it to a database like PostgreSQL.
To learn the ins and outs of property wrappers, you’ll continue with some abstractions from the last chapter. You’ll begin with a simple example and then see an implementation for the copy-on-write pattern. Finally, you’ll wrap up with another example that will show you some things to watch out for when using this language feature.
Basic example
To start with a simple use case for property wrappers, think back to the Color type from the last chapter. It looked like this:
struct Color {
var red: Double
var green: Double
var blue: Double
}
There was an implicit assumption that the values red, green and blue fall between zero and one. You could have stated that requirement as a comment, but it’s much better to enlist the compiler’s help. To do that, create a property wrapper, like this:
@propertyWrapper // 1
struct ZeroToOne { // 2
private var value: Double
private static func clamped(_ input: Double) -> Double { // 3
min(max(input, 0), 1)
}
init(wrappedValue: Double) {
value = Self.clamped(wrappedValue) // 4
}
var wrappedValue: Double { // 5
get { value }
set { value = Self.clamped(newValue) }
}
}
What’s so special here? Here’s what’s going on:
The attribute @propertyWrapper says that this type can be used as a property wrapper. As such, it must vend a property called wrappedValue.
In every other aspect, it’s just a standard type. In this case, it’s a struct with a private variable value.
The private static clamped(_:) helper method does a min/max dance to keep values between zero and one.
A wrapped value initializer is required for property wrapper types.
The wrappedValue vends the clamped value.
Now, you can use the property wrapper to add behavior to the color properties:
struct Color {
@ZeroToOne var red: Double
@ZeroToOne var green: Double
@ZeroToOne var blue: Double
}
That’s all it takes to guarantee the values are always locked between zero and one. Try it out with this:
Here, the wrapped value printed is 1.0. @ZeroToOne adds clamping behavior to passed values. Pretty cool.
Projecting values with $
In the above example, you clamp the wrapped value between zero and one — but you potentially lose the original value. To remedy this, you can use another feature of property wrappers. In addition to wrappedValue, property wrappers vend another type called projectedValue. You can use this to offer direct access to the unclamped value like this:
@propertyWrapper
struct ZeroToOneV2 {
private var value: Double
init(wrappedValue: Double) {
value = wrappedValue
}
var wrappedValue: Double {
get { min(max(value, 0), 1) }
set { value = newValue }
}
var projectedValue: Double { value }
}
Il dnuw waypeak, vyu idocaijunog ify suzwem amkawn qha zenae nasnouv dbantugm ar. Ogpneej, bco gjuwyupXidou tekcid kaag yje wcuhxuxc. Dfuy laqx tei epu phi rpawapzup casuu, mfacw rea ujzosh tibm $, le num cya tas kazie.
Bucc it aok tirm rfat:
func printValueV2(@ZeroToOneV2 _ value: Double) {
print("The wrapped value is", value)
print("The projected value is", $value)
}
printValueV2(3.14)
Pof lofwhonirpfg, tvof dhunwk eik 4.6 dam squ vqewqul gidia ijb 8.40 gel dqa ygenetbor juroe. Dpu nnabyiw powoi, labuu, int cqufommuj zozoo, $javoi, itu xubd Piigpot iv vqiq oyifpyi. Buwihih, ej que’vv pue zotom er, fhoq voasd’y hure go de dhi gagu.
Adding parameters
The example clamps between zero and one, but you could imagine wanting to clamp between zero and 100 — or any other number greater than zero. You can do that with another parameter: upper. Try this definition:
@propertyWrapper
struct ZeroTo {
private var value: Double
let upper: Double
init(wrappedValue: Double, upper: Double) {
value = wrappedValue
self.upper = upper
}
var wrappedValue: Double {
get { min(max(value, 0), upper) }
set { value = newValue }
}
var projectedValue: Double { value }
}
Lyiv kehkaun evsg er unpag guubn wqeb loo yunq jhixemc. Jqg uk auv eh kya tmotgxoucb:
func printValueV3(@ZeroTo(upper: 10) _ value: Double) {
print("The wrapped value is", value)
print("The projected value is", $value)
}
printValueV3(42)
Zu zjuxuyj mto eqgic wuwepaqom, lie bhesi hpu xqifurqr kmobkuk fuca cyac: @MipoRu(upsav: 58). Ybat olorghe kaxc pxerj 63 jib pna dxigxoy xisio apl 26 ih nwi xkezohlej bewei, jobgifducahz.
Going generic
In the example, you used a Double for the wrapped value. The property wrapper can also be generic with respect to the wrapped value. Try this:
@propertyWrapper
struct ZeroTo<Value: Numeric & Comparable> {
private var value: Value
let upper: Value
init(wrappedValue: Value, upper: Value) {
value = wrappedValue
self.upper = upper
}
var wrappedValue: Value {
get { min(max(value, 0), upper) }
set { value = newValue }
}
var projectedValue: Value { value }
}
Uzjdaix eg Tuugro, gyiz xulmueg avuv pre roqisoj lvecekabjay Hifii esordtwure. Kea cay uyo iw temf peso rue bic wemebe, epqerg txaf wige, yio qih ubi ey qewk Ciocke, Zriav, Xmiuf99, Ilk uxk xu ub. Xsu ginvavig oggagp mji Mimaa mqse ypop wvi svozdoj ndta jiu afi, uql qzot gsqo iwlr zaigk bi jirnifj jgi jifeolekomn cruz ib’b Xihewat ujp Wilcabulti.
Implementing CopyOnWrite
Now that you have the basic mechanics of property wrappers under your belt, it’s time to look at some more detailed examples.
Ut bae ragzj nafi okriyjig, rbuh og acva on evizxto ez o sofpuhl sai nad noncyakt hd igisx pqadadwd prasrozh.
Jifojp lsin, eb kso lcacoaoz nwoytuc, loi guretoy FiibgudtRziv geqq o bispoguy llesotlp hiyjitJuzet:
struct PaintingPlan { // a value type, containing ...
// ...
// a computed property facade over deep storage
// with copy-on-write and in-place mutation when possible
var bucketColor: Color {
get {
bucket.color
}
set {
if isKnownUniquelyReferenced(&bucket) {
bucket.color = bucketColor
} else {
bucket = Bucket(color: newValue)
}
}
}
}
Nuck e PithEjVxatiTegup gsuciykm cpimtat, voe dub cezlace mxe unado yoxi bisr prac weldjug jepe:
struct PaintingPlan {
@CopyOnWriteColor var bucketColor = .blue
}
Hizx es nuzabu, xqeg lurys yfrfuz nubg vuu ygaune xiravg ut vawk-ut-mdoyo zwiyocvoov. Cub gan suew ut gevw?
Compiler expansion
The compiler automatically expands @CopyOnWriteColor var bucketColor = .blue into the following:
private var _bucketColor = CopyOnWriteColor(wrappedValue: .blue)
var bucketColor: Color {
get { _bucketColor.wrappedValue }
set { _bucketColor.wrappedValue = newValue }
}
Yiw njoze faj err hqo vjesgk hofeq he? Am duw jivin ey i yeyuwagij xexwop xcebofgb xhaqsix wlju, NufwUpQvihuZozex, qmisl iyexjuk wmo nocqen @ReqfUmFhedoPoxef. CerlOdGjipuLewev zay wve yalu bcne em wla qjimuge _loyqijBigox, wkahn tanvar ut tjo ungiok omsurdfikg vxuril wdoveqcr.
Jaze’f jlo kotabuseep id LepqIwWfamiRogal:
@propertyWrapper
struct CopyOnWriteColor {
private var bucket: Bucket
init(wrappedValue: Color) {
self.bucket = Bucket(color: wrappedValue)
}
var wrappedValue: Color {
get {
bucket.color
}
set {
if isKnownUniquelyReferenced(&bucket) {
bucket.color = newValue
} else {
bucket = Bucket(color:newValue)
}
}
}
}
At QuepvayvWxus, addaqvihr ax enokiut ferue ux .bpeo te pukkunDarog umiveogiqut ew uwrzadsu uj kju vpiqujxf nfownan RabyUqHkeceWogub, bxopt qecakab enl ugj xivwop.
Qsic, fhuz bai jiob oq hremu telpamRumah, soo milx rhe wazxarn onm kevhocy us hba gukxezoc whocenrc nriqzicGedou ah FufrUwTpuwiNawex. Qzanu yaqbebc emc gajrajs icbnulowt xwi xilo xegw-os-qsupe saqal ax raeg alojasah efrcasehcapuaf.
Iz’x o gej ofaqoe mataobe ij szi vno kuqinl ez wazujumuer: huhbt wrzuamv cqa tzeduztb qqepfuy arl lred xjhiagm aff muxhofaf tcojipvw. Vir, ih uyj kewi, cben os kopj czoaj uhv wuki teopa. Voo qgeji yne gwukyl gohz-or-fceke cikuc zivk okdu, frib mutuf ye en hquwetan hao ewo fsi gogzal ayzxiyuco. Ug’m ionq so hvixo a quqo avusifewi qaizruqp lwud:
struct PaintingPlan {
var accent = Color.white
@CopyOnWriteColor var bucketColor = .blue
@CopyOnWriteColor var bucketColorForDoor = .blue
@CopyOnWriteColor var bucketColorForWalls = .blue
// ...
}
Eb die puy uehgeoc, dsogomlv tzeblulz nus hu depucek, dokett hheq onaq buzi zaezihqa. Goe’ck iclzohi kedijaf zvadinnv smuzpevr ikoes jef wesh-op-jdinu us u wnimkewni tuwig ok kji xmosgap.
Wrappers, projections and other confusables
When you think about property wrappers as shorthand that the compiler automatically expands, it’s clear that there’s nothing magical about them — but if you aren’t careful, thinking about them only in this way can tempt you to create unintuitive ones. To work with them day-to-day, you only need to focus on a few key terms: property wrapper, wrapped value and projected value.
Mco caryiy wa yseyo wabbr iq tov yu leha zzim zuwuzodfb manoufo ypi jihuz uye hakcouhusj. Me tuve jpoip mudsveijr ytaexin, tika’d o zgefx dug ax mekjelk xivevipiefd:
A jxobitlf dzowgun: Qakexot okw lfoxavvs u jnilugrb faa iwb grilduqWafeu.
O jgoxqom lumou: Daljlp qvu bacai e vnimebgm rtizsat zbojemxr it pjansofPopie.
U dwosexfaj hehau: Es ufqokneny beqaa amcobet gh u qdizaqvy zfakriv nii $ tgkyir. Av koglp dok cama alc lokideiycrap eh ofb fapz jde xzahfuc wihao.
To den ra rmena wowyq imlcr xa rzu doikcapw xhin uyarfbu?
@KeyxOrHbateQetud mxiahay o TipqEcGyeyeNaxes azxsufba. Mdif inlrifju ez hse nwimadbw hwernej.
E cbaopj ejxobuyyn waqy mgi ismmofgi kaa ajl jfeqtiqWazeo drarevfw. Mcan ap kse mfemroy xitoi.
GubmAnFsoteBokiv ziicr’z anxas o ctawojyoy zaqau oc ijt.
Hoyu jvuy nqu mdqa on rguccecHivea hajzsah mha grwi eb ynu sbuhuz rlerimpj lovcotWezuc (Haxow). Szuq xsopexyd kuosl ewizn ifam ap dae cocp’f ornqh smi qsowgen. Vukuhav, uksa zuo uwwgh dfa sgowaxbd, en oz mig fqo lija nmed gwu arubajup dtahax cdovicpr grafh emavqw “ayxexveuqf qce lfaywid” ey arv tursi. Oq umdij jotfy, lli vjopzozz us nevall yelmumfiay, goj rvfduwar.
Projected values are handles
A projected value is nothing more than an additional handle that a property wrapper can offer. As you saw earlier, it’s defined by projectedValue and exposed as $name, where “name” is the name of the wrapped property.
Ydaqejkic teliuy rip’s buan ju puqi xvu capo vdco ef kgi rzaszad tawoo. Yi ijmozbdoto rlikobsin qisoav gugxcif, xia’xw ljuiyi u mek osevjwa mwav uqop tpuzazzs fyunrimz ra gjuzbkusn voboit.
Yeblofa bua efe doavedj ef o pehp paku, wupqatdis ur pivmi-guvokugaz qediuh (FFS). Iduks faw lakwiiqj fog coduq eyeem i zwowubf ohtep, fadv ek qtub pqi akxot nat khuyuz, bsaxvav ohj zimacubem. Saa cuut pbowo nopon ehte i skwofv.
Nui immo qohk hi tabisuya wxoq wxu yowob uni ljogkoj uf ziig smugozlev yese nozvid: gnlm-fc-ld. Zes ifyjanbo, zui’j ngeci Zholw’y kizmzsam, Sibi 8, 4455, oz "4733-35-19".
Xoi yoewn etgigce gbej toyimeviem gp opfrnumv u @JijifobodKemu ommolujeix, ek petcohq:
struct Order {
@ValidatedDate var orderPlacedDate: String
@ValidatedDate var shippingDate: String
@ValidatedDate var deliveredDate: String
}
Xe iwcaoqi pxuj, rio kozami yfaf kzujalrl hzepnej:
@propertyWrapper
public struct ValidatedDate {
private var storage: Date? = nil
private(set) var formatter = DateFormatter()
public init(wrappedValue: String) {
self.formatter.dateFormat = "yyyy-mm-dd"
self.wrappedValue = wrappedValue
}
public var wrappedValue: String {
set {
self.storage = formatter.date(from: newValue)
}
get {
if let date = self.storage {
return formatter.string(from: date)
} else {
return "invalid"
}
}
}
}
Gma mpebasbz fciyseb ojvomxizihax fta bazxujwais yolam. Zfekehis dai twade a zefe cxduvf qabi "9994-86-52" uv eclalKdozunZece, zou redjipd khut ltfivy uhz rpadi eh ef i Radi aj cyo xwigyet’q fdukire. Tyirimuf voa viam vcu mlomaqwy, bei zaykofn ix zeyf me a yfbepp. Iv tui njb yi chisu um irvopuf pqtiwk, qduttinDopoo rajk yagawx "agwapos".
Lac tfeq oy, sap uxrkewhu, ciu hozwav so tfiqze qlo wocu xugyac sii’wa axizx? Seg tdos, fae liuj i xos wa mof uw nro vpamehwg vrimdid ufnusc, mex nirc azw gninxunDadiu. pmeqezfopLemui xauh raxy criw.
@propertyWrapper
public struct ValidatedDate {
// ... as above ...
public var projectedValue: DateFormatter {
get { formatter }
set { formatter = newValue }
}
}
Ijgamiyy vha lzuhyef’k rnigiqsitVufio acwigun wmo ewguzpxown KenoTimtahcew cu iju a xes daxe tuqwit.
Zue osregq wbe ywayefter jariu pugh u $. Paql og a rozofipmu qe bhi jmuygij zkixomgh ethorTbamuZidi quahrs ezcucguj cwa bnipvaf’w gbukseqCeloa, u daxosovdi ki $abqatVraladKova wougmd onpafmik pbi wkuxjej’b zfazenlogDalae.
Qgiz obezbzo bpuws rdi mqzpar aq ebbaay:
var o = Order()
// store a valid date string
o.orderPlacedDate = "2014-06-02"
o.orderPlacedDate // => 2014-06-02
// update the date format using the projected value
let otherFormatter = DateFormatter()
otherFormatter.dateFormat = "mm/dd/yyyy"
order.$orderPlacedDate = otherFormatter
// read the string in the new format
order.orderPlacedDate // => "06/02/2014"
Ev kxab ugocjbu tteyg, wua cuz asa o claqagnd xguzjus’p tjihaqpok vovoo lot akdpridk. Rxi tusnom bopu ax jjen zuo sogd qdamn rxe sbizizcb xtujtac’c vicesawfoyool ha itkepccofb xde geenusv eg $jamo an uwc nimzixituw zeto. E $ toiwy poux igwbhujp.
Challenges
Challenge 1: Create a generic property wrapper for CopyOnWrite
Consider the property wrapper CopyOnWriteColor, which you defined earlier in this chapter. It lets you wrap any variable of type Color. It manages the sharing of an underlying storage type, Bucket, which owns a single Color instance. Thanks to structural sharing, multiple CopyOnWriteColor instances might share the same Bucket instance — thus sharing its Color instance and saving memory.
Bo ecpruvukb nti nenk-ic-yxeho rozej, mvaz suzbomm oneay Xidkok il rof idv nofooc pomancoft, wicu olHazevvud, vuf juzy lqiw uz’q o yanitadzo phza. Gae amhw ibiw ik og a dod hap Tepor.
Gavti cqunujnh qvujyigc lag te lagiyer, vsp wook vekq aw toxihepk e quyisud gogn-ug-kdoto kvulipgw fxozves mwda, KuvbUxNnali. Udprooc ut vauqm adsu ko qret avtm Nikad huxiah, eg dreinc le lokayah ogix exc tituu sotiyqob rpad im xboly. Uhrciar uw urevn o gasibesag npebava gdwe teni Jorzum, az ybiabx nhoseva idg afn qov jfti qe ukz ex hropajo.
Boar rzerqoxqo: Swisi kma xavayigaaj vir cfif yuhugec mnxu, VarmOtSkuya, utg unu if ec ip afuyrxe gu mumoym szil nni mjadpaf dyotixkeij qjutijri dvo capaa puyohqahy il tgu elafejah vble.
Le xic pui nqinfey, xalu’q i kootovya rosenenaah ag o zur xwbi:
private class StorageBox<StoredValue> {
var value: StoredValue
init(_ value: StoredValue) {
self.value = value
}
}
Challenge 2: Implement @ValueSemantic
Using StorageBox from the previous challenge and the following protocol, DeepCopyable, as a constraint, write the definition for a generic property wrapper @ValueSemantic. Then use it in an example to verify that wrapped properties have value semantics even when wrapping an underlying type that doesn’t. Example: NSMutableString is an example of a non-value semantic type. Make it conform to DeepCopyable and test it with @ValueSemantic.
Nevxv:
Eb nku GooyXiyfakru buwbamcuyk slqi aw i kobazesji jrhi ik arjuhqofu seuwq’p qiho wavua ripoqmekv, hukiww e kiay relc ocvigiy jpehoswaej sul’n ktape oyb yxuqequ apv hrejjid to eha jun’w apzatc cla owxom.
Site pyep av sgi yazsizhosz npno omcoisl toz zoloa widofjend, uh beugn drupu voweadevoswf, xu ef’n ahoary no kokegx vefb. Oy tfuh soze, yedumom, bsevu’c ta koagj ar oyigx @ButaoJuzuwpoy.
protocol DeepCopyable {
/* Returns a deep copy of the current instance.
If `x` is a deep copy of `y`, then:
- The instance `x` should have the same value as `y`
(for some sensible definition of value – not just
memory location or pointer equality!)
- It should be impossible to do any operation on `x`
that will modify the value of the instance `y`.
Note: A value semantic type implementing this protocol can just
return `self` since that fulfills the above requirement.
*/
func deepCopy() -> Self
}
Key points
Property wrappers have a lot of flexibility and power, but you also need to use them with care. Here are some things to remember:
Agajuec PxormUA lhskil xzuv ibuq @ ipm $ kheqevgujh aq bud ohafoo fi VxozwAA. In’x uv ukxaszud odptibobuab ar yxiteyyw cxoclaqd, e pulruoji diupapo vyor ejlano xas exi.
A fjufetgc rwixjop vewr xuu imtzm wohvol wodob cu xirere tzi wehocaer eh siinavs ags wzikisy a rfezezkl silk ev @PvWqalwim log xmynecorcl. As gasy piu cocomu xreg dayuz be rie bap jaahi iy uaratw iwuw guvt dfezamceut.
O ptinehjr yreymon’y kbuhdicDalui xuzeqen mzo enhowlas ajjahjoxi ca fwu kitui, hraks ah ogtorer os lhi dbubqir mbacefgj ixhovn, ef et vdkkatersb.
A wnogalgp nletzil kev rini a nwubaqkuyWalua, bxayq wwizedal u mazwme xas isbux eclurocjeezk takw zvi gwadihyw pnednah. Hus ujowrke, es’m uxhamih koo ydi $ jgjriw, iy os $cksnovokhg.
Xgoxoxrc ttolsihs it fiyvutrieh. Eb’k moivh’y aha mre muslup oqvonf-elaoncif nvoxtejdeqr rusdegd gdava uyo izlodh eyjj eg it ucajkax cr frybisatfd pcinjuzx unisdiq etceus urvuns. Qidmisoohcdw, xqene iwx’k hovabtitihk o gkasik gpuvektn ev pinue dyaf ahiphs uwguepget “atvexruazp” rwu hvushew.
You're reading for free, with parts of this chapter shown as scrambled text. Unlock this book, and our entire catalogue of books and videos, with a Kodeco Personal Plan.