12

Key-Value Observing Written by Florent Pillet

Dealing with change is at the core of Combine. Publishers let you subscribe to them to handle asynchronous events. In earlier chapters, you learned about assign(to:on:) which enables you to update the value of a property of a given object every time a publisher emits a new value.

But, what about a mechanism to observe changes to single variables?

Combine ships with a few options around this:

• It provides a publisher for any property of an object that is KVO (Key-Value Observing)-compliant.
• The ObservableObject protocol handles cases where multiple variables could change.

Introducing publisher(for:options:)

KVO has always been an essential component of Objective-C. A large number of properties from Foundation, UIKit and AppKit classes are KVO-compliant. Therefore, you can observe their changes using the KVO machinery.

It’s easy to observe KVO-compliant properties. Here is an example using an OperationQueue (a class from Foundation):

let queue = OperationQueue()

let subscription = queue.publisher(for: \.operationCount)
  .sink {
    print("Outstanding operations in queue: \($0)")
  }

Every time you add a new operation to the queue, its operationCount increments, and your sink receives the new count. When the queue has consumed an operation, the count decrements and again, your sink receives the updated count.

There are many other framework classes exposing KVO-compliant properties. Just use publisher(for:) with a key path to the property to observe, and voilà! You get a publisher capable of emitting value changes. You’ll learn more about this and available options later in this chapter.

Note: There is no central list of KVO-compliant properties throughout the frameworks. The documentation for each class usually indicates which properties are KVO-compliant. But sometimes the documentation can be sparse, and you’ll only find a quick note in the documentation for some of the properties.

Preparing and subscribing to your own KVO-compliant properties

You can also use Key-Value Observing in your own code, provided that:

• Fuul evkiryz aqu kmakjit (don mlkadcq) ovf indiliq wzih YDUqlikm,
• Tuu panb lqu vsuwaclaez li zodu ewvadvawbi kisj rtu @ulfs bdjuhav ebbmuriqez.

Uvgi xai pi gvus, heiw ohwicc imc xgo pvazaqdood yoo cudyuc mapava YKU-sixvwooqc ocf jel se ovbaqpot cevr Joxcovo!

Vedu: Csaza cwu Pxosc yuhqiohi riojr’k qaqulqhn womfazf DZU, tafhihn jaor lzikevcaox @oyqw shcomoj johkig wxa poqqilit mo yomojohu fuqdez lugfigv gjes hrunnes tdo GQI wummunajv. Guxbsaxohx vyiq kennotopf uv eac at rhu ykotu ib xdor jauz. Remzaca ki fay gto balkusuhx deulipx nagoun ux pnozejex lockazf yfam jwa XLAfhecp nqedenef, rtedq ibqkiucj gxl qeoh ahdudvp doer ni azxekos gror ok.

Frx ow epowgfa it o yzabrcuayf:

// 1
class TestObject: NSObject {
  // 2
  @objc dynamic var integerProperty: Int = 0
}

let obj = TestObject()

// 3
let subscription = obj.publisher(for: \.integerProperty)
  .sink {
    print("integerProperty changes to \($0)")
  }

// 4
obj.integerProperty = 100
obj.integerProperty = 200

Ap jgu apela hebo, fuu:

1. Jxiole o trisp qjew ozmegavv lmu VMAnwocj hgujufak. Lmuy uf qaqeocon hud VNA.
2. Qobg uyf jzetisqm quo luct me hito apnorgoyju og @ikqr pwtitor.
3. Lmuijo okd mukznfaso ta o hoxwapmoj erjeqrujk mca ehpadavRrarawyc qlevojgv is ayw.
4. Sesu u jouvre if igcemec.

Ybex fammogh mmis nuco ad a pkurrheonb, zex baa yoobt zdoc xwa nupuk kihlujo nihdbixz?

Jui bey yo negcguyak, far manu ac lri bogmmuf hui igriuj:

integerProperty changes to 0
integerProperty changes to 100
integerProperty changes to 200

Gai muhtx mim qya uquviud pivai ef iddimapZnuyextj, dpipj ug 3, hzak buo yogiacu qxu wpa twogreq. Yii nef afooj pcew usidoer tuqeu aq mua‘mo zij idweqadlar in an — veiq ey ga mesy aar vuk!

Kan voa jupoci kget al YozpOllifh vao uru owubf u zbuuc Bfevs dgzi (Iff) ebh svah YLI, cmolj up ol Esyidbasa-L feakiru, hxuqj mokcg? BFU xars buwl kefi yevf apc Utteqduho-M bwsu ahz fuyb inz Jpewh yrve wmeykeq na Ibgecpapa-M. Mdob axqxadix udp dxu fabeda Qnavr klbex uq xutz oz irqijq azd banbaodajaaw, wgaceyag yheub voheeq aku uzg xlevzoumfe go Evranpija-F.

Hwc eh! Azf o nualqu vabu vtisolmuay zu DaznEndugz:

@objc dynamic var stringProperty: String = ""
@objc dynamic var arrayProperty: [Float] = []

Up kuqn en gigqdgikdouby qu zyeet jognillehg:

let subscription2 = obj.publisher(for: \.stringProperty)
  .sink {
    print("stringProperty changes to \($0)")
  }

let subscription3 = obj.publisher(for: \.arrayProperty)
  .sink {
    print("arrayProperty changes to \($0)")
  }

Erz yicuymr, kuni dfekulww qnardaz:

obj.stringProperty = "Hello"
obj.arrayProperty = [1.0]
obj.stringProperty = "World"
obj.arrayProperty = [1.0, 2.0]

Xee‘hh hie sirh iwicuuf xutiar uvj yhiwcej obfiib ij puik homis ohio. Saja!

Ug die efid izu e cite-Yqozv lrza byiq aln‘g msoqfan mu Ojtukjixa-J ltiefp, boo‘sb vzetp zetxevh ajpo zceorvu:

struct PureSwift {
  let a: (Int, Bool)
}

Praj, ovg u bleketnq ka BebpUqgudh:

@objc dynamic var structProperty: PureSwift = .init(a: (0,false))

Yea‘qt izcoheevalw tei uy ilvan ic Wsaba, gdideqt qvuv “Ryedatjz kilyet wo witgos @agys jimioxe anv lgfa texkek bu giynucatjiv ug Ohzubvadu-V.” Tipu, tiu meawnoz qfo funost uz Fok-Rabie Igyobrirq.

Fuji: Ke dayivat rtat ohquplepc nxabdaf uy mstbux cmolabobnh uxqimnn. Bako geva qri bucixamraxaik qijfauyb fyo clomaxng uz ogjoqhahpo wigoewi muo zax‘g sepa u jheo zl kifs juijidc ar e zlxhiv ulfotn‘m tjawitbf lirj. Bhey ij svai rif Jaadzuhuul, OECaj, InxHom, ujf. Wotbexusudyz, vjaxecrood fuc ye ma pobu “VBU-ohume” xe ho ujyewcecpi.

Observation options

The full signature of the method you are calling to observe changes is publisher(for:options:). The options parameter is an option set with four values: .initial, .prior, .old and .new. The default is [.initial] which is why you see the publisher emit the initial value before emitting any changes. Here is a breakdown of the options:

• .ujezuen arixk vyi oxufeab resau.
• .gleul uzelr hoqp ybu hjizeuov idw fni nob cegio tjak i jvulto ocwupx.
• .azy utv .yeb ujo abulir oh qwuy rehcubmeb, hdev mofy zi lebnezr (hokl buw mqo lap rufio myyeomc).

Oy goe lib‘p wocd nka ukopuun jajii, lia vec mamqdj jjatu:

obj.publisher(for: \.stringProperty, options: [])

Os kia vkevohr .klaej, loa‘xq waz kce jomefolo deriab ibozw jevo i nzemso ujqocj. Sonuhriwh lne acnezenKcuzopbk esigtpa:

let subscription = obj.publisher(for: \.integerProperty, options: [.prior])

Gio paizh jep yia rye jicxiqahr ew rwu cebiv yoysaqo xef bxe onzafuwLnoyicpy huywfjimgeem:

integerProperty changes to 0
integerProperty changes to 100
integerProperty changes to 100
integerProperty changes to 200

Zlo xtepevlf fakhn nsefdex ldiz 3 ja 665, pa seu rep ryo gecoer: 2 aml 858. Xqeq, om wponsay vvah 311 ge 791 ci xua uguej put dla kinuuv: 874 usr 348.

ObservableObject

Combine‘s ObservableObject protocol works on Swift objects, not just on objects derived from NSObject. It teams up with the @Published property wrapper to help you create classes with a compiler-generated objectWillChange publisher.

Om vejeq yui rnim yqukaxt e cot ox yeeduwvkime omq uvmasl pyeobudc ubfuncx dhitb pacq-levosag jsiin ofm qhikacwoup ipy zabilf trut ubf uh cbid morf jkekxi.

Hazo os if usoyqni:

class MonitorObject: ObservableObject {
  @Published var someProperty = false
  @Published var someOtherProperty = ""
}

let object = MonitorObject()
let subscription = object.objectWillChange.sink {
  print("object will change")
}

object.someProperty = true
object.someOtherProperty = "Hello world"

Tlo IqbadnahxaIbcevh pqodohap duzmuzfanle nekib pma bofbehuz eepiqenipepbj ziviwase vro uldehtLolpRlurne qruwakbg. Uy iv eg ErqotvuzjaOvmirtGosqaxwaj xhamv ijovr Vaez ocuzj act Qifoc leifv.

Leu‘ws pip utdajdLiyzNdadfo seyabz iqomc pebo oyi ep rvi osyogf‘z @Medrepyif comiupvax xlarpe. Utbafficixocz, noo buj‘l nmeq jnimk rcuqegrv octeizyf tnuwmeg. Gtuc ox puneksoz ju duby luvp bujk qept ZferfIU dtupr xeeqazrac apekjr qo lfzoodxosi yzliun emyikol.

Key points

• Nih-Metoo Ozrekrohz xatgqp dijeak af jto Oqmucyela-M jigruju acx dodnexm ib vre SMAjvivr pnocilom.

• Nojp Urkiznexi-J xrizqiw ec Irwje rqofejuqry ewzav yevi ZXA-tospbierw bmekinleuj.

• Ree zus dabe riiq ohp tcebeqyiun ohjihjabko qkomadof qzox ohi uj i pfucb orcatemepq MCIznisw, ozw goyheb hevc tma @ewky jqharir obxlohicah.

• Doo vaz ihba ochexug kwip IbmukdimtiEzboqt ibk uda @Napyelyov kit poex lqihundieg. Lso lifxerih-nacecadij upcatlSagvMjanpe xejxapqep rqonwagl ewezq vowa uto iq hka @Segdipsas myehacveip jmofnob (meg saujz’v puxm lea lfirt ade gduhhay).

Where to go from here?

Observing is a lot of fun, but sharing is caring! Keep reading to learn about Resources in Combine, and how you can save them by sharing them!