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Troubled Waters: Bridging platform-native SDKs with Kotlin Multiplatform Kevin Galligan

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Partner at Kotlin GDE

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Touchlab

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Common JVM JS Native

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Common JVM JS Native Firebase/JVM Firebase/JS Firebase/iOS

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Common JVM JS Native Firebase/JVM Firebase/JS Firebase/iOS

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Common Android iOS Firebase/JVM Firebase/iOS

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JNI

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Basic Topics • Tell Kotlin about the native code • Link native code into Kotlin Xcode Framework • API Design

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Simple First pass it in

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Interface Driven • 💰 Don’t need cinterop • 💰 No weird linking • 💰 No extra binary size • 🗑 Not easily testable • 🗑 Impractical for complex situations • 🗑 Bad for libraries

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interface KotlinAnalytics { fun logEvent(name: String, parameters: Map) }

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interface KotlinAnalytics { fun logEvent(name: String, parameters: Map) } fun initKoinIos( userDefaults: NSUserDefaults, appInfo: AppInfo, analytics: KotlinAnalytics, doOnStartup: () -> Unit ) ( // Etc )

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class IosAnalytics: KotlinAnalytics { func logEvent(name: String, parameters: [String : Any]) { Analytics.logEvent(name, parameters: parameters) } }

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class IosAnalytics: KotlinAnalytics { func logEvent(name: String, parameters: [String : Any]) { Analytics.logEvent(name, parameters: parameters) } } KoinIOSKt.doInitKoinIos( userDefaults: userDefaults, appInfo: iosAppInfo, analytics: IosAnalytics(), doOnStartup: doOnStartup )

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fun withCallback(callback: (Breed) -> Unit) { }

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viewModel.withCallback(callback: {[weak self] (breed:Breed) -> Void in self?.callMe(breed: breed) })

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cinterop kotlin calling c/objc

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cinterop kotlin calling c/objc

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Call C/Obj-C some swift, some C++

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*.h cinterop Kotlin/iOS

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*.h cinterop Kotlin/iOS Binary ? ? ? ? ? ? ? ? ?? ? ?

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package = co.touchlab.whatever language = Objective-C headers = whatever.h whatever.def

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other config

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package = co.touchlab.whatever language = Objective-C headers = whatever.h whatever.def

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extraOpts = listOf("-mode", "sourcecode")

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*.h cinterop Kotlin/iOS Binary ? ? ? ? ? ? ? ? ?? ? ?

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Where are these *.h files? multiple options

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In Your Repo copy/paste

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Script Downloads grab headers

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Truncated Headers feels like cheating a bit :)

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Cocoapods Dependency the hammer approach

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kotlin { //etc cocoapods { summary = "Common library for the KaMP starter kit" homepage = "https://github.com/touchlab/KaMPKit" ios.deploymentTarget = "12.4" podfile = project.file("../ios/Podfile") pod("FirebaseAnalytics") } }

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Docs on Cocoapods/Kotlin https://kotlinlang.org/docs/native-cocoapods-libraries.html

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cocoapods.FirebaseAnalytics .FIRAnalytics.logEventWithName("someevent", mapOf(/*data*/))

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Makes project weird can’t run tests, dependency in podspec

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Carthage ❤ Kotlin https://github.com/wireapp/carthage-gradle-plugin

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SPM? not yet

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Linking where is the binary?

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*.h cinterop Kotlin/iOS Binary ? ? ? ? ? ? ? ? ?? ? ?

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complex-ish

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Linking Basics Binary Framework

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Static Linking Binary Static Framework

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Dynamic Linking Binary Dynamic Framework

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Assemble Static Static Framework *.h cinterop Kotlin/iOS Binary ? ? ? ? ? ? ? ??? ? ?

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Assemble Dynamic *.h cinterop Kotlin/iOS Binary ? ? ? ? ? ? ? ??? ? ? Dynamic Framework

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Libraries == More Complex recency bias

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Kermit Crashlytics Firebase.h Firebase

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Kermit Crashlytics Firebase.h Firebase Dynamic

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Undefined symbols for architecture x86_64: "_OBJC_CLASS_$_FIRStackFrame", referenced from: objc-class-ref in libco.touchlab:kermit-crashlytics-cache.a(result.o) "_OBJC_CLASS_$_FIRExceptionModel", referenced from: objc-class-ref in libco.touchlab:kermit-crashlytics-cache.a(result.o) "_OBJC_CLASS_$_FIRCrashlytics", referenced from: objc-class-ref in libco.touchlab:kermit-crashlytics-cache.a(result.o) ld: symbol(s) not found for architecture x86_64

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Undefined symbols for architecture x86_64: "_OBJC_CLASS_$_FIRStackFrame", referenced from: objc-class-ref in libco.touchlab:kermit-crashlytics-cache.a(result.o) "_OBJC_CLASS_$_FIRExceptionModel", referenced from: objc-class-ref in libco.touchlab:kermit-crashlytics-cache.a(result.o) "_OBJC_CLASS_$_FIRCrashlytics", referenced from: objc-class-ref in libco.touchlab:kermit-crashlytics-cache.a(result.o) ld: symbol(s) not found for architecture x86_64

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stay one lesson ahead…

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Kotlin/iOS Cocoapods *.h Binary

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Kotlin/iOS Some Code *.h *.c/*.m

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Kotlin/iOS *.h *.c/*.m cinterop clang

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Kotlin/iOS *.h *.c/*.m cinterop

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Kotlin/iOS *.h *.c/*.m cinterop cklib

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cklib Links • https://github.com/touchlab/cklib • https://github.com/cashapp/zipline/blob/trunk/ zipline/build.gradle.kts#L157 • https://github.com/touchlab/Kermit/blob/main/ kermit-crashlytics-test/build.gradle.kts#L69

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Linking Links • https://bpoplauschi.github.io/2021/10/24/Intro-to-static-and-dynamic- libraries-frameworks.html • https://bpoplauschi.github.io/2021/10/25/Advanced-static-vs-dynamic- libraries-and-frameworks.html

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Testing Binary Framework

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Kotlin/iOS test.kexe

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Kotlin/iOS test.kexe Cocoapods *.h Binary Some Code *.h *.c/*.m

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Kotlin/iOS test.kexe *.h Binary Firebase.h Firebase

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Kotlin/iOS test.kexe *.h Binary Firebase.h Firebase Stub Binary

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Kotlin/iOS test.kexe *.h Binary Firebase.h Firebase Stub Binary

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stay one lesson ahead…

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API Design thoughts on the common language

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Uncanny Valley they do the same thing, but…

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Common Android iOS Firebase/JVM Firebase/iOS

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Common Android iOS Firebase/JVM Firebase/iOS

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Common Android iOS C code

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Common Android iOS C code JNI cinterop bridge

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Common Android iOS C code JNI cinterop bridge

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Rules are difficult “it depends” is everything

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Don’t default expect/actual can be good, can be frustrating

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expect/actual is less flexible harder to test, etc

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expect class KotlinAnalytics() { fun logEvent(name: String, parameters: Map) }

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interface KotlinAnalytics { fun logEvent(name: String, parameters: Map) } expect fun platformAnalyticsFactory(): KotlinAnalytics

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expect fun platformLogWriter(): LogWriter

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expect class CrashlyticsLogWriter( minSeverity: Severity = Severity.Info, minCrashSeverity: Severity = Severity.Warn, printTag: Boolean = true ) : LogWriter

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expect fun crashlyticsLogWriter( minSeverity: Severity = Severity.Info, minCrashSeverity: Severity = Severity.Warn, printTag: Boolean = true ) : LogWriter

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Service Objects/Small Graph interface and delegates work well

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Parent<->Child/Larger Graph interface-with-delegates starts looking ugly

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Order Big Graph/Data Objects getProducts() Product Product Product Supplier

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Order Big Graph/Data Objects getProducts() Product Product Product Supplier

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actual typealias with great power…

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/** * Multiplatform AtomicInt implementation */ expect class AtomicInt(initialValue: Int) { fun get(): Int fun set(newValue: Int) fun incrementAndGet(): Int fun decrementAndGet(): Int fun addAndGet(delta: Int): Int fun compareAndSet(expected: Int, new: Int): Boolean }

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JVM Side?

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import java.util.concurrent.atomic.AtomicInteger actual typealias AtomicInt = AtomicInteger

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Actual needs to match because obviously it does

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import kotlin.native.concurrent.AtomicInt actual class AtomicInt actual constructor(initialValue:Int){ private val atom = AtomicInt(initialValue) actual fun get(): Int = atom.value actual fun set(newValue: Int) { atom.value = newValue } actual fun incrementAndGet(): Int = atom.addAndGet(1) actual fun decrementAndGet(): Int = atom.addAndGet(-1) actual fun addAndGet(delta: Int): Int = atom.addAndGet(delta) actual fun compareAndSet(expected: Int, new: Int): Boolean = atom.compareAndSet(expected, new) }

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import kotlin.native.concurrent.AtomicInt actual class AtomicInt actual constructor(initialValue:Int){ private val atom = AtomicInt(initialValue) actual fun get(): Int = atom.value actual fun set(newValue: Int) { atom.value = newValue } actual fun incrementAndGet(): Int = atom.addAndGet(1) actual fun decrementAndGet(): Int = atom.addAndGet(-1) actual fun addAndGet(delta: Int): Int = atom.addAndGet(delta) actual fun compareAndSet(expected: Int, new: Int): Boolean = atom.compareAndSet(expected, new) }

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Platform Affinity android iOS ?

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Platform Affinity android iOS android-y

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typealias can be brittle

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typealias can be brittle

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Platform Affinity android iOS android-y JS

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Parallel Delegates android iOS android-y

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Empty Typealias everything is extensions

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expect class QuerySnapshot expect val QuerySnapshot.documentChanges_:List expect fun QuerySnapshot.getDocumentChanges_(…):List expect val QuerySnapshot.documents_:List expect val QuerySnapshot.metadata: SnapshotMetadata expect val QuerySnapshot.query: Query expect val QuerySnapshot.empty: Boolean expect val QuerySnapshot.size: Int

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expect class QuerySnapshot expect val QuerySnapshot.documentChanges_:List expect fun QuerySnapshot.getDocumentChanges_(…):List expect val QuerySnapshot.documents_:List expect val QuerySnapshot.metadata: SnapshotMetadata expect val QuerySnapshot.query: Query expect val QuerySnapshot.empty: Boolean expect val QuerySnapshot.size: Int

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actual typealias QuerySnapshot = FIRQuerySnapshot actual val QuerySnapshot.documentChanges_: List get() = documentChanges as List actual val QuerySnapshot.documents_: List get() = documents as List actual fun QuerySnapshot.getDocumentChanges_(metadataChanges: Metadata documentChangesWithIncludeMetadataChanges(metadataChanges == Metad iOS

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actual typealias QuerySnapshot = FIRQuerySnapshot actual val QuerySnapshot.documentChanges_: List get() = documentChanges as List actual val QuerySnapshot.documents_: List get() = documents as List actual fun QuerySnapshot.getDocumentChanges_(metadataChanges: Metadata documentChangesWithIncludeMetadataChanges(metadataChanges == Metad iOS

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actual typealias QuerySnapshot = FIRQuerySnapshot actual val QuerySnapshot.documentChanges_: List get() = documentChanges as List actual val QuerySnapshot.documents_: List get() = documents as List actual fun QuerySnapshot.getDocumentChanges_(metadataChanges: Metadata documentChangesWithIncludeMetadataChanges(metadataChanges == Metad iOS

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expect open class DocumentSnapshot DocumentSnapshot actual typealias DocumentSnapshot = FIRDocumentSnapshot iOS common actual typealias DocumentSnapshot = com.google.firebase.firestore.DocumentSnapshot Android

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actual typealias QuerySnapshot = com.google.firebase.firestore.QuerySn actual val QuerySnapshot.documentChanges_: List get() = documentChanges actual val QuerySnapshot.documents_: List get() = documents actual fun QuerySnapshot.getDocumentChanges_(metadataChanges: Metadata getDocumentChanges(metadataChanges.toJvm()) Android

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Platform Affinity

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Platform Agnostic

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Order Big Graph/Data Objects getProducts() Product Product Product Supplier

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Order Big Graph/Data Objects getProducts() Product Product Product Supplier

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Again, that’s rare mostly just use interfaces and delegates!

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Empty Typealias • https://vimeo.com/371460823 • https://github.com/touchlab-lab/FirestoreKMP

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API *almost* designs itself you’re mapping existing APIs

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Iterative Design tweak for platform specifics

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Platform-Specific Init then common

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Whatever/ Common Whatever/ Android Whatever/ iOS Whatever AAR Whatever.h Init Init

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Whatever/ Common Whatever/ Android Whatever/ iOS Whatever AAR Whatever.h Init Init Other Calls

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Simple First pass it in

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That was a lot! it’s not all that bad

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At Touchlab • Big team build tools • Better iOS Dev ex • Swift code generation • Other things

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If you’re facing these issues… @kpgalligan Kotlin Slack

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Thanks! @kpgalligan

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Thanks! @kpgalligan Join the team !