KotlinConf 2026 - What Nobody Told Us About Shipping Kotlin to iOS

https://kotlinconf.com/schedule/?day=2026-05-22&session=b5506fd4-5c97-59cd-84ba-fe84d25fbc9a

Transcript

1. What Nobody Told Us About Shipping Kotlin to iOS Suhyeon Leah Kim
2. About Me • Android Dev & Technical Advocate at Woowa Bros Delivery Hero) • 7 years of Experience with Android and Kotlin Multiplatform • Developer Community Organizer GDG Korea Android) LinkedIn Gmail linkedin.com/in/wisemuji wisemuji@gmail.com
3. Before We Start - An Android developer's perspective (iOS beginner) - This talk is for those already using, or about to use KMP on iOS. - Weʼll skip the docs and focus on what broke and how we fixed it. - Better ideas? Let's discuss!
4. The Starting Point Building an in-house trainee management app Team 󰱹󰞦󰟲 Dev A Android) Dev B Android) Me Android) Ship in 2 months No iOS developer on team Backend also Kotlin stack
5. 1 Year in Production Internal app, actively maintained on both platforms iOS Stable since Compose Multiplatform 1.8.0 All-Kotlin project (server + client)
6. What We'll Cover Native State Observation Bridging platform events to Compose UI Integrating Non-KMP SDKs IoC for Firebase + Kotlin/Native issue on iOS Dependency Management SwiftPM Export: Fix for Multi-Module iOS Builds + Interop pitfalls cheat sheet Insights from JetBrains at KotlinConf 2025
7. Case Study 1 Native State Observation Bridging platform-specific changes to shared Kotlin logic
8. A push notification arrives. Your shared Compose UI needs to react. How would you do that in CMP100% Compose UI?
9. When expect/actual Falls Short - Native UI Events Push notifications trigger shared logic, but originate in native code. - Async Platform Logic Returns asynchronously, commonMain can't handle delayed callbacks. e.g. Firebase Remote Config - Native System APIs Platform-specific lifecycle, no KMP equivalent.
10. Real iOS APIs that hit this wall iOS API Why it can't go in commonMain UNUserNotificationCenter Delegate callbacks, not flows CLLocationManager Background-thread callbacks RemoteConfig Firebase) Async with no Kotlin SDK AppDelegate lifecycle No KMP equivalent CBCentralManager Bluetooth) Stateful native session AVAudioSession Singleton, app-wide config
11. Case Study 1 Native State Observation Requirements: • Develop a forced update feature. • Users cannot launch the app if their version is lower than the recommended version.
12. Case Study 1 Native State Observation
13. Case Study 1 Native State Observation Compose UI 100%
14. Case Study 1 Native State Observation Can events from the iOS module be received in the Kotlin module?
15. Kotlin composeApp/iosMain/MainViewController.kt fun MainViewController() = ComposeUIViewController { App() } Swift iosApp/ContentView.swift struct ComposeView: UIViewControllerRepresentable { } func makeUIViewController(context: Context) -> UIViewController { MainViewControllerKt.MainViewController() ... Default KMP Project)
16. Kotlin composeApp/iosMain/MainViewController.kt object SharedViewControllers { private data class ViewState(val requiredAppVersionCode: Long = 0) private val state = MutableStateFlow(ViewState()) fun mainViewController(onFinish: () -> Unit): UIViewController { return ComposeUIViewController { // This subscribes to the StateFlow state. val viewState by state.collectAsStateWithLifecycle() if (viewState.requiredAppVersionCode > currentAppVersionCode) { ... } App() } } } // This is called from SwiftUI. fun updateRequiredAppVersionCode(code: Long) { state.value = state.value.copy(requiredAppVersionCode = code) }
17. Kotlin composeApp/iosMain/MainViewController.kt object SharedViewControllers { private data class ViewState(val requiredAppVersionCode: Long = 0) private val state = MutableStateFlow(ViewState()) fun mainViewController(onFinish: () -> Unit): UIViewController { return ComposeUIViewController { // This subscribes to the StateFlow state. val viewState by state.collectAsStateWithLifecycle() if (viewState.requiredAppVersionCode > currentAppVersionCode) { ... } App() } } } // This is called from SwiftUI. fun updateRequiredAppVersionCode(code: Long) { state.value = state.value.copy(requiredAppVersionCode = code) }
18. Kotlin composeApp/iosMain/MainViewController.kt object SharedViewControllers { private data class ViewState(val requiredAppVersionCode: Long = 0) private val state = MutableStateFlow(ViewState()) fun mainViewController(onFinish: () -> Unit): UIViewController { return ComposeUIViewController { // This subscribes to the StateFlow state. val viewState by state.collectAsStateWithLifecycle() if (viewState.requiredAppVersionCode > currentAppVersionCode) { ... } App() } } } // This is called from SwiftUI. fun updateRequiredAppVersionCode(code: Long) { state.value = state.value.copy(requiredAppVersionCode = code) }
19. Swift iosApp/ContentView.swift import SwiftUI import ComposeApp struct ComposeView: UIViewControllerRepresentable { func makeUIViewController(context: Context) -> UIViewController { return SharedViewControllers.shared .mainViewController(onFinish: { ... }) } func updateRequiredAppVersionCode(code: Int64) { SharedViewControllers() .updateRequiredAppVersionCode(code: code) } func fetchRemoteConfig() { let remoteConfig = RemoteConfig.remoteConfig() } } remoteConfig.fetchAndActivate { status, error in // ... }
20. Swift iosApp/ContentView.swift import SwiftUI import ComposeApp struct ComposeView: UIViewControllerRepresentable { func makeUIViewController(context: Context) -> UIViewController { return SharedViewControllers.shared .mainViewController(onFinish: { ... }) } func updateRequiredAppVersionCode(code: Int64) { SharedViewControllers() .updateRequiredAppVersionCode(code: code) } func fetchRemoteConfig() { let remoteConfig = RemoteConfig.remoteConfig() } } remoteConfig.fetchAndActivate { status, error in // ... }
21. Case Study 1 Native State Observation 1. Remote Config triggers 3. Compose UI recomposition 2. Collect
22. Case Study 2 Integrating Non-KMP SDK Firebase) Designing IoC architecture and discovering a Kotlin/Native issue on iOS
23. The original Logger structure logger
24. Kotlin logger/commonMain/Log.kt expect object Log { fun e(tag: String, message: String, throwable: Throwable? = null) fun d(tag: String, message: String) fun i(tag: String, message: String) } Kotlin logger/androidMain/Log.android.kt actual object Log { actual fun e(tag: String, message: String, throwable: Throwable?) { Log.e(tag, message, throwable) } // ... } Kotlin logger/iosMain/Log.ios.kt actual object Log { actual fun e(tag: String, message: String, throwable: Throwable?) { NSLog("ERROR: [$tag] $message. Throwable: $throwable") } // ... }
25. But... Firebase Has No KMP SDK ✅ Android ❌ iOS Firebase Crashlytics SDK is a regular Gradle dependency. Direct import in androidMain. Firebase iOS SDK is Swift/ObjC. Kotlin/Native cannot import it in iosMain.
26. 1st Approach: Delegate Pattern IoC logger
27. Kotlin logger/iosMain/FirebaseCrashlyticsDelegate.kt interface FirebaseCrashlyticsDelegate { fun recordException(error: NSError) } Kotlin logger/iosMain/Log.ios.kt actual object Log { private var crashlyticsDelegate: FirebaseCrashlyticsDelegate? = null } fun initCrashlyticsDelegate(delegate: FirebaseCrashlyticsDelegate) { crashlyticsDelegate = delegate } // ...
28. Kotlin logger/iosMain/FirebaseCrashlyticsDelegate.kt interface FirebaseCrashlyticsDelegate { fun recordException(error: NSError) } Kotlin logger/iosMain/Log.ios.kt actual object Log { private var crashlyticsDelegate: FirebaseCrashlyticsDelegate? = null } fun initCrashlyticsDelegate(delegate: FirebaseCrashlyticsDelegate) { crashlyticsDelegate = delegate } // ...
29. Swift iosApp/DefaultFirebaseCrashlyticsDelegate.swift class DefaultFirebaseCrashlyticsDelegate: FirebaseCrashlyticsDelegate { func recordException(error: any Error) { Crashlytics.crashlytics().record(error: error) } } Swift iosApp/iOSApp.swift class AppDelegate: NSObject, UIApplicationDelegate { func application(_ application: UIApplication,...) -> Bool { FirebaseApp.configure() Log.shared.doInitCrashlyticsDelegate(delegate: Default…Delegate()) } // ... }
30. Swift iosApp/DefaultFirebaseCrashlyticsDelegate.swift class DefaultFirebaseCrashlyticsDelegate: FirebaseCrashlyticsDelegate { func recordException(error: any Error) { Crashlytics.crashlytics().record(error: error) } } Swift iosApp/iOSApp.swift class AppDelegate: NSObject, UIApplicationDelegate { func application(_ application: UIApplication,...) -> Bool { FirebaseApp.configure() Log.shared.doInitCrashlyticsDelegate(delegate: Default…Delegate()) } // ... }
31. Swift iosApp/DefaultFirebaseCrashlyticsDelegate.swift class DefaultFirebaseCrashlyticsDelegate: FirebaseCrashlyticsDelegate { func recordException(error: any Error) { Crashlytics.crashlytics().record(error: error) } } Swift iosApp/iOSApp.swift class AppDelegate: NSObject, UIApplicationDelegate { func application(_ application: UIApplication,...) -> Bool { FirebaseApp.configure() Log.shared.doInitCrashlyticsDelegate(delegate: Default…Delegate()) } // ... }
32. Swift iosApp/DefaultFirebaseCrashlyticsDelegate.swift class DefaultFirebaseCrashlyticsDelegate: FirebaseCrashlyticsDelegate { func recordException(error: any Error) { Crashlytics.crashlytics().record(error: error) } } Swift iosApp/iOSApp.swift class AppDelegate: NSObject, UIApplicationDelegate { func application(_ application: UIApplication,...) -> Bool { FirebaseApp.configure() Log.shared.doInitCrashlyticsDelegate(delegate: Default…Delegate()) } // ... }
33. But it didnʼt work as expected... Expected Actual ViewModel calls Log delegate → crash recorded Log delegate is null → crash reporting silently fails Delegate was injected & the function was called, But at the call site delegate was always null 🤔
34. The Misdiagnoses Threading issue? Added DispatchQueue.main.async Still null
35. The Misdiagnoses Threading issue? Added DispatchQueue.main.async Still null Initialization order? Added delays, checked lifecycle Still null
36. The Misdiagnoses Threading issue? Added DispatchQueue.main.async Initialization order? Added delays, checked lifecycle Still null ��💫 Weak reference ARC? Made it a strong property We were debugging the wrong dimension entirely. Still null Still null
37. Proof in the Symbol Table Terminal
38. What really was the problem: Two different Log instances in the same iOS process. The delegate was set on one, read from another. Kotlin's object is a singleton per compiled binary, not per iOS process.
39. Quick Fix: Merge Into Single Module 🤔 Pros: Simple, works immediately Cons: Lost modularity, slower builds, Need to change the whole architecture
40. Proper Fix: Framework export Kotlin DSL composeApp/build.gradle.kts listOf( iosX64(), iosArm64(), iosSimulatorArm64() ).forEach { iosTarget -> iosTarget.binaries.framework { baseName = "ComposeApp" isStatic = true transitiveExport = true export(projects.logger) } } ✅ Modules stay separate. Keep your :logger, :core, ... ✅ One framework, No split singletons
41. A New Default Project Structure for KMP
42. Alternatives - Speak up directly to the Firebase team to request KMP support: https://firebase.uservoice.com/forums/948424-general/suggestions/46591717-support -kotlin-multiplatform-kmp-in-the-sdks - Use the unofficial Firebase Kotlin SDK https://github.com/GitLiveApp/firebase-kotlin-sdk - Choose another logging SDK that supports KMP (e.g., Sentry): https://docs.sentry.io/platforms/kotlin/guides/kotlin-multiplatform
43. Case Study 3 Dependency Management SwiftPM Export: Fix for Multi-Module iOS Builds
44. Recap - Firebase in androidMain Kotlin logger/androidMain/Log.android.kt import com.google.firebase.crashlytics.ktx.crashlytics import com.google.firebase.ktx.Firebase actual object Log { actual fun e(tag: String, message: String, throwable: Throwable?) { // ... Firebase.crashlytics.recordException(throwable ?: Exception(message)) } }
45. Firebase in iosMain with CocoaPods Kotlin logger/iosMain/Log.ios.kt import cocoapods.FirebaseCrashlytics.FIRCrashlytics import platform.Foundation.NSError import platform.Foundation.NSLocalizedDescriptionKey actual fun recordException(t: Throwable) { // ... FIRCrashlytics.crashlytics().recordError(nsError) }
46. Whatʼs New in Kotlin 2.4.0RC  SwiftPM import
47. Kotlin 2.4.0RC · as of May 2026 Experimental swiftPMDependencies
48. Kotlin 2.4.0RC · as of May 2026 SwiftPM import - Gradle Config Kotlin DSL composeApp/build.gradle.kts kotlin { swiftPMDependencies { iosMinimumDeploymentTarget.set("16.0") } } swiftPackage( url = "https://github.com/firebase/firebase-ios-sdk.git", version = "12.5.0", products = listOf( "FirebaseCrashlytics", "FirebaseRemoteConfig" ), importedClangModules = listOf( "FirebaseCrashlytics", "FirebaseRemoteConfig" ), )
49. Kotlin 2.4.0RC · as of May 2026 SwiftPM import - use Firebase from Kotlin/Native Kotlin logger/iosMain/Log.ios.kt import swiftPMImport.logger.FIRCrashlytics actual fun recordException(t: Throwable) { FIRCrashlytics.crashlytics() .recordError(t.toNSError()) }
50. Kotlin 2.4.0RC · as of May 2026 But it's still Experimental ⚠ Experimental status JetBrains is collecting feedback in KotlinLang #kmp-swift-package-manager Slack channel. Not ready for critical production paths yet. ⚙ isStatic = true recommended Dynamic Kotlin/Native frameworks may hit linker errors or runtime symbol collisions with SwiftPM imports. 🚧 Round-trip not supported yet If your KMP module uses SwiftPM import, you can't yet re-export that module as a Swift package. Ref: kotlinlang.org/docs/multiplatform/multiplatform-spm-import.html
51. Result: ✅ Single framework, no split singletons ✅ Native SDKs called from Kotlin via SwiftPM import
52. Bonus: Swift ↔ Kotlin Interop Pitfalls Traps at the language boundary
53. Pitfall #1 Main Thread Guarantee Firebase callbacks → main thread (safe). But: CLLocationManager · CBCentralManager · URLSession · and more → background threads! Swift Kotlin DispatchQueue.main.async { fun updateFromBackground(code: Long) { SharedViewControllers.shared CoroutineScope(Dispatchers.Main).launch { .updateRequiredAppVersionCode (code: code) state.update { } it.copy(requiredAppVersionCode = code) } } }
54. Pitfall #2 Type Mapping Traps Swift // Remember from Case Study 1: // Swift Int != Kotlin Long func updateRequiredAppVersionCode(code: Int64) { SharedViewControllers() .updateRequiredAppVersionCode(code: code) } Kotlin Swift Trap Long Int64 Swift Int ≠ Kotlin Long Int Int32 Swift Int is 64-bit! Long? NSNumber? Boxed, loses type info Throwable KotlinThrowable Not NSError!
55. Interop Pitfalls Cheat Sheet Pitfall Symptom Fix BG thread → StateFlow UI warning, race condition DispatchQueue.main.async Int vs Int32/Int64 Compiler error / truncation Explicit Int32 / Int64 Long?  NSNumber? Wrong value, type confusion use .int64Value, not .intValue Throwable ≠ NSError Crash or unhandled exception Manual .toNSError() converter NSError → any Error Protocol stub mismatch Use Xcode auto-generated stub ARC deallocates bridge Crash, no stack trace StateObject or strong ref 🔗 Full reference: github.com/kotlin-hands-on/kotlin-swift-interopedia
56. Wrapping up Why is the community important 8
57. Back in the KotlinConf 2025... I marched into the JetBrains booth. And then...
58. Back in the KotlinConf 2025...
59. Back in the KotlinConf 2025... KMP Community is still evolving, and we need your help!
60. Wrap-up: What we learned in 1 year What we covered 1. Native State Observation StateFlow + ComposeUIViewController to bridge native events into shared Compose UI 2. Integrating Non-KMP SDKs One framework, one binary, one singleton IoC, framework export) 3. Dependency Management CocoaPods today, SwiftPM import tomorrow Kotlin 2.4.0 One Year In 100% Compose UI · 98.2% Kotlin KMP on iOS works in production. 🎉
61. linkedin.com/in/wisemuji LinkedIn Thank You, and Donʼt Forget to Vote