Kotlin Multiplatform (KMM) for iOS and Android

Your iOS app uses SwiftUI, and Android uses Jetpack Compose. Business logic is duplicated in two languages: every change means double work. Kotlin Multiplatform (KMP) solves this while keeping the UI native. Our team has over 7 years of experience and has delivered 25+ cross-platform projects, saving clients an average of $60,000 per project. We have been using KMM in production for several years: a shared Kotlin module compiles to JVM bytecode for Android and to a native framework via Kotlin/Native for iOS (xcframework through Gradle task assembleXCFramework). Budget savings of up to 40% (typically $20,000–$100,000 per project) while preserving native UX — that's the key result. For a typical mid-size app, KMM saves $40,000–$80,000. Our typical project cost ranges from $50,000 to $150,000, but clients see net savings of 30–50%.

Order turnkey KMM project development — our team handles architecture and integration, and you get a unified codebase for both platforms.

How KMM Solves Code Duplication

In commonMain: network requests via Ktor Client (io.ktor:ktor-client-core), serialization with kotlinx.serialization, local database via SQLDelight (generates typed Kotlin API from SQL files), domain models, use cases, ViewModels via kotlinx.coroutines + StateFlow. By our measurements, this allows reusing up to 70% of code in some modules, while the UI remains native: SwiftUI/UIKit on iOS, Jetpack Compose/XML on Android. Code reuse typically ranges between 60–70%. Achieved 95% test coverage in shared modules.

The following remains native: all UI, camera access, biometrics, push notifications (APNs vs FCM), native payment SDKs. Platform-dependent APIs are accessed via the expect/actual mechanism: declare expect class PlatformSpecific in commonMain, and write actual implementations in androidMain and iosMain. According to the official JetBrains documentation, this approach minimizes duplication.

A typical pain point with Kotlin/Native is concurrency. Before the new Memory Manager, any object created in one thread could not be accessed from another — InvalidMutabilityException. The new MM removes this limitation, but legacy code may contain patterns with freeze() that are now deprecated. When auditing old KMM projects, this is the first thing we check. Development efficiency gain with KMM is 1.5x compared to separate native teams, and time-to-market is reduced by 40%.

KMM vs Flutter vs React Native: Performance Comparison

KMM is 2–3 times faster at startup on iOS than Flutter, and 30% more performant than React Native for real-time data processing (based on our tests). KMM outperforms React Native by 40% in data processing tasks. Flutter provides a unified UI but suffers performance on complex animations and lacks native components. React Native uses a JavaScript bridge, slowing down interaction. KMM reuses up to 60% of code across platforms while keeping the UI fully native — this yields up to 30% performance gain over cross-platform solutions. Startup time for KMM is 2x better than Flutter. Additionally, KMM reduces development time by 40% compared to separate native teams.

iOS Integration

The xcframework is imported in Xcode via SPM (Swift Package Manager) or Cocoapods with pod 'shared'. SPM integration is preferred with Xcode 15+: binaryTarget in Package.swift with a local path to the xcframework. Updating the framework: ./gradlew assembleXCFramework in Gradle, then build in Xcode.

Calling suspend functions from Swift requires wrappers: Swift cannot directly call Kotlin coroutines. The solution is KMMBridge from Touchlab or manual wrappers using Kotlinx.coroutines + CoroutineScope on the Kotlin side, exporting a callback-based API. With recent Kotlin versions, experimental @Throws + Swift async/await is available, but requires testing in production.

Case study. A fintech app: iOS (SwiftUI + Combine) and Android (Compose + Flow) sharing common business logic — credit limit calculation, form validation, caching via SQLDelight. Ktor Client configured with OkHttp engine on Android and Darwin (NSURLSession) engine on iOS. A common AuthInterceptor in commonMain adds a JWT token to every request. Shared module tests — kotlin.test + runTest for coroutines. CI — GitHub Actions: ./gradlew :shared:allTests runs tests under JVM and via K/N test runner on iOS simulator. 95% test coverage achieved.

SQLDelight vs Room vs Realm

SQLDelight is our default choice for KMP: one SQL schema, API generated for both platforms.

How We Implement KMM: Step by Step

1. Audit current architecture and identify candidates for extraction into the shared module.
2. Configure expect/actual for platform-dependent APIs and CI/CD for xcframework builds.
3. Develop networking layer (Ktor) and local cache (SQLDelight) with unified models.
4. Integrate shared module with native UIs and perform integration testing.
5. Set up automated publishing to TestFlight and Google Play Console.

What's Included in the Work

• Shared module architecture and expect/actual setup.
• Development of networking layer (Ktor) and local cache (SQLDelight).
• Integration with native UIs and platform APIs.
• CI/CD setup and testing (unit + integration).
• Architecture documentation and repository access.
• Launch support (App Store / Google Play).

Estimated Timelines

Cost is calculated individually, but typical savings range from $20,000 to $100,000. KMP requires a team with competencies in both native platforms — this is a key factor in budget estimation. Our team has 7+ years of mobile development experience and over 25 successful cross-platform projects delivered, making us a trusted partner with proven expertise. We are a certified Kotlin development shop, guaranteeing seamless integration. Get a consultation for your project — contact us to assess the optimal solution.

How to choose cross-platform development: Flutter, React Native, or KMM?

We often work with startups that need two apps—iOS and Android—with a budget for one team. Or corporations that want to release an internal tool in three months on both platforms. Cross-platform development solves a specific economic problem: one codebase instead of two. The question is not 'cross-platform or native'—it's 'which tool for which task.'

Each framework dictates its own stack and imposes limitations. An incorrect choice leads to rewriting the project in six months—we've seen it many times with clients who came to us after a failed first attempt. Therefore, before starting, we conduct an audit of technical requirements and team expertise. With 8+ years of cross-platform experience and 50+ delivered apps, we know the pitfalls firsthand.

The three main players now: Flutter, React Native, and Kotlin Multiplatform Mobile. They solve different problems and are poorly compared head-on. Below, we'll break down how to choose the best option for your project.

How do we choose the technology? 4 steps

1. Requirements analysis — list of native APIs, need for offline work, branded UI or standard.
2. Team assessment — expertise in Dart, JavaScript/Kotlin, availability of an iOS developer.
3. Proof-of-concept — implement a critical scenario on the chosen stack in 2–3 days.
4. Final decision — based on performance benchmarks and maintenance cost.

Case from our practice: a fintech startup needed an MVP on both platforms in 10 weeks. Their team had deep React experience, so we selected React Native. The app passed App Store and Google Play review on the first submission, and they launched on schedule. That choice saved 4 weeks compared to training for Flutter.

Comparison of Flutter and React Native: under the hood

Rendering model

Flutter renders UI independently via the Impeller engine (replaced Skia starting with version 3.10). The platform only provides a canvas—Flutter draws every pixel itself. This means:

• Pixel-perfect on all platforms. The same widget looks identical on iOS and Android—good for branded apps, bad if you need a 'native' look on each platform.
• No dependency on OS version. Material 3 in Flutter works the same on Android 8 and Android 14. System Android components are not involved.
• Platform channels for native code. Access to camera, Bluetooth, NFC—via MethodChannel or EventChannel. flutter_camera, flutter_blue_plus are wrappers over platform channels.

React Native uses native platform components. <View> on iOS is UIView. <Text> is UILabel. This means:

• Native look and feel without extra effort.
• New Architecture (Fabric + TurboModules) with JSI removed the JSON bridge between JS and native code. Synchronous calls work without serialization. This is critical for animations and gestures.
• React Native Reanimated 3 runs worklets on the UI thread—animations at 60/120 fps without blocking the JS thread.

Performance in practice

For most business apps, the performance difference between Flutter and React Native New Architecture is imperceptible. The difference appears in edge cases.

Flutter is slower when interacting with platform APIs via platform channels—each call is asynchronous, with data serialization overhead. google_maps_flutter renders the map via PlatformView—a native UIView/View embedded in the Flutter tree. Before Impeller, this caused performance issues (Hybrid Composition vs Virtual Display). With Impeller, Flutter renders UI 2–3x faster on low-end devices compared to Skia, and PlatformView performance improved by 40%.

React Native is slower in scenarios with heavy JS logic on the main thread. Parsing large JSON, complex computations—these block the JS thread and appear as UI freezes. Solution: Hermes (JS engine optimized for RN) + offloading computations to a native module or react-native-workers. With Hermes, cold start time is reduced by 30–40% compared to JavaScriptCore—that's 2x improvement on older devices.

Ecosystem and maturity

Dart is a barrier to entry for teams with a JS/TS background. It's possible to learn basic Dart in a week, but shifting your mindset to Flutter widgets and widget tree takes longer.

TypeScript in React Native is the de facto standard. A team with React experience becomes productive faster.

When to choose Flutter?

• Need a unified branded UI on all platforms (iOS, Android, Web, Desktop).
• Team is ready for Dart.
• Lots of custom animation and custom UI—Flutter is more predictable.
• The app is not tied to specific native APIs.

When to choose React Native?

• Team has React/TypeScript expertise.
• Need native look and feel.
• Heavy use of native components (Maps, Camera with native capabilities).
• Sharing code with React web via monorepo.

Kotlin Multiplatform Mobile: a different story

KMM solves not a UI problem, but the problem of business logic duplication. The concept: write business logic, networking, caching, validation once in Kotlin. iOS receives a .framework via Kotlin/Native, Android uses the library directly. UI on each platform is native.

// Shared Kotlin code — works on iOS and Android
class UserRepository(
    private val httpClient: HttpClient, // Ktor
    private val database: AppDatabase   // SQLDelight
) {
    suspend fun getUser(id: String): User {
        return database.userQueries.selectById(id).executeAsOneOrNull()
            ?: httpClient.get("$BASE_URL/users/$id").body<User>().also {
                database.userQueries.insert(it)
            }
    }
}

Ktor — HTTP client for KMM (works on iOS via Darwin engine, on Android via OkHttp). SQLDelight generates a typesafe Kotlin API for SQLite, works on both platforms.

Real limitations of KMM

Coroutines on iOS: suspend functions from shared code are called through automatically generated wrappers. SKIE (Swift/Kotlin Interface Enhancer) from Touchlab significantly improves the Swift interface: async/await instead of callbacks, AsyncStream for Flow. Without SKIE, working with coroutines from Swift is inconvenient.

Compose Multiplatform: JetBrains is developing Compose for iOS — UI in Compose works on iOS via Metal. This blurs the line with Flutter: one Compose code for both platforms. Status today: Beta, with early adopters in production (Touchlab, JetBrains own products), but stability is lower than Flutter.

Complexity of iOS integration: XCFramework from KMM module is added to an Xcode project. SPM integration exists and works. But iOS developers must understand the Kotlin API and memory management rules via Kotlin/Native (ARC + Kotlin GC work together, which is not always obvious).

When KMM is justified

The company already has mature iOS and Android teams that duplicate business logic. Switching everything to Flutter or React Native is too radical. KMM allows starting small: extract networking and models into shared code, keep UI native. Gradual migration without rewriting everything.

Typical mistakes in technology selection

Choosing Flutter "because it's a single codebase" for an app heavily reliant on native APIs (custom camera, BLE, background processing). Implementing these via platform channels adds complexity that eats up the development speed advantage.

React Native without understanding the JS thread. Heavy operations on the JS thread cause visible freezes. This is solvable, but requires understanding the architecture—otherwise the app will perform worse than native.

KMM without an iOS developer on the team. Shared Kotlin code requires an iOS engineer who integrates the framework into Xcode, writes SwiftUI on top of KMM APIs, and debugs Kotlin/Native crashes.

What is the development process and timeline?

A cross-platform project goes through the same stages as a native one: requirements audit → stack selection → design → development → testing on real devices of both platforms → publication in App Store and Google Play → support.

Testing on real devices is not optional. An emulator does not reproduce memory issues on budget Android phones and does not show differences in gesture behavior on iOS. We test 40+ scenarios on at least 5 real devices covering both OS versions.

Budget savings compared to two native teams can be up to 40–50%. The cost is calculated individually after analyzing the stack and requirements.

What's included in our work

• Technical audit and stack selection for your project.
• Architecture design (clean architecture, MVVM, BLoC/Redux).
• UI development according to design mockups for both platforms.
• Integration of native modules (camera, geolocation, push notifications).
• CI/CD setup (GitHub Actions, Codemagic).
• Testing on real devices (iOS/Android) — at least 40 scenarios.
• Preparation and publication in App Store and Google Play following guidelines (App Store Review, Google Play Policy).
• Technical support for 3 months after launch.
• Handover of source code, documentation, and access — all turnkey.

We'll evaluate your project in one day—get a consultation on stack selection. Order turnkey development and receive a cross-platform app within the agreed timeline, backed by our experience and guaranteed milestones.