Kotlin Multiplatform Cheat Sheet

// commonMain/kotlin/Platform.ktexpect class Platform() {    val name: String}expect fun getPlatform(): Platform// androidMain/kotlin/Platform.android.ktactual class Platform actual constructor() {    actual val name: String = "Android ${android.os.Build.VERSION.SDK_INT}"}actual fun getPlatform(): Platform = Platform()// iosMain/kotlin/Platform.ios.ktactual class Platform actual constructor() {    actual val name: String = UIDevice.currentDevice.systemName()}actual fun getPlatform(): Platform = Platform()

kotlin {    androidTarget()    iosX64()    iosArm64()    iosSimulatorArm64()    jvm("desktop")    sourceSets {        val commonMain by getting {            dependencies {                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0")            }        }        val androidMain by getting        val iosMain by creating {            dependsOn(commonMain)        }        val iosX64Main by getting { dependsOn(iosMain) }        val iosArm64Main by getting { dependsOn(iosMain) }    }}

// commonMain: shared business logic, no platform APIsclass UserRepository(private val api: ApiClient) {    suspend fun getUser(id: String) = api.fetchUser(id)}// commonMain can use expect declarations for anything platform-specific,// like file storage, HTTP engines (via Ktor's multiplatform client), or// platform-specific date/time formatting.// androidMain / iosMain: actual implementations, plus platform-only code// that isn't part of any expect declaration at all (e.g. Android Activity// classes or iOS SwiftUI views) lives entirely in its own source set.

kotlin {    // Kotlin 1.9+ applies the default hierarchy template automatically,    // creating intermediate source sets like `appleMain`/`nativeMain`    // wherever two or more matching targets are declared    androidTarget()    iosX64()    iosArm64()    iosSimulatorArm64()    macosX64()    macosArm64()    linuxX64()    // appleMain (iosMain + macosMain) and nativeMain (appleMain + linuxMain)    // are created for you; code shared by all Apple targets goes here    sourceSets {        val appleMain by getting {            dependencies {                implementation("io.ktor:ktor-client-darwin:2.3.11")            }        }        val nativeMain by getting    }}

// commonMain: expect a typealias mapping to a platform-native typeexpect class AtomicCounter(initial: Int) {    fun incrementAndGet(): Int}// commonMain: expect object for a platform-specific singletonexpect object Logger {    fun log(message: String)}// commonMain: expect enum, actual enum must declare the same entriesexpect enum class Theme { LIGHT, DARK, SYSTEM }// androidMainactual typealias AtomicCounter = java.util.concurrent.atomic.AtomicIntegeractual object Logger {    actual fun log(message: String) = android.util.Log.d("App", message)}actual enum class Theme { LIGHT, DARK, SYSTEM }

# src/nativeInterop/cinterop/zlib.defheaders = zlib.hheaderFilter = zlib.hlinkerOpts = -lz# build.gradle.ktskotlin {    iosArm64 {        compilations.getByName("main") {            cinterops {                val zlib by creating {                    defFile(project.file("src/nativeInterop/cinterop/zlib.def"))                }            }        }    }}# Usage in iosArm64Main:# import zlib# val level = Z_BEST_COMPRESSION

// commonTest/kotlin/UserRepositoryTest.ktimport kotlin.test.Testimport kotlin.test.assertEqualsclass UserRepositoryTest {    @Test    fun `formats display name correctly`() {        val repo = UserRepository(FakeApiClient())        assertEquals("Ana D.", repo.displayName("Ana", "Diaz"))    }}// Platform-specific behavior can still be tested with expect/actual test// utilities, e.g. an `expect fun runBlockingTest(block: suspend () -> Unit)`// implemented differently per target when a common coroutine test API// isn't available on every platform yet.