Compose Multiplatform Agent Skill

Compose Multiplatform

> Compose Multiplatform (CMP) and Kotlin Multiplatform (KMP) patterns for cross-platform UI.

> Loaded for projects with org.jetbrains.compose plugin.

> Foundation: ../compose-motion/SKILL.md for animation API; this file covers what's specific to writing one Compose codebase for Android + iOS + Desktop + Web.

KMP vs CMP - quick clarification

KMP (Kotlin Multiplatform) is the language and build infrastructure: shared Kotlin code compiled to JVM, Native (iOS, macOS, Linux, Windows), and Wasm. CMP (Compose Multiplatform) is the UI framework on top of KMP, built by JetBrains as a port of Jetpack Compose. You write a single Compose codebase in commonMain that runs on Android, iOS, Desktop (JVM), and Web (Wasm). Platform-specific code lives in androidMain, iosMain, desktopMain, wasmJsMain and is wired in via expect/actual declarations.

Project structure

composeApp/
├── src/
│   ├── commonMain/        ← shared Compose code (most of the app)
│   │   └── kotlin/
│   ├── androidMain/       ← Android-specific (uses Activity, Context)
│   ├── iosMain/           ← iOS-specific (uses UIKit/UIView interop)
│   ├── desktopMain/       ← JVM desktop (uses java.awt/swing if needed)
│   └── wasmJsMain/        ← Wasm web target
├── build.gradle.kts
iosApp/                    ← Xcode project consuming the generated framework
androidApp/                ← Android Application module (often merged into composeApp)

The commonMain folder should hold 80-95% of your code in a well-architected CMP project. If iosMain or androidMain start growing past a few hundred lines, you're probably leaking platform concerns into UI logic that could stay shared.

expect/actual pattern

The KMP escape hatch when you genuinely need different implementations per target. Declare the contract once in commonMain, implement it once per target.

// commonMain
expect fun openShareSheet(text: String)

// androidMain
actual fun openShareSheet(text: String) {
    val intent = Intent(Intent.ACTION_SEND).apply {
        type = "text/plain"
        putExtra(Intent.EXTRA_TEXT, text)
    }
    context.startActivity(Intent.createChooser(intent, null))
}

// iosMain
actual fun openShareSheet(text: String) {
    val activityVC = UIActivityViewController(
        activityItems = listOf(text),
        applicationActivities = null
    )
    UIApplication.sharedApplication.keyWindow
        ?.rootViewController
        ?.presentViewController(activityVC, true, null)
}

expect/actual works for top-level functions, classes, type aliases, and properties. The signature in actual must match exactly, including modifiers and default values.

expect/actual for composables

Composables follow the same rules. Useful when a feature needs a platform-specific Compose API (Android RuntimeShader, iOS UIKitView, Desktop SwingPanel).

// commonMain
@Composable
expect fun PlatformBlur(modifier: Modifier = Modifier, content: @Composable () -> Unit)

// androidMain (uses RuntimeShader on Android 13+)
@Composable
actual fun PlatformBlur(modifier: Modifier, content: @Composable () -> Unit) {
    Box(modifier.graphicsLayer { renderEffect = blurEffect }) { content() }
}

// iosMain (uses UIVisualEffectView via UIKitView)
@Composable
actual fun PlatformBlur(modifier: Modifier, content: @Composable () -> Unit) {
    Box(modifier) {
        UIKitView(
            factory = { UIVisualEffectView(effect = UIBlurEffect.systemMaterial()) },
            modifier = Modifier.matchParentSize()
        )
        content()
    }
}

Rule: expect composables should be the exception, not the rule. Most "platform feel" differences can be tuned via tokens (colors, corner radii, spring stiffness) in commonMain, not via separate code paths.

LocalDensity cross-platform

On Android, LocalDensity.current.density reflects the device DPI bucket (1.0, 1.5, 2.0, 3.0...). On iOS, density is computed from UIScreen.scale (typically 2.0 or 3.0 on Retina). On Desktop, density depends on the screen scaling factor (1.0 by default; 2.0 on Retina-class displays; user-configurable on Windows). On Wasm, density follows window.devicePixelRatio.

Don't hardcode Dp to pixel ratios; trust Dp and LocalDensity to handle conversion. If you need an exact pixel value (e.g., for a Canvas draw operation), do the conversion explicitly:

val density = LocalDensity.current
val pxValue = with(density) { 16.dp.toPx() }

Avoid reading density inside hot loops; cache the conversion.

LocalConfiguration and platform-aware UI

LocalConfiguration.current is Android-only and lives in androidMain. For CMP, prefer the cross-platform alternatives:

• LocalWindowInfo.current.containerSize - the window/screen size as IntSize, available in commonMain.
• LocalDensity.current - density, available in commonMain.
• LocalLayoutDirection.current - LTR / RTL.
• BoxWithConstraints { ... } - read maxWidth / maxHeight directly inside layout.

If you need real device characteristics (orientation, idiom, model), wrap the access in expect/actual and pass a typed object like PlatformInfo to the common layer.

Fonts cross-platform via Compose Resources

org.jetbrains.compose.resources is the shared resources plugin. Drop fonts in commonMain/composeResources/font/, and the Gradle plugin generates a typed Res accessor.

composeApp/src/commonMain/composeResources/
├── font/
│   ├── Inter-Regular.ttf
│   └── Inter-Bold.ttf
├── drawable/
│   └── logo.svg
└── values/
    ├── strings.xml          ← default locale
    └── strings.fr.xml       ← French overrides

Usage in commonMain:

import myproject.composeapp.generated.resources.Inter_Regular
import myproject.composeapp.generated.resources.Inter_Bold
import myproject.composeapp.generated.resources.Res

val InterFamily = FontFamily(
    Font(Res.font.Inter_Regular, FontWeight.Normal),
    Font(Res.font.Inter_Bold, FontWeight.Bold),
)

Text("Hello", fontFamily = InterFamily)

Same pattern for Res.drawable.logo (image), Res.string.app_name (localized string via stringResource(...)), Res.file.config (raw bytes via Res.readBytes(...)).

iOS interop with SwiftUI

CMP produces a UIViewController you can drop into a SwiftUI app. KMP generates a top-level Kotlin function (commonly named MainViewController() or ComposeUIViewController { ... }) that returns a UIViewController. Wrap it with UIViewControllerRepresentable.

// iosMain/kotlin/main.ios.kt
fun MainViewController(): UIViewController = ComposeUIViewController {
    AppContent()  // commonMain composable
}

// iOS app target
import SwiftUI
import ComposeApp  // KMP-generated framework

struct ComposeContent: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> UIViewController {
        Main_iosKt.MainViewController()
    }
    func updateUIViewController(_ uiViewController: UIViewController, context: Context) {}
}

struct ContentView: View {
    var body: some View { ComposeContent().ignoresSafeArea() }
}

The Kotlin function name gets mangled to Main_iosKt.MainViewController() because the file is main.ios.kt. Check the generated framework headers if the symbol name surprises you.

Android entry point

No interop ceremony on Android. The Activity hosts the common composable directly via setContent { ... }.

// androidApp/src/main/kotlin/MainActivity.kt
class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            AppContent()  // commonMain composable
        }
    }
}

If you need to pass Context or Activity into commonMain, expose it via a DI graph or an expect class PlatformContext in commonMain with actual class PlatformContext(val context: Context) in androidMain.

Embedding SwiftUI/UIKit inside a Compose iOS view (the reverse direction)

Use UIKitView for a UIView factory or UIKitViewController for a UIViewController factory.

// iosMain
UIKitView(
    factory = {
        UISwitch().apply {
            addTarget(target, action = NSSelectorFromString("onToggle:"), forControlEvents = UIControlEventValueChanged)
        }
    },
    modifier = Modifier.size(48.dp, 32.dp)
)

For SwiftUI views: wrap them in a UIHostingController exposed via a Swift @objc bridge function, then call from Kotlin via the generated headers (cinterop). See references/cmp-interop.md for the full pattern.

Animation cross-platform

All animation APIs (animate*AsState, AnimatedVisibility, updateTransition, SharedTransitionLayout) work identically across targets in CMP 1.7+. Spring tuning written in commonMain produces the same physics on Android and iOS. Gestures (Modifier.draggable, Modifier.pointerInput) work cross-platform with the same API surface.

The animation primer lives in ../compose-motion/SKILL.md. Cross-platform deltas to keep in mind:

• iOS first-frame is slower (Skia bootstrap); a 200ms enter animation feels tighter on Android, slightly delayed on iOS cold start.
• Wasm motion can stutter on first frame (JIT warmup); pre-warm critical paths or hide motion until interactive.

What does NOT work (gotchas)

• Drawer state on iOS: native ModalNavigationDrawer swipe-to-open from the leading edge conflicts with iOS's back-swipe gesture. Use a button trigger or move the swipe area inward 30dp+.
• LayoutDirection.Rtl quirks: Android handles RTL natively, iOS Compose had bugs in 1.6 (text alignment, padding inversions). Improved in 1.7+ but verify with real Arabic/Hebrew strings.
• Soft keyboard handling: imePadding() works on Android out of the box. On iOS Compose 1.6+, it requires IOSKeyboardEventListener setup or a WindowInsets observer wired through the platform layer.
• Color.parseHex(...) does not exist in Compose. Use Color(0xFFRRGGBB) or write a tiny extension.
• System fonts on iOS via Compose: do not fallback to FontFamily.SansSerif and expect SF Pro. Compose on iOS ships its own font fallback chain. Either bundle SF Pro via Compose Resources (license-permitting) or use UIKitView to drop a native UILabel for system-font text.
• Animations on Web (Wasm): heavier startup, occasional first-frame stutter; profile with browser devtools and lazy-load heavy animation graphs.
• java.util.UUID, java.io.File and other JVM-only APIs are forbidden in commonMain if you ship to iOS or Wasm. Use kotlinx.uuid, kotlinx-io, or the okio multiplatform port.

CMP version notes

• Compose Multiplatform 1.7 stable: SharedTransitionLayout cross-platform, improved iOS keyboard handling, lifecycle observability via LocalLifecycleOwner on iOS.
• Kotlin 2.0+ required (K2 compiler).
• Some Material 3 components have platform-specific look (e.g., Switch on iOS auto-renders with iOS-style proportions; DatePicker stays Material across all targets).
• compose-multiplatform-resources plugin is the standard for assets; the older moko-resources is no longer recommended for new projects.

Performance considerations

• iOS first-frame is slower than Android (Skia bootstrapping ~150-300ms cold). Keep your splash visible until the first composition emits, or pre-warm with a transparent root composable.
• Wasm bundle size: aim for <2MB compressed. Tree-shake heavy deps, lazy-load secondary screens via kotlinx.coroutines deferred composition, and inspect the .wasm output in wasmJsBrowserDistribution.
• Desktop: cold start is fast on JVM; AOT compilation via Kotlin/Native is overkill for desktop unless you need a single-file binary.
• Android: same baseline as Jetpack Compose - profile with the Compose compiler stability metrics and Layout Inspector recomposition counts.

Anti-Patterns

| BAD | GOOD | Why |

|---|---|---|

| Reflection trick or System.getProperty("os.name") to detect platform inside commonMain | expect/actual with a typed Platform object | Reflection breaks on Wasm/Native; expect/actual is the contract the compiler enforces |

| Assuming Android Context is reachable in commonMain | Inject a typed dependency via expect class PlatformContext or a DI scope | Context does not exist on iOS/Desktop/Wasm; the code will not compile for those targets |

| Hardcoding Material colors that look great on Android but jarring on iOS | Define a commonMain design system, then optionally adjust 2-3 tokens via actual | Cross-platform consistency is good, but iOS users notice when a Material blue feels alien on iPhone |

| LaunchedEffect(Unit) { while(true) { delay(16); ... } } in commonMain | rememberInfiniteTransition() or scope to lifecycle events | Tight coroutine loops drain battery on iOS; infinite transitions pause when offscreen |

Quick Reference: Loading Sub-skills

| Need | Load |

|---|---|

| iOS / Android interop deep-dive | references/cmp-interop.md |

| Per-platform behavior catalog | references/cmp-platform-quirks.md |

| Animation API | ../compose-motion/SKILL.md |

| Advanced graphics (M3 Expressive, AGSL on Android only) | ../compose-graphics/SKILL.md |

| iOS-side native interop with SwiftUI | ../swiftui-motion/SKILL.md (when target is iOS and SwiftUI native blend wanted) |

| Mobile UX context | ../mobile-principles/SKILL.md |

| Desktop UX context | ../desktop-principles/SKILL.md |

| Foundation | ../motion-principles/SKILL.md |

Sources

• Meet-Miyani/compose-skill (KMP/CMP comprehensive)
• JetBrains Compose Multiplatform
• Compose Multiplatform docs
• skydoves/Orbital (KMP-aware shared transitions)