Kotlin Multiplatform UI: Mastering Compose for Cross-Platform Apps

Compose Multiplatform revolutionizes cross-platform development by letting you build native UIs for Android, iOS, desktop, and web with a single codebase. In this guide, we’ll explore its core concepts—from declarative UI design to state management—so you can create performant, beautiful apps faster than ever.

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What Makes Compose Multiplatform Unique?

• True Native Rendering: Unlike Flutter’s Skia engine, Compose uses platform-native components.
• Shared UI Logic: Write once, deploy everywhere while maintaining platform-specific look/feel.
• Hot Reload: See changes instantly during development.

Project Setup

Step 1: Create a KMP project in Android Studio with the Compose Multiplatform template.
Step 2: Configure build.gradle.kts:

plugins {  
    id("org.jetbrains.compose") version "1.6.0"  
    kotlin("multiplatform")  
}  

kotlin {  
    androidTarget()  
    ios()  
    jvm("desktop")  

    sourceSets {  
        val commonMain by getting {  
            dependencies {  
                implementation(compose.runtime)  
                implementation(compose.foundation)  
                implementation(compose.material3)  
            }  
        }  
    }  
}  

Your First Composable

@Composable  
fun App() {  
    var count by remember { mutableStateOf(0) }  

    Column(  
        modifier = Modifier.fillMaxSize().padding(16.dp),  
        horizontalAlignment = Alignment.CenterHorizontally  
    ) {  
        Text("Welcome!", style = MaterialTheme.typography.headlineMedium)  
        Button(onClick = { count++ }) { Text("Clicked $count times") }  
    }  
}  

Platform Entry Points:

• Android: setContent { App() } in MainActivity.
• iOS: Wrap in ComposeViewControllerToSwiftUI.
• Desktop: Use application { Window { App() } }.

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Composables: UI Building Blocks

@Composable  
fun Greeting(name: String) {  
    Text("Hello, $name!") // Updates automatically when 'name' changes  
}  

• Key Insight: Composables rebuild only when their inputs change.

State Management

Stateful Example (Avoid):

@Composable  
fun Counter() {  
    var count by remember { mutableStateOf(0) }  
    Button(onClick = { count++ }) { Text("Count: $count") }  
}  

Stateless Best Practice (State Hoisting):

@Composable  
fun CounterApp() {  
    var count by remember { mutableStateOf(0) }  
    CounterDisplay(count) { count++ }  
}  

@Composable  
fun CounterDisplay(count: Int, onIncrement: () -> Unit) {  
    Button(onClick = onIncrement) { Text("Count: $count") }  
}  

3. Layout Systems

Layout	Purpose	Example
Column	Vertical stacking	Column { Text("A"); Text("B") }
Row	Horizontal alignment	Row { Text("Left"); Text("Right") }
Box	Overlapping elements	Box { Image(); Text() }
LazyColumn	Optimized lists	LazyColumn { items(100) { ... } }

4. Modifiers: Styling Powerhouse

Text(  
    "Styled Text",  
    modifier = Modifier  
        .padding(16.dp)  
        .background(Color.Blue, RoundedCornerShape(8.dp))  
        .clickable { /* Handle click */ }  
)  

Common Modifiers:

• .padding(), .size(), .background()
• .fillMaxWidth(), .weight() (for responsive layouts)

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Key Takeaways

1. Compose Multiplatform enables true native UIs across platforms.
2. State hoisting makes components reusable and testable.
3. Use LazyColumn/LazyRow for performant lists.
4. Modifiers unlock pixel-perfect designs without CSS/XML.

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• Target keywords: “Compose Multiplatform Tutorial,” “KMP Declarative UI,” “Cross-Platform State Management.”
• Link to JetBrains Compose docs for authority.