Jetpack Compose and Kotlin Multiplatform

Jetpack Compose has revolutionized Android UI development by providing a modern, declarative way to build interfaces. Simultaneously, Kotlin Multiplatform (KMP) has emerged as a powerful tool for sharing code across multiple platforms, including Android, iOS, web, and desktop. Combining these two technologies can lead to efficient and maintainable cross-platform applications. This post will explore how to integrate Jetpack Compose with Kotlin Multiplatform to create UI components that can be shared across platforms.

What is Kotlin Multiplatform (KMP)?

Kotlin Multiplatform allows developers to write code once in Kotlin and share it across multiple target platforms. With KMP, you can share business logic, data models, and even UI code (with some limitations) between platforms, reducing development time and ensuring consistency across your applications.

Why Combine Jetpack Compose with Kotlin Multiplatform?

• Code Sharing: Share UI components and business logic across Android, iOS, web, and desktop.
• UI Consistency: Maintain a consistent look and feel across platforms.
• Reduced Development Time: Write UI code once and reuse it on multiple platforms.
• Maintainability: Simplify updates and bug fixes by maintaining a single codebase.

Setting Up a Kotlin Multiplatform Project with Jetpack Compose

Step 1: Project Setup

First, you need to create a Kotlin Multiplatform project. You can use the Kotlin Multiplatform wizard in IntelliJ IDEA to set up the basic project structure.

Step 2: Configure build.gradle.kts Files

Ensure your project’s build.gradle.kts files are properly configured. Here’s an example setup for the shared module (the KMP module):

plugins {
    kotlin("multiplatform") version "1.9.21"
    id("org.jetbrains.compose") version "1.6.0"
}

group = "org.example"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
    maven("https://maven.pkg.jetbrains.space/public/p/compose/dev")
    google()
}

kotlin {
    androidTarget {
        compilations.all {
            kotlinOptions {
                jvmTarget = "1.8"
            }
        }
    }
    
    jvm("desktop")

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation(compose.runtime)
                implementation(compose.foundation)
                implementation(compose.material)
                implementation(compose.uiToolingPreview)
                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0") // Ensure consistent version
            }
        }
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test"))
            }
        }
        val androidMain by getting {
            dependencies {
                implementation("androidx.compose.ui:ui-tooling-preview-android")
                implementation("androidx.compose.ui:ui-android")
            }
        }
        val desktopMain by getting {
            dependencies {
                implementation(compose.desktop.common)
                implementation(compose.desktop.currentOs)
            }
        }
    }
}

android {
    namespace = "org.example.kmpsample"
    compileSdk = 34
    defaultConfig {
        minSdk = 24
        targetSdk = 34
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }
}

Key points in this setup:

• Applied the Kotlin Multiplatform and Compose plugins.
• Configured both Android and JVM (desktop) targets.
• Added Compose dependencies in the commonMain source set for cross-platform UI components and in the platform-specific source sets as needed.
• Set the JVM target to “1.8” for compatibility.

Step 3: Create Shared UI Components

Create your Jetpack Compose UI components in the commonMain source set. These components will be shared across all target platforms.

// In commonMain/kotlin/MyComposable.kt
import androidx.compose.material.Text
import androidx.compose.runtime.Composable

@Composable
fun MySharedComposable(text: String) {
    Text("Hello from shared Compose: $text")
}

Step 4: Use Shared Components in Platform-Specific Code

In your Android and desktop applications, you can now use the shared UI components.

Android:

// In Android's MainActivity.kt
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material.MaterialTheme
import androidx.compose.material.Surface
import androidx.compose.ui.Modifier

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                Surface(modifier = Modifier.fillMaxSize(), color = MaterialTheme.colors.background) {
                    MySharedComposable("Android")
                }
            }
        }
    }
}

Desktop:

// In Desktop's Main.kt
import androidx.compose.desktop.ui.tooling.preview.Preview
import androidx.compose.runtime.Composable
import androidx.compose.ui.window.Window
import androidx.compose.ui.window.application

fun main() = application {
    Window(onCloseRequest = ::exitApplication, title = "Desktop App") {
        MySharedComposable("Desktop")
    }
}

@Preview
@Composable
fun AppPreview() {
    MySharedComposable("Desktop Preview")
}

Step 5: Handling Platform-Specific Dependencies and APIs

Not all Compose functionalities and dependencies are available across all platforms. Use expect and actual declarations to handle platform-specific implementations.

// In commonMain/kotlin/PlatformSpecific.kt
expect fun platformName(): String

// In androidMain/kotlin/PlatformSpecific.kt
actual fun platformName(): String = "Android"

// In desktopMain/kotlin/PlatformSpecific.kt
actual fun platformName(): String = "Desktop"

// In commonMain/kotlin/MyComposable.kt
import androidx.compose.material.Text
import androidx.compose.runtime.Composable

@Composable
fun MySharedComposable(text: String) {
    Text("Hello from " + platformName() + ": " + text)
}

Best Practices and Considerations

• UI Abstraction: Focus on sharing UI logic and data models rather than trying to share complex UI components directly.
• Platform-Specific Adaptations: Be prepared to adapt your UI components for each platform to ensure they look and feel native.
• Dependency Management: Carefully manage dependencies to avoid conflicts between platforms.
• Testing: Thoroughly test your UI components on each target platform to ensure they work correctly.
• Performance: Optimize your code for performance on each platform, as performance characteristics may vary.

Advanced Techniques

Using Modifiers Effectively

When sharing UI components, use modifiers carefully to allow customization in platform-specific implementations.

import androidx.compose.foundation.layout.padding
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp

@Composable
fun MyCustomText(text: String, modifier: Modifier = Modifier) {
    Text(text = text, modifier = modifier.padding(8.dp))
}

Now, you can customize the appearance of the text in the platform-specific code by passing a custom modifier.

Implementing Custom Renderers (Experimental)

While not yet fully mature, custom renderers can allow you to use Compose code to render UI on platforms like iOS. This typically involves creating platform-specific implementations of Compose primitives and managing the rendering pipeline. More information and experimental support can be found in Kotlin/Native repositories.

Conclusion

Integrating Jetpack Compose with Kotlin Multiplatform offers a powerful way to share UI components and business logic across multiple platforms. By following best practices and carefully managing platform-specific adaptations, you can create efficient, maintainable, and consistent cross-platform applications. While there are challenges to consider, the benefits of code sharing and UI consistency make this combination a valuable tool for modern app development.