Advanced Features in Compose Multiplatform for Cross-Platform Development

Jetpack Compose has revolutionized Android UI development with its declarative approach and Kotlin-first syntax. Building on this foundation, Compose Multiplatform extends its capabilities beyond Android, enabling developers to create applications for various platforms, including iOS, web, desktop, and more, from a single codebase. In this comprehensive guide, we’ll explore advanced features in Compose Multiplatform, enabling you to leverage its full potential for creating efficient, maintainable, and platform-agnostic applications.

Introduction to Compose Multiplatform

Compose Multiplatform is a Kotlin-based UI framework that enables code sharing across multiple platforms. Built on Jetpack Compose, it allows developers to write UI code once and deploy it on various operating systems and devices. This substantially reduces development time and costs while ensuring consistency in user experience across platforms.

Setting Up Your Project

Before diving into advanced features, let’s set up a basic Compose Multiplatform project.

Step 1: Configure the Gradle Build Script

Create a new Kotlin Multiplatform project using the IntelliJ IDEA wizard or manually set up the build.gradle.kts file with the necessary configurations for target platforms.

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

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

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

kotlin {
    jvm {
        withJava()
    }
    iosArm64()
    iosX64()
    iosSimulatorArm64()

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation(compose.runtime)
                implementation(compose.foundation)
                implementation(compose.ui)
            }
        }
        val androidMain by getting {
            dependencies {
                implementation("androidx.appcompat:appcompat:1.6.1")
                implementation("androidx.core:core-ktx:1.12.0")
                implementation(compose.uiToolingPreview)
                implementation(compose.material)
            }
        }
        val iosMain by getting {
            dependencies {
                implementation(compose.ui)
            }
        }
        val jvmMain by getting {
            dependencies {
                implementation(compose.desktop.ui)
            }
        }
    }
}

This setup targets JVM (desktop), iOS (simulator, ARM64, X64), and Android, establishing the multiplatform capability of our project.

Step 2: Project Structure

The basic project structure consists of platform-specific source sets and a shared commonMain source set.

src/
├── commonMain/
│   └── kotlin/          # Shared code goes here
├── androidMain/
│   └── kotlin/          # Android-specific code
├── iosMain/
│   └── kotlin/          # iOS-specific code
└── jvmMain/
    └── kotlin/          # Desktop-specific code

Advanced Features in Compose Multiplatform

Now, let’s explore the advanced features that make Compose Multiplatform a compelling choice for cross-platform development.

1. Custom Renderers and Platform-Specific Components

Compose Multiplatform enables you to define custom renderers for each platform. This is useful when certain UI elements must leverage platform-specific APIs or components. Custom renderers allow you to create a common abstraction in the shared code while implementing the actual rendering using native elements on each platform.

For instance, consider creating a native map view component. The interface in commonMain would look like this:

// commonMain
import androidx.compose.runtime.Composable

@Composable
expect fun NativeMapView(latitude: Double, longitude: Double)

Next, implement the platform-specific rendering for Android:

// androidMain
import androidx.compose.runtime.Composable
import androidx.compose.ui.viewinterop.AndroidView
import com.google.android.gms.maps.MapView

actual @Composable fun NativeMapView(latitude: Double, longitude: Double) {
    AndroidView({ context ->
        MapView(context).apply {
            // Initialize and configure the map
        }
    }, update = { mapView ->
        // Update map view with new coordinates
    })
}

Similarly, implement it for iOS using MapKit:

// iosMain
import androidx.compose.runtime.Composable
import platform.MapKit.MKMapView
import platform.UIKit.UIView
import androidx.compose.ui.interop.UIKitView

actual @Composable fun NativeMapView(latitude: Double, longitude: Double) {
    UIKitView(factory = {
        MKMapView().apply {
            // Initialize and configure the map
        }
    }, update = { mapView ->
        // Update map view with new coordinates
    })
}

2. Interop with Native Code

Interoperability with existing native codebases is crucial when migrating or integrating Compose Multiplatform into legacy projects. Kotlin/Native facilitates seamless interaction with platform-specific APIs, whether Objective-C/Swift on iOS or Java/Kotlin on Android.

Example: Invoking a native iOS function:

// iosMain
import kotlinx.cinterop.*
import platform.Foundation.*

fun getSystemVersion(): String {
    return NSString.stringWithCString(NSProcessInfo.processInfo.operatingSystemVersionString.UTF8String, encoding = NSUTF8StringEncoding) as String
}

To invoke Android-specific Kotlin code:

// androidMain
import android.content.Context

fun getApplicationName(context: Context): String {
    val applicationInfo = context.applicationInfo
    val stringId = applicationInfo.labelRes
    return if (stringId == 0) applicationInfo.nonLocalizedLabel.toString() else context.getString(stringId)
}

Now, call these functions from common code:

// commonMain
expect fun platformName(): String

fun printPlatformName() {
    println("Running on ${platformName()}")
}

Implement the actual call on each platform:

// androidMain
actual fun platformName(): String = "Android"

// iosMain
actual fun platformName(): String = "iOS"

3. Dependency Injection with Koin

Dependency Injection (DI) helps manage dependencies and improve the testability of your code. Koin is a lightweight DI framework fully compatible with Kotlin Multiplatform.

First, add Koin dependencies to your build.gradle.kts:

dependencies {
    implementation("io.insert-koin:koin-core:3.5.3")
    implementation("io.insert-koin:koin-android:3.5.3")  // For Android only
}

Define a Koin module:

import org.koin.dsl.module

val appModule = module {
    single { Database() }  // Singleton instance
    factory { UserRepository(get()) }  // Factory instance
}

Then, start Koin in your platform-specific code:

// androidMain
import org.koin.android.ext.koin.androidContext
import org.koin.core.context.startKoin

class MyApplication : Application() {
    override fun onCreate() {
        super.onCreate()
        startKoin {
            androidContext(this@MyApplication)
            modules(appModule)
        }
    }
}

Finally, inject the dependencies in your composables:

import androidx.compose.runtime.Composable
import org.koin.compose.koinInject

@Composable
fun UserProfileScreen() {
    val userRepository: UserRepository = koinInject()
    // Use userRepository
}

4. Platform-Specific Styling and Theming

Achieving a consistent look and feel while accommodating platform-specific UI/UX guidelines is vital. Compose Multiplatform enables the implementation of conditional styling or platform-specific theming strategies.

Define common and platform-specific themes:

// commonMain
import androidx.compose.runtime.Composable

expect @Composable fun PlatformSpecificTheme(content: @Composable () -> Unit)

// androidMain
import androidx.compose.material.MaterialTheme
import androidx.compose.runtime.Composable

actual @Composable fun PlatformSpecificTheme(content: @Composable () -> Unit) {
    MaterialTheme {
        content()
    }
}

// iosMain
import androidx.compose.ui.window.ComposeUIViewController

fun MainViewController() = ComposeUIViewController {
    PlatformSpecificTheme {
        // Your app's UI
    }
}

5. Concurrent Programming and Coroutines

Kotlin Coroutines provide a structured and efficient approach to managing concurrency, making them essential for handling background tasks, asynchronous operations, and managing state across platforms. Compose Multiplatform seamlessly supports coroutines.

Here is how to define and use a coroutine in shared code:

import kotlinx.coroutines.*

// Function to fetch data asynchronously
suspend fun fetchData(): String {
    delay(1000)  // Simulate network delay
    return "Data fetched successfully"
}

// Use coroutine scope for managing the coroutine lifecycle
val coroutineScope = CoroutineScope(Dispatchers.Default)

fun executeDataFetch() {
    coroutineScope.launch {
        val result = fetchData()
        println("Result: $result")
    }
}

And within a Composable:

import androidx.compose.runtime.*

@Composable
fun DataDisplay() {
    var data by remember { mutableStateOf("Loading...") }

    LaunchedEffect(Unit) {
        data = fetchData()
    }

    Text("Data: $data")
}

Conclusion

Compose Multiplatform opens a vast array of possibilities for creating versatile and maintainable applications. By leveraging its advanced features, such as custom renderers, interoperability with native code, dependency injection with Koin, platform-specific styling, and concurrent programming with coroutines, you can create compelling cross-platform applications that share a significant portion of their codebase while providing a tailored experience for each platform. Understanding and utilizing these features is crucial for developers aiming to maximize efficiency and deliver exceptional user experiences across various operating systems.