Compose Multiplatform: Comprehensive Overview in Jetpack Compose

Jetpack Compose has revolutionized Android UI development with its declarative approach. Now, the introduction of Compose Multiplatform takes this innovation further by allowing developers to build UIs that target multiple platforms, including Android, iOS, desktop (JVM), and web (using WebAssembly). This comprehensive guide provides an in-depth Compose Multiplatform Overview in Jetpack Compose, covering its benefits, setup, architecture, and practical examples.

What is Compose Multiplatform?

Compose Multiplatform is a Kotlin-based UI framework developed by JetBrains that leverages the Jetpack Compose declarative UI paradigm to enable code sharing across multiple platforms. It allows you to write UI code once and deploy it on Android, iOS, desktop (Windows, macOS, Linux), and web (via WebAssembly).

Benefits of Using Compose Multiplatform

• Code Sharing: Substantially reduces code duplication by sharing UI code across platforms.
• Consistent UI: Ensures a consistent look and feel across different devices and operating systems.
• Declarative UI: Uses Jetpack Compose’s modern, declarative UI approach for easier development and maintenance.
• Kotlin Native: Employs Kotlin Native for native performance on iOS and other platforms.
• Faster Development: Accelerates the development process with hot reloading and live previews.

Setting Up a Compose Multiplatform Project

To start with Compose Multiplatform, you need to set up a new project using Kotlin Multiplatform Gradle plugin.

Prerequisites

• Kotlin: Ensure you have Kotlin 1.9.20 or later installed.
• JDK: Java Development Kit (JDK) version 11 or later.
• Android Studio: Latest version of Android Studio for Android development.
• Xcode: Xcode for iOS development.

Step-by-Step Setup

Follow these steps to create a Compose Multiplatform project:

Step 1: Create a New Kotlin Multiplatform Project

Use the Kotlin Multiplatform wizard in IntelliJ IDEA or Android Studio.

• Open IntelliJ IDEA or Android Studio.
• Select “New Project.”
• Choose “Kotlin” from the left panel.
• Select “Compose Multiplatform” project template.
• Configure the project settings (name, location, etc.) and click “Create.”

Step 2: Project Structure

The project structure includes several modules:

- shared  (Common code for all platforms)
- androidApp (Android-specific code)
- iosApp (iOS-specific code)
- desktopApp (Desktop-specific code)
- webApp (Web-specific code)

Step 3: Add Dependencies

In the shared/build.gradle.kts file, add necessary dependencies:

kotlin {
    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation(compose.components.core)
                implementation(compose.components.foundation)
                implementation(compose.components.material)
            }
        }
        val androidMain by getting {
            dependencies {
                implementation("androidx.appcompat:appcompat:1.6.1")
                implementation("androidx.core:core-ktx:1.12.0")
                implementation("androidx.lifecycle:lifecycle-runtime-ktx:2.6.2")
                implementation("androidx.activity:activity-compose:1.8.2")
                implementation(compose.components.uiToolingPreview)
                debugImplementation(compose.components.uiTooling)
            }
        }
        val iosMain by getting {
            dependencies {
                // iOS specific dependencies
            }
        }
        val desktopMain by getting {
            dependencies {
                 implementation(compose.desktop.components.ui)
            }
        }
        val webMain by getting {
            dependencies {
                implementation(compose.web.components.core)
            }
        }
    }
}

Compose Multiplatform Architecture

The architecture of Compose Multiplatform is designed to maximize code sharing while allowing platform-specific customizations.

Common Module (shared)

The shared module contains the core UI logic, composable functions, and business logic shared across all platforms.

// shared/src/commonMain/kotlin/App.kt
import androidx.compose.material.Text
import androidx.compose.runtime.Composable

@Composable
fun App() {
    Text("Hello, Compose Multiplatform!")
}

Platform-Specific Modules

Each platform-specific module (androidApp, iosApp, desktopApp, webApp) uses the code from the shared module and provides platform-specific implementations and adaptations.

// androidApp/src/main/java/MainActivity.kt
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            App() // Use the shared composable
        }
    }
}

Basic Compose Multiplatform Example

Let’s create a simple counter app that runs on Android and desktop.

Step 1: Define the UI in the shared Module

// shared/src/commonMain/kotlin/CounterApp.kt
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.Spacer
import androidx.compose.foundation.layout.width
import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.getValue
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.unit.dp

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

    Column(horizontalAlignment = Alignment.CenterHorizontally) {
        Text("Count: $count")
        Row {
            Button(onClick = { count-- }) {
                Text("-")
            }
            Spacer(width = 16.dp)
            Button(onClick = { count++ }) {
                Text("+")
            }
        }
    }
}

Step 2: Implement in androidApp

// androidApp/src/main/java/MainActivity.kt
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            CounterApp() // Use the shared composable
        }
    }
}

Step 3: Implement in desktopApp

// desktopApp/src/jvmMain/kotlin/Main.kt
import androidx.compose.ui.window.Window
import androidx.compose.ui.window.application

fun main() = application {
    Window(title = "Compose Desktop Counter") {
        CounterApp() // Use the shared composable
    }
}

Advanced Features and Considerations

Platform-Specific UI

Use expect/actual declarations to provide platform-specific implementations for UI elements that require native components.

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

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

// desktopApp/src/jvmMain/kotlin/PlatformSpecific.kt
actual fun platformName(): String = "Desktop"

State Management

Use Kotlin’s State and MutableState or state management libraries like Decompose or Koin for managing the app’s state in a multiplatform environment.

Navigation

Implement navigation using libraries like Decompose, which provides a multiplatform navigation solution.

Conclusion

Compose Multiplatform brings the power of Jetpack Compose to a wide range of platforms, enabling developers to write UIs once and deploy them everywhere. By understanding the architecture, setup, and basic implementation, developers can create efficient, consistent, and maintainable multiplatform applications. This Compose Multiplatform Overview in Jetpack Compose has provided a comprehensive guide to get started with this exciting technology.