Compose for iOS Rendering in Jetpack Compose: A Comprehensive Guide

Jetpack Compose, Google’s modern UI toolkit for building native Android applications, has revolutionized Android UI development by offering a declarative approach to UI creation. A lesser-known but powerful capability is its capacity to render UI elements for iOS through Kotlin Multiplatform (KMP). This allows developers to share a significant portion of their UI code between Android and iOS platforms. In this blog post, we’ll delve into how you can leverage Jetpack Compose for iOS rendering within a Jetpack Compose project.

Understanding Kotlin Multiplatform (KMP)

Before diving into the specifics, let’s understand Kotlin Multiplatform (KMP). KMP is a Kotlin feature that allows developers to write code that can be compiled and run on multiple platforms, including Android, iOS, JavaScript, and more. The core idea is to maximize code reuse while still allowing platform-specific code where necessary.

Why Use Jetpack Compose for iOS Rendering?

• Code Reuse: Share UI logic and design between Android and iOS.
• Consistency: Ensure a consistent look and feel across platforms.
• Efficiency: Reduce development time and effort by writing code once and deploying it on multiple platforms.

Setting Up a Kotlin Multiplatform Project with Jetpack Compose

To use Jetpack Compose for iOS rendering, you’ll need to set up a Kotlin Multiplatform project.

Step 1: Create a New Kotlin Multiplatform Project

You can create a new KMP project using IntelliJ IDEA or Android Studio with the Kotlin Multiplatform plugin installed. Choose the “Kotlin Multiplatform App” template when creating the project.

Step 2: Configure Gradle Files

Ensure that your build.gradle.kts files are correctly configured. You’ll need to define the targets for both Android and iOS. Here’s an example:

plugins {
    kotlin("multiplatform") version "1.9.21"
    id("com.android.application")
    id("org.jetbrains.compose") version "1.6.1"
}

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

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

kotlin {
    androidTarget {
        compilations.all {
            kotlinOptions {
                jvmTarget = "1.8"
            }
        }
    }
    
    iosArm64()
    iosX64()
    iosSimulatorArm64()

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation(compose.runtime)
                implementation(compose.foundation)
                implementation(compose.material)
            }
        }
        val androidMain by getting {
            dependencies {
                implementation("androidx.core:core-ktx:1.9.0")
                implementation("androidx.appcompat:appcompat:1.6.1")
                implementation("com.google.android.material:material:1.10.0")
            }
        }
        val iosX64Main by getting
        val iosArm64Main by getting
        val iosSimulatorArm64Main by getting
        val iosMain by getting {
            dependsOn(commonMain)
        }
    }
}

android {
    namespace = "org.example.myapplication"
    compileSdk = 34

    defaultConfig {
        applicationId = "org.example.myapplication"
        minSdk = 24
        targetSdk = 34
        versionCode = 1
        versionName = "1.0"

        testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            isMinifyEnabled = false
            proguardFiles(getDefaultProguardFile("proguard-android-optimize.txt"), "proguard-rules.pro")
        }
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
    }
    buildFeatures {
        compose = true
    }
    composeOptions {
        kotlinCompilerExtensionVersion = "1.6.1"
    }
    packaging {
        resources {
            excludes += "/META-INF/{AL2.0,LGPL2.1}"
        }
    }
}

Step 3: Define Shared UI Components

In the commonMain source set, define your Jetpack Compose UI components that you want to share between Android and iOS. Here’s an example of a simple composable function:

// in commonMain
import androidx.compose.material.Text
import androidx.compose.runtime.Composable

@Composable
fun Greeting(name: String) {
    Text("Hello, $name!")
}

Rendering Jetpack Compose on iOS

To render Jetpack Compose UI on iOS, you’ll need to use a library like Compose for iOS (previously known as Jetpack Compose for iOS), which leverages the Skia graphics engine. Note that while there are advancements, direct Compose interoperability remains a complex and evolving area. As of late 2024, the practical approach focuses more on shared logic and rendering custom components using platform-specific APIs.

Step 1: Set Up Skia Integration

Include the necessary Skia bindings for iOS. You’ll typically need to use CocoaPods for this setup. Create a Podfile in your iosApp directory:

platform :ios, '14.0'
require_relative '../shared/build/cocoapods/pods.rb'

target :ComposeForIosExample do
  use_frameworks!
  
  # Pods for ComposeForIosExample
  pod 'skiko'
  pod 'skiko-resources'
  pod 'skiko-awt-resources'

  pod 'MultiplatformSettings', :git => 'https://github.com/russhwolf/multiplatform-settings', :tag => '1.0.0'
  pod 'Kermit', :git => 'https://github.com/touchlab/Kermit', :tag => '1.2.2'

  kotlin_multiplatform_setup()
end

Then, run pod install in the iosApp directory.

Step 2: Create iOS Views

Create a native iOS UIView that hosts the Jetpack Compose content. This involves using Skia to draw the Compose UI elements. This part usually involves quite a bit of custom code. Here’s a basic illustration:

import UIKit
import shared
import Skia

class ComposeViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let skiaView = SkiaUIView()
        skiaView.backgroundColor = .white

        self.view.addSubview(skiaView)

        skiaView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            skiaView.topAnchor.constraint(equalTo: self.view.topAnchor),
            skiaView.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),
            skiaView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor),
            skiaView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor)
        ])
    }
}

class SkiaUIView : UIView {
    override func draw(_ rect: CGRect) {
        guard let context = UIGraphicsGetCurrentContext() else { return }
        
        // This is a placeholder. You need to properly integrate Skia and draw
        // the Compose UI elements using Skia rendering APIs.
        UIColor.red.setFill()
        context.fill(rect)
        
        // Placeholder text drawing using UIKit (replace with Skia drawing)
        let text = "Hello from Compose on iOS!"
        let font = UIFont.systemFont(ofSize: 20)
        let attributes = [NSAttributedString.Key.font: font, NSAttributedString.Key.foregroundColor: UIColor.black]
        let textSize = text.size(withAttributes: attributes)
        
        let x = (rect.width - textSize.width) / 2
        let y = (rect.height - textSize.height) / 2
        
        text.draw(at: CGPoint(x: x, y: y), withAttributes: attributes)
    }
}

Note: The actual rendering of Compose UI with Skia involves a complex integration and is typically accomplished using libraries that handle the Skia surface management and rendering loop.

Step 3: Integrate Compose UI Logic

In your iOS code, you can now call the shared composable functions defined in your commonMain source set. Remember that the UI drawing part has to be adapted to the Skia environment or use the rendered bitmaps provided by Kotlin Compose.

import shared
import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let label = UILabel()
        label.text = CommonKt.createHelloWorldText() // An example function defined in commonMain
        label.textAlignment = .center
        label.frame = CGRect(x: 0, y: 0, width: 300, height: 50)
        label.center = view.center
        
        view.addSubview(label)
    }
}

Practical Considerations and Challenges

While sharing code between platforms offers many benefits, keep these points in mind:

• UI/UX Consistency: Ensure your shared UI components adhere to the platform-specific UI/UX guidelines.
• Performance: Optimize your Compose code for performance on both Android and iOS.
• Third-party Libraries: Some Android-specific libraries might not be directly available on iOS. Use KMP-compatible libraries or create platform-specific implementations.

Alternatives and Best Practices

• Declarative UI Approach: Embracing a declarative UI paradigm will ease your cross-platform implementations.
• Platform-Specific Components: Use conditional compilation (expect/actual) to create platform-specific components when needed.
• Incremental Migration: Gradually migrate your existing Android app to KMP instead of a complete rewrite.

Conclusion

Leveraging Jetpack Compose for iOS rendering through Kotlin Multiplatform opens exciting possibilities for cross-platform development. By sharing UI code, developers can reduce development time, ensure consistency, and improve efficiency. Although setting up and integrating Compose for iOS requires careful configuration and consideration of platform-specific nuances, the benefits of code reuse and a unified codebase can be significant. As the tooling and libraries around Kotlin Multiplatform and Jetpack Compose continue to evolve, expect even greater ease and flexibility in sharing UI code across platforms.