Kotlin iOS Development

Introduction

Kotlin Multiplatform (KMP) allows developers to share code between different platforms, including iOS. This enables you to write business logic, networking, data processing, and other non-UI components once in Kotlin and reuse them across both Android and iOS applications.

In this guide, you'll learn how to set up Kotlin for iOS development, understand the architecture of a KMP project targeting iOS, and see practical examples of code sharing between platforms.

Prerequisites

Before diving into Kotlin iOS development, ensure you have:

macOS (required for iOS development)
Android Studio
Xcode
JDK 11 or newer
Kotlin Multiplatform plugin

Setting Up Your First Kotlin iOS Project

Step 1: Create a new Kotlin Multiplatform Mobile project

In Android Studio:

Go to File > New > New Project
Select "Kotlin Multiplatform App" and click Next
Configure your project with:
- Name: MyKmpApp
- Package name: com.example.mykmpapp
- Shared Module name: shared

kotlin
// The generated project structure will look like this:
// androidApp/ - Android application
// iosApp/     - iOS application
// shared/     - Shared Kotlin code

Step 2: Understanding the Project Structure

The generated project consists of three main modules:

androidApp: Android-specific code (Activities, Fragments, etc.)
iosApp: iOS application written in Swift using UIKit or SwiftUI
shared: Common Kotlin code that will be shared between platforms

Let's look at the shared module's build script:

kotlin
plugins {
    kotlin("multiplatform")
    id("com.android.library")
}

kotlin {
    android()
    
    listOf(
        iosX64(),
        iosArm64(),
        iosSimulatorArm64()
    ).forEach {
        it.binaries.framework {
            baseName = "shared"
        }
    }
    
    sourceSets {
        val commonMain by getting {
            dependencies {
                // Common dependencies go here
            }
        }
        
        val androidMain by getting {
            dependencies {
                // Android-specific dependencies
            }
        }
        
        val iosMain by getting {
            dependencies {
                // iOS-specific dependencies
            }
        }
    }
}

This build script configures three main source sets:

commonMain: Contains code shared between Android and iOS
androidMain: Contains Android-specific implementations
iosMain: Contains iOS-specific implementations

Writing Shared Code for iOS

Creating a Simple Shared Data Class

Let's create a simple data class in the shared module:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/data/User.kt
package com.example.mykmpapp.data

data class User(
    val id: Int,
    val username: String,
    val email: String
)

Creating a Shared Repository

Now, let's create a repository that could be used by both platforms:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/repository/UserRepository.kt
package com.example.mykmpapp.repository

import com.example.mykmpapp.data.User

interface UserRepository {
    fun getUsers(): List<User>
    fun getUserById(id: Int): User?
}

class UserRepositoryImpl : UserRepository {
    // A simple in-memory database for demonstration
    private val users = listOf(
        User(1, "john_doe", "[email protected]"),
        User(2, "jane_doe", "[email protected]")
    )
    
    override fun getUsers(): List<User> {
        return users
    }
    
    override fun getUserById(id: Int): User? {
        return users.find { it.id == id }
    }
}

Platform-Specific Implementations

Sometimes you need to implement platform-specific code. You can use the expect/actual pattern for this:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/platform/Platform.kt
package com.example.mykmpapp.platform

expect class Platform() {
    val platform: String
}

// In shared/src/androidMain/kotlin/com/example/mykmpapp/platform/Platform.kt
package com.example.mykmpapp.platform

actual class Platform actual constructor() {
    actual val platform: String = "Android ${android.os.Build.VERSION.SDK_INT}"
}

// In shared/src/iosMain/kotlin/com/example/mykmpapp/platform/Platform.kt
package com.example.mykmpapp.platform

import platform.UIKit.UIDevice

actual class Platform actual constructor() {
    actual val platform: String = UIDevice.currentDevice.systemName() + " " + UIDevice.currentDevice.systemVersion
}

Using Kotlin Code from Swift

Exposing a Shared Module API

To make your Kotlin code easily accessible from Swift, it's helpful to create a central entry point:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/Greeting.kt
package com.example.mykmpapp

import com.example.mykmpapp.platform.Platform
import com.example.mykmpapp.repository.UserRepository
import com.example.mykmpapp.repository.UserRepositoryImpl

class Greeting {
    private val platform: Platform = Platform()
    private val userRepository: UserRepository = UserRepositoryImpl()
    
    fun greeting(): String {
        return "Hello from ${platform.platform}!"
    }
    
    fun getUsers() = userRepository.getUsers()
    
    fun getUserById(id: Int) = userRepository.getUserById(id)
}

Accessing Kotlin Code in Swift

In your iOS application, you can now use the shared code like this:

swift
// In iosApp/iosApp/ContentView.swift
import SwiftUI
import shared

struct ContentView: View {
    private let greeting = Greeting()
    @State private var users: [User] = []
    
    var body: some View {
        VStack {
            Text(greeting.greeting())
                .padding()
            
            List(users, id: \.id) { user in
                VStack(alignment: .leading) {
                    Text(user.username)
                        .font(.headline)
                    Text(user.email)
                        .font(.subheadline)
                }
            }
            .onAppear {
                users = greeting.getUsers()
            }
        }
    }
}

Working with Concurrency

Handling asynchronous operations across platforms requires special attention. KMP provides Dispatchers to manage threading:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/repository/RemoteUserRepository.kt
package com.example.mykmpapp.repository

import com.example.mykmpapp.data.User
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.withContext

class RemoteUserRepository {
    suspend fun fetchUsers(): List<User> = withContext(Dispatchers.Default) {
        // Simulating network request delay
        kotlinx.coroutines.delay(1000)
        
        // Return mock data
        listOf(
            User(1, "remote_john", "[email protected]"),
            User(2, "remote_jane", "[email protected]")
        )
    }
}

To expose this functionality to iOS, we need to handle the coroutines properly:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/UserService.kt
package com.example.mykmpapp

import com.example.mykmpapp.data.User
import com.example.mykmpapp.repository.RemoteUserRepository
import kotlinx.coroutines.MainScope
import kotlinx.coroutines.launch

class UserService {
    private val repository = RemoteUserRepository()
    private val scope = MainScope()
    
    fun fetchUsers(callback: (List<User>) -> Unit) {
        scope.launch {
            val users = repository.fetchUsers()
            callback(users)
        }
    }
}

In Swift, you can use this like:

swift
// In iosApp/iosApp/RemoteUsersView.swift
import SwiftUI
import shared

struct RemoteUsersView: View {
    private let userService = UserService()
    @State private var users: [User] = []
    @State private var isLoading = false
    
    var body: some View {
        VStack {
            if isLoading {
                ProgressView()
            } else {
                List(users, id: \.id) { user in
                    VStack(alignment: .leading) {
                        Text(user.username)
                            .font(.headline)
                        Text(user.email)
                            .font(.subheadline)
                    }
                }
            }
        }
        .onAppear(perform: loadUsers)
    }
    
    private func loadUsers() {
        isLoading = true
        userService.fetchUsers { fetchedUsers in
            self.users = fetchedUsers
            self.isLoading = false
        }
    }
}

Integrating with iOS UI Frameworks

Using Kotlin with SwiftUI

SwiftUI works well with Kotlin Multiplatform. You can create view models in Kotlin and observe them in SwiftUI:

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/viewmodel/UserViewModel.kt
package com.example.mykmpapp.viewmodel

import com.example.mykmpapp.data.User
import com.example.mykmpapp.repository.UserRepository
import com.example.mykmpapp.repository.UserRepositoryImpl
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.asStateFlow
import kotlinx.coroutines.launch

class UserViewModel {
    private val repository: UserRepository = UserRepositoryImpl()
    private val _users = MutableStateFlow<List<User>>(emptyList())
    val users: StateFlow<List<User>> = _users.asStateFlow()
    
    fun loadUsers() {
        val loadedUsers = repository.getUsers()
        _users.value = loadedUsers
    }
}

To use this with SwiftUI, you can create a wrapper:

swift
// In iosApp/iosApp/UserListViewModel.swift
import Foundation
import shared
import Combine

class UserListViewModel: ObservableObject {
    private let viewModel = UserViewModel()
    @Published var users: [User] = []
    
    init() {
        // Set up observation
        // Note: This is simplified. In a real app, you'd use a library 
        // like KMPNativeCoroutinesCombine for proper Flow observation
    }
    
    func loadUsers() {
        viewModel.loadUsers()
        users = viewModel.users.value
    }
}

Best Practices for Kotlin iOS Development

Keep platform-specific code minimal: Focus on sharing business logic, networking, and data manipulation.
Use proper architecture: Follow Clean Architecture or MVVM to make code more maintainable and shareable.
Plan for testability: Write unit tests for your shared code.
Handle memory appropriately: Be aware of memory management differences between Kotlin and Swift.
Expose simple interfaces: Make your shared API clean and straightforward for Swift to consume.
Use CocoaPods integration: For larger projects, use CocoaPods to manage your Kotlin dependencies.

Real-world Example: A Weather App

Let's build a simple weather app that demonstrates the concepts we've learned.

Shared Data Model

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/weather/WeatherData.kt
package com.example.mykmpapp.weather

data class WeatherData(
    val city: String,
    val temperature: Double,
    val description: String,
    val humidity: Int
)

Shared Repository and API

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/weather/WeatherRepository.kt
package com.example.mykmpapp.weather

interface WeatherRepository {
    suspend fun getWeatherForCity(cityName: String): WeatherData
}

class MockWeatherRepository : WeatherRepository {
    override suspend fun getWeatherForCity(cityName: String): WeatherData {
        // Simulate API delay
        kotlinx.coroutines.delay(1000)
        
        return WeatherData(
            city = cityName,
            temperature = 20.0 + (-5..5).random(),
            description = listOf("Sunny", "Cloudy", "Rainy", "Windy").random(),
            humidity = (30..90).random()
        )
    }
}

Weather Service for iOS

kotlin
// In shared/src/commonMain/kotlin/com/example/mykmpapp/weather/WeatherService.kt
package com.example.mykmpapp.weather

import kotlinx.coroutines.MainScope
import kotlinx.coroutines.launch

class WeatherService {
    private val repository: WeatherRepository = MockWeatherRepository()
    private val scope = MainScope()
    
    fun getWeatherForCity(city: String, callback: (WeatherData) -> Unit, onError: (String) -> Unit) {
        scope.launch {
            try {
                val weatherData = repository.getWeatherForCity(city)
                callback(weatherData)
            } catch (e: Exception) {
                onError(e.message ?: "An unknown error occurred")
            }
        }
    }
}

Using the Weather Service in SwiftUI

swift
// In iosApp/iosApp/WeatherView.swift
import SwiftUI
import shared

struct WeatherView: View {
    private let weatherService = WeatherService()
    @State private var city: String = "London"
    @State private var weatherData: WeatherData?
    @State private var isLoading = false
    @State private var errorMessage: String?
    
    var body: some View {
        VStack(spacing: 20) {
            TextField("City", text: $city)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding()
            
            Button("Get Weather") {
                fetchWeather()
            }
            .padding()
            .disabled(isLoading)
            
            if isLoading {
                ProgressView()
            } else if let error = errorMessage {
                Text(error)
                    .foregroundColor(.red)
            } else if let weather = weatherData {
                WeatherDetailView(weather: weather)
            }
            
            Spacer()
        }
        .padding()
    }
    
    private func fetchWeather() {
        isLoading = true
        errorMessage = nil
        
        weatherService.getWeatherForCity(
            city: city,
            callback: { data in
                weatherData = data
                isLoading = false
            },
            onError: { error in
                errorMessage = error
                isLoading = false
            }
        )
    }
}

struct WeatherDetailView: View {
    let weather: WeatherData
    
    var body: some View {
        VStack(alignment: .leading, spacing: 10) {
            Text(weather.city)
                .font(.largeTitle)
            
            HStack {
                Text("\(Int(weather.temperature))°C")
                    .font(.system(size: 50))
                
                Spacer()
                
                VStack(alignment: .trailing) {
                    Text(weather.description)
                        .font(.title2)
                    Text("Humidity: \(weather.humidity)%")
                }
            }
        }
        .padding()
        .background(Color.blue.opacity(0.1))
        .cornerRadius(10)
        .padding()
    }
}

Summary

Kotlin Multiplatform offers a powerful way to share code between Android and iOS platforms, allowing you to write business logic, networking, and data processing once and reuse it across both platforms. By leveraging KMP:

You can save development time by writing shared code once
Maintain consistency between platforms
Keep platform-specific code where it belongs
Leverage existing Swift and iOS frameworks

In this guide, we've covered:

Setting up a Kotlin Multiplatform project
Understanding the project structure
Writing shared code with the expect/actual pattern
Exposing Kotlin code to Swift
Handling concurrency across platforms
Integrating with iOS UI frameworks
Building a real-world weather app example

Additional Resources

Exercises

Basic: Create a simple to-do list app that shares data models and repository logic between Android and iOS.
Intermediate: Extend the weather app to include a 5-day forecast and location search.
Advanced: Implement a note-taking app that syncs with a backend API, sharing all network code, data models, and business logic between platforms.

If you spot any mistakes on this website, please let me know at [email protected]. I’d greatly appreciate your feedback! :)

Introduction​

Prerequisites​

Setting Up Your First Kotlin iOS Project​

Step 1: Create a new Kotlin Multiplatform Mobile project​

Step 2: Understanding the Project Structure​

Writing Shared Code for iOS​

Creating a Simple Shared Data Class​

Creating a Shared Repository​

Platform-Specific Implementations​

Using Kotlin Code from Swift​

Exposing a Shared Module API​

Accessing Kotlin Code in Swift​

Working with Concurrency​

Integrating with iOS UI Frameworks​

Using Kotlin with SwiftUI​

Best Practices for Kotlin iOS Development​

Real-world Example: A Weather App​

Shared Data Model​

Shared Repository and API​

Weather Service for iOS​

Using the Weather Service in SwiftUI​

Summary​

Additional Resources​

Exercises​