Kotlin Multiplatform: Data Serialization and Ktor Networking Made Simple

Efficient data handling and networking are the backbone of modern cross-platform apps. In Kotlin Multiplatform (KMP), tools like kotlinx.serialization and Ktor Client streamline these tasks, enabling seamless data exchange across Android, iOS, and web. This chapter dives into serialization, HTTP requests, and asynchronous operations in KMP—all while keeping your codebase clean and platform-agnostic.

1. Serialization with kotlinx.serialization

Kotlin’s kotlinx.serialization library simplifies converting objects to formats like JSON, ProtoBuf, or XML.

Setup in KMP

Add these dependencies to build.gradle.kts:

// Shared module  
plugins {  
    kotlin("plugin.serialization") version "1.9.20"  
}  

dependencies {  
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0")  
}  
Code language: JavaScript (javascript)

Define a Serializable Class

import kotlinx.serialization.Serializable  

@Serializable  
data class User(val id: Int, val name: String, val email: String)  
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Convert Between JSON and Objects

Serialize to JSON:val user = User(1, "Alex", "alex@example.com") val json = Json.encodeToString(user) // Output: {"id":1,"name":"Alex","email":"alex@example.com"}
Deserialize from JSON:val jsonData = """{"id":1,"name":"Alex","email":"alex@example.com"}""" val userObj = Json.decodeFromString<User>(jsonData)

Handling Missing Fields:

val json = Json { ignoreUnknownKeys = true } // Skips unknown keys during deserialization  
Code language: JavaScript (javascript)

2. Networking with Ktor Client

Ktor is a multiplatform HTTP client for handling API requests.

Setup Ktor in KMP

Add platform-specific dependencies:

// Shared module  
dependencies {  
    implementation("io.ktor:ktor-client-core:2.3.6")  
    implementation("io.ktor:ktor-client-content-negotiation:2.3.6")  
    implementation("io.ktor:ktor-serialization-kotlinx-json:2.3.6")  
}  

// Platform modules  
androidMain {  
    implementation("io.ktor:ktor-client-okhttp:2.3.6")  
}  
iosMain {  
    implementation("io.ktor:ktor-client-darwin:2.3.6")  
}  
jsMain {  
    implementation("io.ktor:ktor-client-js:2.3.6")  
}  
Code language: JavaScript (javascript)

Create an HTTP Client

import io.ktor.client.*  
import io.ktor.client.plugins.contentnegotiation.*  

val httpClient = HttpClient {  
    install(ContentNegotiation) {  
        json(Json { ignoreUnknownKeys = true })  
    }  
}  
Code language: JavaScript (javascript)

Fetch Data from an API

suspend fun fetchUser(): User {  
    return httpClient.get("https://api.example.com/user/1").body()  
}  

// Usage  
suspend fun printUser() {  
    val user = fetchUser()  
    println(user) // User(id=1, name=Alex, email=alex@example.com)  
}  
Code language: JavaScript (javascript)

Error Handling

suspend fun fetchUserSafe(): User? {  
    return try {  
        httpClient.get("https://api.example.com/user/1").body()  
    } catch (e: ClientRequestException) {  
        println("Error: ${e.response.status}")  
        null  
    } catch (e: ServerResponseException) {  
        println("Server error: ${e.response.status}")  
        null  
    }  
}  
Code language: JavaScript (javascript)

3. Working with Data Formats

KMP supports multiple formats via kotlinx.serialization:

Format	Use Case	Dependency
JSON	Web APIs	`kotlinx-serialization-json`
ProtoBuf	High-performance systems	`kotlinx-serialization-protobuf`
CBOR	Compact binary data	`kotlinx-serialization-cbor`

ProtoBuf Example

// Add dependency  
implementation("org.jetbrains.kotlinx:kotlinx-serialization-protobuf:1.6.0")  

// Serialize  
val data = SensorData(1675038210, 24.5)  
val bytes: ByteArray = ProtoBuf.encodeToByteArray(data)  

// Deserialize  
val decodedData = ProtoBuf.decodeFromByteArray<SensorData>(bytes)  
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4. Asynchronous Operations with Coroutines

Kotlin Coroutines handle async tasks without blocking the main thread.

Parallel Requests

suspend fun fetchMultipleUsers(): List<User> = coroutineScope {  
    val user1 = async { fetchUser() }  
    val user2 = async { fetchUser() }  
    listOf(user1.await(), user2.await())  
}  
Code language: JavaScript (javascript)