Networking

Most apps fetch data from a REST API. The common stack is Retrofit (HTTP) + kotlinx.serialization (JSON). Ktor’s client is a multiplatform alternative.

Serialization

Annotate models with @Serializable. The kotlinx.serialization compiler plugin generates the parsing code:

// build.gradle.kts
plugins { id("org.jetbrains.kotlin.plugin.serialization") }
dependencies { implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.7.x") }

@Serializable
data class ArticleDto(val id: Int, val title: String, val body: String)

val json = Json { ignoreUnknownKeys = true }
val articles: List<ArticleDto> = json.decodeFromString(responseBody)

The example app uses kotlinx.serialization to parse a JSON payload — that part is CI-tested.

Retrofit

Declare the API as an interface with suspend functions:

interface ArticleApi {
    @GET("articles")
    suspend fun articles(): List<ArticleDto>

    @GET("articles/{id}")
    suspend fun article(@Path("id") id: Int): ArticleDto
}

Build it with the kotlinx.serialization converter:

val retrofit = Retrofit.Builder()
    .baseUrl("https://example.com/api/")
    .addConverterFactory(json.asConverterFactory("application/json".toMediaType()))
    .build()

val api: ArticleApi = retrofit.create(ArticleApi::class.java)

Error handling and DTO → domain mapping

Wrap calls and map transport models (ArticleDto) to domain models (Article) in the repository:

class RemoteArticleRepository(private val api: ArticleApi) : ArticleRepository {
    override suspend fun articles(): List<Article> =
        api.articles().map { Article(it.id, it.title, it.body) }
}

// at the call site
val result = runCatching { repository.articles() }
result.onFailure { /* show error state */ }

Run network calls off the main thread — Dispatchers.IO (or let Retrofit’s suspend functions handle it, which they do).

Ktor client (alternative)

val client = HttpClient(CIO) {
    install(ContentNegotiation) { json() }
}
val articles: List<ArticleDto> = client.get("https://example.com/api/articles").body()

Exercises

1. Define a @Serializable UserDto and parse a JSON array of users with Json.decodeFromString.

2. Write a Retrofit ApiService interface with a suspend GET endpoint.

---

Previous: Local Persistence with Room · Next: Dependency Injection with Hilt