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Networking with Retrofit

June 02, 2026 • 1 min read

Most apps talk to a server. Retrofit turns a REST API into a simple Kotlin interface and (with a converter) parses JSON into your data classes.

Define the API

interface ApiService {
    @GET("users/{id}")
    suspend fun getUser(@Path("id") id: Int): User

    @GET("products")
    suspend fun products(@Query("page") page: Int): List<Product>
}

Build Retrofit

val api = Retrofit.Builder()
    .baseUrl("https://api.example.com/")
    .addConverterFactory(GsonConverterFactory.create())
    .build()
    .create(ApiService::class.java)

Call it safely

viewModelScope.launch {
    try {
        val user = api.getUser(42)   // suspend, off main thread
        _state.value = UiState.Success(user)
    } catch (e: Exception) {
        _state.value = UiState.Error(e.message)
    }
}
Common mistake: Add the INTERNET permission in the manifest, and always wrap network calls in try/catch — networks fail often.

Summary

Retrofit + a JSON converter makes API calls declarative. Call suspend functions from a coroutine and handle errors gracefully.