Retrofit CallAdapters

Retrofit defines CallAdapter to transform its internal Call into whatever return type you want. CallAdapter is about adapting return types at runtime.

Retrofit supports Kotlin suspend functions natively.

Retrofit still uses external adapters for:

RxJava

Java 8 CompletableFuture

Guava

Those are still implemented as CallAdapter.Factory extensions you add manually.

Docs and Android style guidelines favor Flow/suspend at the repository layer.

Example

interface ApiService {
    @GET("users")
    suspend fun getUsers(): List
}

No Call and no adapter needed for coroutines.

Build Retrofit

val retrofit = Retrofit.Builder()
    .baseUrl("https://example.com/")
    .addConverterFactory(GsonConverterFactory.create())
    .build()

RxJava Example

val retrofit = Retrofit.Builder()
    .baseUrl("https://example.com/")
    .addCallAdapterFactory(RxJava3CallAdapterFactory.create())
    .addConverterFactory(GsonConverterFactory.create())
    .build()