Retrofit Suspend Kotlin Result Adapter

A Retrofit 2 CallAdapter.Factory for Kotlin suspend functions that use the standard library kotlin.Result as return value. The call adapter catches network failures / deserialization errors so that there is no need to try catch suspend functions at call sites.

Many useful methods are available on the Result type. Once Kotlin allows null-safety operators ?., ?: and !! on Result, the ergonomics of using this type will be greatly improved.

Note that using kotlin.Result as function return type requires Kotlin 1.5.

Usage

val retrofit = Retrofit.Builder()
  .baseUrl("https://example.com")
  .addCallAdapterFactory(SuspendResultCallAdapterFactory())
  .build()

Your retrofit service can now use suspend functions with Result<T> return type.

interface MyService {
  @GET("/user")
  suspend fun getUser(): Result<User>
}

On Android, you can call suspend functions on the main thread without try catch.

lifecycleScope.launch {
  retrofit.create<MyService>()
    .getUser()
    .getOrNull()?.let {
      // update UI
      binding.nameLabel.value = it
    }
}

Failure handler

You can register a default failure handler through the constructor. This is a good place for configuring global error handling logic.

val retrofit = Retrofit.Builder()
  .baseUrl("https://example.com")
  .addCallAdapterFactory(SuspendResultCallAdapterFactory {
    report(it)
  })
  .addConverterFactory(MoshiConverterFactory.create(moshi))
  .build()

Custom converter for Result<T>

If your API returns data wrapped in an "envelop", it may be a good idea to unwrap the data and place it in Result directly.

Suppose the API response looks like this. You can still use Result<User> as return type.

{
  "errcode": 0,
  "data": {
    "id": 1,
    "name": "Peter"
  }
}

To achieve this, write a custom converter. Checkout CustomConverterTest for an example.