EitherNet

A pluggable sealed API result type for modeling Retrofit responses.

Usage

By default, Retrofit uses exceptions to propagate any errors. This library leverages Kotlin sealed types
to better model these responses with a type-safe single point of return and no exception handling needed!

The core type for this is ApiResult<out T, out E>, where T is the success type and E is a possible
error type.

ApiResult has two sealed subtypes: Success and Failure. Success is typed to T with no
error type and Failure is typed to E with no success type. Failure in turn is represented by
four sealed subtypes of its own: Failure.NetworkFailure, Failure.ApiFailure, Failure.HttpFailure,
and Failure.UnknownFailure. This allows for simple handling of results through a consistent,
non-exceptional flow via sealed when branches.

when (val result = myApi.someEndpoint()) {
  is Success -> doSomethingWith(result.response)
  is Failure -> when (result) {
    is NetworkFailure -> showError(result.error)
    is HttpFailure -> showError(result.code)
    is ApiFailure -> showError(result.error)
    is UnknownFailure -> showError(result.error)
  }
}

Usually, user code for this could just simply show a generic error message for a Failure
case, but the sealed subtypes also allow for more specific error messaging or pluggability of error
types.

Simply change your endpoint return type to the typed ApiResult and include our call adapter and
delegating converter factory.

interface TestApi {
  @GET("/")
  suspend fun getData(): ApiResult<SuccessResponse, ErrorResponse>
}

val api = Retrofit.Builder()
  .addConverterFactory(ApiResultConverterFactory)
  .addCallAdapterFactory(ApiResultCallAdapterFactory)
  .build()
  .create<TestApi>()

If you don't have custom error return types, simply use Nothing for the error type.

Decoding Error Bodies

If you want to decode error types in HttpFailures, annotate your endpoint with @DecodeErrorBody:

interface TestApi {
  @DecodeErrorBody
  @GET("/")
  suspend fun getData(): ApiResult<SuccessResponse, ErrorResponse>
}

Now a 4xx or 5xx response will try to decode its error body (if any) as ErrorResponse. If you want to
contextually decode the error body based on the status code, you can retrieve a @StatusCode annotation
from annotations in a custom Retrofit Converter.

// In your own converter factory.
override fun responseBodyConverter(
  type: Type,
  annotations: Array<out Annotation>,
  retrofit: Retrofit
): Converter<ResponseBody, *>? {
  val (statusCode, nextAnnotations) = annotations.statusCode()
    ?: return null
  val errorType = when (statusCode.value) {
    401 -> Unauthorized::class.java
    404 -> NotFound::class.java
    // ...
  }
  val errorDelegate = retrofit.nextResponseBodyConverter<Any>(this, errorType.toType(), nextAnnotations)
  return MyCustomBodyConverter(errorDelegate)
}

Note that error bodies with a content length of 0 will be skipped.

Plugability

A common pattern for some APIs is to return a polymorphic 200 response where the data needs to be
dynamically parsed. Consider this example:

{
  "ok": true,
  "data": {
    ...
  }
}

The same API may return this structure in an error event

{
  "ok": false,
  "error_message": "Please try again."
}

This is hard to model with a single concrete type, but easy to handle with ApiResult. Simply
throw an ApiException with the decoded error type in a custom Retrofit Converter and it will be
automatically surfaced as a Failure.ApiFailure type with that error instance.

@GET("/")
suspend fun getData(): ApiResult<SuccessResponse, ErrorResponse>

// In your own converter factory.
class ErrorConverterFactory : Converter.Factory() {
  override fun responseBodyConverter(
    type: Type,
    annotations: Array<out Annotation>,
    retrofit: Retrofit
  ): Converter<ResponseBody, *>? {
    // This returns a `@ResultType` instance that can be used to get the error type via toType()
    val (errorType, nextAnnotations) = annotations.errorType() ?: return null
    return ResponseBodyConverter(errorType.toType())
  }

  class ResponseBodyConverter(
    private val errorType: Type
  ) : Converter<ResponseBody, *> {
    override fun convert(value: ResponseBody): String {
      if (value.isErrorType()) {
        val errorResponse = ...
        throw ApiException(errorResponse)
      } else {
        return SuccessResponse(...)
      }
    }
  }
}

Installation

dependencies {
  implementation("com.slack.eithernet:eithernet:<version>")
}

Snapshots of the development version are available in [Sonatype's snapshots repository][snap].

GitHub

https://github.com/slackhq/EitherNet