Supercharging Retrofit with Kotlin – The Coinbase Weblog


Constructing a Kotlin-esque API for Retrofit

At Coinbase we use Retrofit and Sq.’s RxJava 2 Adapter as our API to the wire. Retrofit makes networking a breeze, however all through our app we discovered ourselves writing code like this:

Dealing with the results of a community name

This works, however there’s just a few tough edges:

  • Our API isn’t declarative. To find out what state we’re in we now have to null examine a bunch of issues and it’s simple to overlook a case
  • We’ve inadvertently leaked our community serializer (moshi) into our software layer to deserialize error our bodies
  • Streams get torn down when a community error happens. This isn’t a giant deal right here, but when we begin combining community streams with different Observables we possible don’t need community errors to terminate the ensuing stream

Let’s take a look at how we will use Retrofit’s CallAdapter API to nerf down these edges. We’ll use Sealed Classes to symbolize the results of community calls and construct error physique deserialization into Retrofit.

Sealed Lessons + Typed Error Physique Deserialization — Networking Nirvana

To have Retrofit return an occasion of NetworkResponse when the getTokens() API is invoked, we now have to jot down a customized CallAdapter.Factory. The CallAdapter.Manufacturing facility under says, “I understand how to create cases of NetworkResponse which might be emitted to RxJava streams.”

When the getTokens() API is invoked, this CallAdapter.Manufacturing facility:

  • Delegates to an adapter that is aware of how you can make an occasion of the kind Observable<AccessToken> (line 46)
  • Asks for a converter able to serializing the kind Error (line 54)
  • Creates a KotlinRxJava2CallAdapter (line 60). This adapter deserializes error our bodies and can adorn the stream of Observable<AccessToken> right into a Single<NetworkResponse<AccessToken,Error>>

To make use of our customized CallAdapter.Manufacturing facility we now have to plug it into our Retrofit occasion. Adapter registration order is essential since we’ve written a delegating CallAdapter; we should register our adapter earlier than every other adapters it could delegate to. Within the snippet under we delegate the creation of Observable<AccessToken> to Sq.’s RxJava Adapter.

val retrofit = Retrofit.Builder()
.baseUrl("https://api.coinbase.com/v2/")
.addCallAdapterFactory(KotlinRxJava2CallAdapterFactory())
.addCallAdapterFactory(RxJava2CallAdapterFactory.create())
.construct()

When you’re focused on having a Kotlin-esque API to the wire that gives error physique deserialization, you could find the complete code for our Adapter here.

P.S. We’re hiring!

Particular due to Jesse Wilson for catching a bug within the full code for the adapter; all error codes aren’t assured to return JSON!

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