MainActivity Kotlin

import com.google.gson.GsonBuilder
import io.reactivex.android.schedulers.AndroidSchedulers
import io.reactivex.internal.schedulers.IoScheduler
import kotlinx.android.synthetic.main.activity_main.*
import retrofit2.Retrofit
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory
import retrofit2.converter.gson.GsonConverterFactory

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        rv__list_posts.layoutManager = LinearLayoutManager(this)

        val retrofit = Retrofit.Builder().addConverterFactory(GsonConverterFactory.create(GsonBuilder().create()))
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .baseUrl("https://jsonplaceholder.typicode.com/").build()

        val postsApi = retrofit.create(INetworkAPI::class.java)

        var response = postsApi.getAllPosts()

        response.observeOn(AndroidSchedulers.mainThread()).subscribeOn(IoScheduler()).subscribe {
            rv__list_posts.adapter = PostItemAdapter(it, this)
        }

    }
}

API Interface

import io.reactivex.Observable
import retrofit2.http.GET

interface INetworkAPI {

    @GET("posts/")
    fun getAllPosts(): Observable<List<Post>>
}

Model class Kotlin

I used a plugin in Android studio JSON to Kotlin Data class Converter.

Which automatically converted above JSON List to the below Kotlin Data Class Post.

import com.google.gson.annotations.SerializedName


data class Post(
      @SerializedName("userId") val userId: Int,
      @SerializedName("id") val id: Int,
      @SerializedName("title") val title: String,
      @SerializedName("body") val body: String
)

Kotlin Adapter

class PostItemAdapter(val postList: List<Post>, val context: Context) : 
      RecyclerView.Adapter<PostItemAdapter.ViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {

        return ViewHolder(LayoutInflater.from(context).inflate(R.layout.post_item_layout,
                          parent, false))
    }

    override fun getItemCount(): Int {
        return 10
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {

        holder.itemView.txtPostTitle.text = postList.get(position).title
        holder.itemView.txtPostBody.text = postList.get(position).body

    }
    class ViewHolder(view: View) : RecyclerView.ViewHolder(view)
}