Kotlin Coroutines makes async programming easy, leaving all problems inside library code. Programmer can focused on code writing.
I prepearing simple project: Retrofit Service, 1 Activity, 1 Adapter, 1 Model.

build.gradle in app level: RecyclerView, Coroutines, Retrofit

dependencies {
    // ...
    implementation 'com.android.support:recyclerview-v7:28.0.0'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:0.26.1'
    implementation 'com.squareup.retrofit2:retrofit:2.4.0'
    implementation 'com.squareup.retrofit2:converter-moshi:2.4.0'
    implementation 'com.jakewharton.retrofit:retrofit2-kotlin-coroutines-experimental-adapter:1.0.0'
}

kotlin {
    experimental {
        coroutines "enable"
    }
}

androidExtensions {
    experimental = true
}

Model: (I used old friend – JSONPlaceholder)

data class Post(
        val userId: Long,
        val id: Long,
        val title: String,
        val body: String
): Serializable

Adapter and holder (by the way, take a loot to LayoutContainer, it allows access to View through Kotlin Extensions, but we need  androidExtensions -> experimental = true block in build.gradle)

class MainAdapter(var items: List<Post>): RecyclerView.Adapter<MainAdapter.PostHolder>() {
    override fun getItemCount() = items.size
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = PostHolder(LayoutInflater.from(parent.context).inflate(R.layout.post_item, parent, false))
    override fun onBindViewHolder(holder: PostHolder, position: Int) { holder.bind(items[position]) }

    inner class PostHolder(override val containerView: View): RecyclerView.ViewHolder(containerView), LayoutContainer {
        fun bind(item: Post) {
            postId.text = item.id.toString()
            postTitle.text = item.title
            postBody.text = item.body
        }
    }
}

Retrofit API-interface:

interface JsonPlaceholderApi {

    @GET("/posts")
    fun getPosts(): Deferred<Response<List<Post>>>

    companion object {
        private const val BASE_URL = "https://jsonplaceholder.typicode.com"
        fun getApi(): JsonPlaceholderApi = Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(MoshiConverterFactory.create())
                    .addCallAdapterFactory(CoroutineCallAdapterFactory())
                    .build()
                    .create(JsonPlaceholderApi::class.java)
}

And finally, Activity and layouts:

class MainActivity : Activity() {

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

        val adapter = MainAdapter(listOf())
        recycler.adapter = adapter

        GlobalScope.launch(Dispatchers.Main) {
            progress.visibility = View.VISIBLE
            val postsRequest = JsonPlaceholderApi.getApi().getPosts()
            val postsResponse = postsRequest.await()
            progress.visibility = View.GONE
            if (postsResponse.isSuccessful) {
                adapter.items = postsResponse.body() ?: listOf()
                adapter.notifyDataSetChanged()
            } else {
                Toast.makeText(this@MainActivity, "Error ${postsResponse.code()}", Toast.LENGTH_SHORT).show()
            }
        }
    }
}
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    >

    <ProgressBar
        android:id="@+id/progress"
        android:layout_width="100dp"
        android:layout_height="100dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:visibility="gone"
        />

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layoutManager="android.support.v7.widget.LinearLayoutManager"
        tools:listitem="@layout/post_item"
        />

</android.support.constraint.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp"
    >

    <TextView
        android:id="@+id/postId"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="12sp"
        android:textColor="@android:color/darker_gray"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        tools:text="id"
        />

    <TextView
        android:id="@+id/postTitle"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:textSize="16sp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toStartOf="@id/postId"
        android:layout_marginEnd="10dp"
        tools:text="title"
        />

    <TextView
        android:id="@+id/postBody"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:textSize="14sp"
        app:layout_constraintTop_toBottomOf="@id/postTitle"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginTop="5dp"
        tools:text="body"
        />

</android.support.constraint.ConstraintLayout>

That’s all, folks. All code (with little clarifications) on my GitHub.

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