MainActivity.kt

package em.zed.literallynothing

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ProgressBar
import android.widget.TextView
import kotlinx.coroutines.experimental.*
import kotlinx.coroutines.experimental.android.UI
import kotlinx.coroutines.experimental.channels.BroadcastChannel
import kotlinx.coroutines.experimental.channels.Channel
import kotlinx.coroutines.experimental.channels.SendChannel


typealias Action<T> = T.() -> Unit

fun fetchItems(view: SendChannel<Action<MainActivity>>) = launch(CommonPool) {
    view.send { spin(true) }
    delay(10_000)
    val items = listOf("foo", "bar", "baz", "quux")
    view.send {
        spin(false)
        (rv.adapter as MyAdapter).let {
            it.items = items
            it.notifyDataSetChanged()
        }
    }
}

class Scope {
    val actions = BroadcastChannel<Action<MainActivity>>(Channel.CONFLATED)
    init {
        fetchItems(actions)
    }
}

class MainActivity : AppCompatActivity() {

    internal lateinit var rv: RecyclerView
    internal lateinit var spinner: ProgressBar
    private lateinit var our: Scope
    private lateinit var loop: Job

    override fun onRetainCustomNonConfigurationInstance(): Any = our

    override fun onCreate(savedInstanceState: Bundle?) {
        our = lastCustomNonConfigurationInstance as? Scope ?: Scope()
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        rv = findViewById(R.id.the_list) as RecyclerView
        rv.adapter = MyAdapter()
        rv.layoutManager = LinearLayoutManager(this)
        spinner = findViewById(R.id.the_spinner) as ProgressBar
    }

    override fun onResume() {
        super.onResume()
        loop = launch(UI) {
            our.actions.open().use {
                for (action in it) action()
            }
        }
    }

    override fun onPause() {
        super.onPause()
        loop.cancel(CancellationException("activity paused."))
    }

    internal fun spin(busy: Boolean) {
        if (busy) {
            rv.visibility = View.GONE
            spinner.visibility = View.VISIBLE
        } else {
            rv.visibility = View.VISIBLE
            spinner.visibility = View.GONE
        }
    }
}

class MyAdapter : RecyclerView.Adapter<MyViewHolder>() {
    internal var items: List<String> = listOf()

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = LayoutInflater
            .from(parent.context)
            .inflate(android.R.layout.simple_selectable_list_item, parent, false)
            .let(::MyViewHolder)

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        holder.label.text = items[position]
    }

    override fun getItemCount() = items.size
}

class MyViewHolder(root: View) : RecyclerView.ViewHolder(root) {
    internal val label = root as TextView
}