Kotlin Coroutines でリトライをしながら通信を試行する

MainActivity.kt

package com.example.tomoyashibata.myapplication

import android.graphics.Bitmap
import android.graphics.BitmapFactory
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.util.Log
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_main.*
import kotlinx.coroutines.experimental.CommonPool
import kotlinx.coroutines.experimental.android.UI
import kotlinx.coroutines.experimental.delay
import kotlinx.coroutines.experimental.launch
import java.io.IOException
import java.net.URL
import kotlin.coroutines.experimental.suspendCoroutine


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

        launch(UI) {
            try {
                val bitmap = [email protected]()
                [email protected](bitmap)
            } catch (e: IOException) {
                // エラーを Toast で表示
                Toast.makeText(this@MainActivity, e.toString(), Toast.LENGTH_LONG).show()
            }
        }
    }

    private suspend fun loadAvatarImage(): Bitmap = suspendCoroutine { cont ->
        // URL から末尾の .jpg を取り除いてわざとエラーが起こるようにしている
        val urlConnection = URL("https://pbs.twimg.com/profile_images/637494283365298177/J0t3qvlm_400x400").openConnection()

        launch(CommonPool) {
            try {
                val bitmap = retryIO(times = 10) {
                    urlConnection.connect()
                    val stream = urlConnection.getInputStream()
                    BitmapFactory.decodeStream(stream)
                }
                cont.resume(bitmap)
            } catch (e: IOException) {
                cont.resumeWithException(e)
            }
        }
    }

    // リトライ用メソッド
    // https://stackoverflow.com/questions/46872242/how-to-exponential-backoff-retry-on-kotlin-coroutines
    suspend fun <T> retryIO(
        times: Int = Int.MAX_VALUE,
        initialDelay: Long = 100,
        maxDelay: Long = 1000,
        factor: Double = 2.0,
        block: suspend () -> T): T {
        var currentDelay = initialDelay
        repeat(times - 1) {
            try {
                return block()
            } catch (e: IOException) {
                Log.e("IOException: ", e.toString())
            }
            delay(currentDelay)
            currentDelay = (currentDelay * factor).toLong().coerceAtMost(maxDelay)
        }
        return block()
    }
}