interface AsyncDns {
  /**
   * Query DNS records for `hostname`, in the order they are received.
   */
  fun query(
    hostname: String,
    callback: Callback,
  )

  /**
   * Callback to receive results from the DNS Queries.
   */
  @ExperimentalOkHttpApi
  interface Callback {
    /**
     * Return addresses for a dns query for a single class of IPv4 (A) or IPv6 (AAAA).

import okhttp3.Call
import okhttp3.Callback
import okhttp3.OkHttpClient
import okhttp3.Request
import okhttp3.Response

class AsynchronousGet {
  private val client = OkHttpClient()

  fun run() {
    val request =
      Request.Builder()
        .url("http://publicobject.com/helloworld.txt")
        .build()

    client.newCall(request).enqueue(
      object : Callback {
        override fun onFailure(

    }

  /**
   * A callback to be invoked each time the dispatcher becomes idle (when the number of running
   * calls returns to zero).
   *
   * Note: The time at which a [call][Call] is considered idle is different depending on whether it
   * was run [asynchronously][Call.enqueue] or [synchronously][Call.execute]. Asynchronous calls
   * become idle after the [onResponse][Callback.onResponse] or [onFailure][Callback.onFailure]

## Version 5.0.0-alpha.4

_2022-02-01_

**This release introduces fast fallback to better support mixed IPv4+IPv6 networks.** Fast fallback
is what we're calling our implementation of Happy Eyeballs, [RFC 8305][rfc_8305]. With this
feature OkHttp will attempt both IPv6 and IPv4 connections concurrently, keeping whichever connects
first. Fast fallback gives IPv6 connections a 250 ms head start so IPv6 is preferred on networks

    var responseClosed = false
    var canceled = false
    var afterCallbackOnResponse: () -> Unit = {}

    override fun cancel() {
      canceled = true
    }

    override fun enqueue(responseCallback: Callback) {
      responseCallback.onResponse(this, response)
      afterCallbackOnResponse()
    }
  }

    )
    val call =
      client.newCall(
        Request.Builder()
          .url(server.url("/"))
          .build(),
      )
    val completionLatch = CountDownLatch(1)
    val callback: Callback =
      object : Callback {
        override fun onFailure(
          call: Call,
          e: IOException,
        ) {
          completionLatch.countDown()
        }

        override fun onResponse(

        override fun enqueue(responseCallback: Callback) = TODO()

        override fun cancel() = TODO()

        override fun isExecuted(): Boolean = TODO()

        override fun isCanceled(): Boolean = TODO()

        override fun timeout(): Timeout = TODO()

        override fun clone(): Call = TODO()
      }
  }

  @Test
  fun callback() {
    val callback =
      object : Callback {

  /** The timestamp of the last taken event, used to measure elapsed time between events. */
  private var lastTimestampNs: Long? = null

  /** Confirm that the thread does not hold a lock on `lock` during the callback. */
  fun forbidLock(lock: Any) {
    forbiddenLocks.add(lock)
  }

  /**
   * Removes recorded events up to (and including) an event is found whose class equals [eventClass]
   * and returns it.
   */

import java.net.SocketTimeoutException
import java.util.ArrayDeque
import java.util.Random
import java.util.concurrent.TimeUnit
import java.util.concurrent.TimeUnit.MILLISECONDS
import okhttp3.Call
import okhttp3.Callback
import okhttp3.EventListener
import okhttp3.OkHttpClient
import okhttp3.Protocol
import okhttp3.Request
import okhttp3.Response
import okhttp3.WebSocket
import okhttp3.WebSocketListener

    fun retryOnConnectionFailure(retryOnConnectionFailure: Boolean) =
      apply {
        this.retryOnConnectionFailure = retryOnConnectionFailure
      }

    /**
     * Configure this client to perform fast fallbacks by attempting multiple connections
     * concurrently, returning once any connection connects successfully.
     *
     * This implements Happy Eyeballs ([RFC 6555][rfc_6555]), balancing connect latency vs.