### EventListener.Factory

In the preceding example we used a field, `callStartNanos`, to track the elapsed time of each event. This is handy, but it won’t work if multiple calls are executing concurrently. To accommodate this, use a `Factory` to create a new `EventListener` instance for each `Call`. This allows each listener to keep call-specific state.

    ): Relay {
      val randomAccessFile = RandomAccessFile(file, "rw")
      val result = Relay(randomAccessFile, upstream, 0L, metadata, bufferMaxSize)

      // Write a dirty header. That way if we crash we won't attempt to recover this.
      randomAccessFile.setLength(0L)
      result.writeHeader(PREFIX_DIRTY, -1L, -1L)

      return result
    }

    /**

        }

        if (connectResult.isSuccess) {
          // We have a connected TCP connection. Cancel and defer the racing connects that all lost.
          cancelInFlightConnects()

          // Finish connecting. We won't have to if the winner is from the connection pool.
          if (!connectResult.plan.isReady) {
            connectResult = connectResult.plan.connectTlsEtc()
          }

          if (connectResult.isSuccess) {

saves bandwidth but it costs CPU and memory! If you run into a problem you may need to adjust or
disable the `permessage-deflate` compression settings on your server.

Note that OkHttp won't use compression when sending messages smaller than 1 KiB.

 *  Fix: Don't crash when the URL hostname contains an underscore on Android.

          .build(),
      )
    }
  }

  private fun testServerClosesOutput(socketPolicy: SocketPolicy) {
    server.enqueue(
      MockResponse(
        body = "This connection won't pool properly",
        socketPolicy = socketPolicy,
      ),
    )
    val responseAfter = MockResponse(body = "This comes after a busted connection")
    server.enqueue(responseAfter)

### Conditional Cache Hit
 
When cache flags require checking the cache results are still valid an early cacheConditionalHit event is
received followed by a cache hit or miss.  Critically in the cache hit scenario the server won’t send the response body.

The response will have non-null `cacheResponse` and `networkResponse`. The cacheResponse will be used as the top level
response only if the response code is HTTP/1.1 304 Not Modified.
 
 - CallStart

 * true for systems like Mac, Unix, and Linux that use inodes in their file system interface. It is
 * typically false on Windows.
 *
 * If this returns false we won't permit simultaneous reads and writes. When writes commit we need
 * to delete the previous snapshots, and that won't succeed if the file is open. (We do permit
 * multiple simultaneous reads.)
 *
 * @param file a file in the directory to check. This file shouldn't already exist!

 * connections should expect repetition of stream IDs.
 *
 * Return true to request cancellation of a pushed stream.  Note that this does not guarantee
 * future frames won't arrive on the stream ID.
 */
interface PushObserver {
  /**
   * Describes the request that the server intends to push a response for.
   *
   * @param streamId server-initiated stream ID: an even number.

    assertThat(statusLine.protocol).isEqualTo(Protocol.HTTP_1_1)
    assertThat(statusLine.code).isEqualTo(code)
  }

  /**
   * This is not defined in the protocol but some servers won't add the leading empty space when the
   * message is empty. http://www.w3.org/Protocols/rfc2616/rfc2616-sec6.html#sec6.1
   */
  @Test
  fun emptyMessageAndNoLeadingSpace() {
    val version = 1
    val code = 503

    assertThat(client.canceled).isFalse()

    taskFaker.advanceUntil(ns(RealWebSocket.CANCEL_AFTER_CLOSE_MILLIS))
    assertThat(client.canceled).isTrue()

    client.processNextFrame() // This won't get a frame, but it will get a closed pipe.
    client.listener.assertFailure(IOException::class.java, "canceled")
    taskFaker.runTasks()
  }

  @Test
  fun clientCloseCancelsConnectionAfterCustomTimeout() {