### 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
    }

    /**

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)

   * Returns -1 if no further cleanups are required.
   */
  fun closeConnections(now: Long): Long {
    // Compute the concurrent call capacity for each address. We won't close a connection if doing
    // so would violate a policy, unless it's OLD.
    val addressStates = this.addressStates
    for (state in addressStates.values) {
      state.concurrentCallCapacity = 0
    }

    /**
     * Returns true if the request contains conditions that save the server from sending a response
     * that the client has locally. When a request is enqueued with its own conditions, the built-in
     * response cache won't be used.
     */
    private fun hasConditions(request: Request): Boolean =
      request.header("If-Modified-Since") != null || request.header("If-None-Match") != null
  }

  companion object {

  open fun responseBodyStart(call: Call) {
  }

  /**
   * Invoked immediately after receiving a response body and completing reading it.
   *
   * Will only be invoked for requests having a response body e.g. won't be invoked for a web socket
   * upgrade.
   *
   * If the response body is closed before the response body is exhausted, this is invoked at the

      open = isOpen
    }
    if (cancel) {
      // RST this stream to prevent additional data from being sent. This is safe because the input
      // stream is closed (we won't use any further bytes) and the output stream is either finished
      // or closed (so RSTing both streams doesn't cause harm).
      ******@****.***(ErrorCode.CANCEL, null)
    } else if (!open) {

    // be deleted when those files are closed.
    if (!civilizedFileSystem) {
      if (entry.lockingSourceCount > 0) {
        // Mark this entry as 'DIRTY' so that if the process crashes this entry won't be used.
        journalWriter?.let {
          it.writeUtf8(DIRTY)
          it.writeByte(' '.code)
          it.writeUtf8(entry.key)
          it.writeByte('\n'.code)
          it.flush()
        }

 * this class separates _allocations_ from _streams_. An allocation is created by a call, used for
 * one or more streams, and then released. An allocated connection won't be stolen by other calls
 * while a redirect or authorization challenge is being handled.
 *
 * When the maximum concurrent streams limit is reduced, some allocations will be rescinded.