* #enqueue enqueued} and {@link #dispatch dispatched}.
   */
  public void addListener(L listener, Executor executor) {
    checkNotNull(listener, "listener");
    checkNotNull(executor, "executor");
    listeners.add(new PerListenerQueue<>(listener, executor));
  }

  /**
   * Enqueues an event to be run on currently known listeners.
   *

    }

    if (failFastResponse != null && responseQueue.peek() == null) {
      // Fail fast if there's no response queued up.
      return failFastResponse!!
    }

    val result = responseQueue.take()

    // If take() returned because we're shutting down, then enqueue another dead letter so that any
    // other threads waiting on take() will also return.
    if (result == DEAD_LETTER) responseQueue.add(DEAD_LETTER)

   * that enqueued messages were not transmitted.
   */
  fun queueSize(): Long

  /**
   * Attempts to enqueue `text` to be UTF-8 encoded and sent as a the data of a text (type `0x1`)
   * message.
   *
   * This method returns true if the message was enqueued. Messages that would overflow the outgoing

  private val messageAndCloseQueue = ArrayDeque<Any>()

  /** The total size in bytes of enqueued but not yet transmitted messages. */
  private var queueSize = 0L

  /** True if we've enqueued a close frame. No further message frames will be enqueued. */
  private var enqueuedClose = false

  /** The close code from the peer, or -1 if this web socket has not yet read a close frame. */

  // instance for every unit test.
  MockWebServer server = new MockWebServer();

  // Schedule some responses.
  server.enqueue(new MockResponse.Builder()
      .body("hello, world!")
      .build());
  server.enqueue(new MockResponse.Builder()
      .body("sup, bra?")
      .build());
  server.enqueue(new MockResponse.Builder()
      .body("yo dog")
      .build());

  // Start the server.
  server.start();

   *
   * @throws IllegalStateException when the call has already been executed.
   */
  fun enqueue(responseCallback: Callback)

  /** Cancels the request, if possible. Requests that are already complete cannot be canceled. */
  fun cancel()

  /**
   * Returns true if this call has been either [executed][execute] or [enqueued][enqueue]. It is an
   * error to execute a call more than once.
   */
  fun isExecuted(): Boolean

    dispatcher.maxRequests = 2
    client.newCall(newRequest("http://a/1")).enqueue(callback)
    client.newCall(newRequest("http://b/1")).enqueue(callback)
    client.newCall(newRequest("http://c/1")).enqueue(callback)
    client.newCall(newRequest("http://a/2")).enqueue(callback)
    client.newCall(newRequest("http://b/2")).enqueue(callback)
    dispatcher.maxRequests = 4

  }

  /**
   * Test that MockWebServer blocks for a call to enqueue() if a request is made before a mock
   * response is ready.
   */
  @Test
  fun dispatchBlocksWaitingForEnqueue() {
    Thread {
      try {
        Thread.sleep(1000)
      } catch (ignored: InterruptedException) {
      }
      server.enqueue(MockResponse().setBody("enqueued in the background"))
    }.start()

    /**
     * Used to update and read the latestTaskQueue field. Set to null once the runnable has been run
     * or queued.
     */
    @Nullable ExecutionSequencer sequencer;

    /**
     * Executor the task was set to run on. Set to null when the task has been queued, run, or
     * cancelled.
     */
    @Nullable Executor delegate;

    /**

   * Test that MockWebServer blocks for a call to enqueue() if a request is made before a mock
   * response is ready.
   */
  @Test
  fun dispatchBlocksWaitingForEnqueue() {
    Thread {
      try {
        Thread.sleep(1000)
      } catch (ignored: InterruptedException) {
      }
      server.enqueue(
        MockResponse
          .Builder()
          .body("enqueued in the background")
          .build(),