private static final LazyLogger logger = new LazyLogger(ListenerCallQueue.class);

  // TODO(chrisn): promote AppendOnlyCollection for use here.
  private final List<PerListenerQueue<L>> listeners =
      Collections.synchronizedList(new ArrayList<PerListenerQueue<L>>());

  /** Method reference-compatible listener event. */
  interface Event<L> {
    /** Call a method on the listener. */
    void call(L listener);
  }

  /**