*    If the message spans multiple frames, each interleaved control frame will result in a
   *    corresponding call to [FrameCallback].
   */
  @Throws(IOException::class)
  fun processNextFrame() {
    check(!closed) { "closed" }

    readHeader()
    if (isControlFrame) {
      readControlFrame()
    } else {
      readMessageFrame()
    }
  }