// Masks are only a concern for client writers.
  private val maskKey: ByteArray? = if (isClient) ByteArray(4) else null
  private val maskCursor: Buffer.UnsafeCursor? = if (isClient) Buffer.UnsafeCursor() else null

  /** Send a ping with the supplied [payload]. */
  @Throws(IOException::class)
  fun writePing(payload: ByteString) {
    writeControlFrame(OPCODE_CONTROL_PING, payload)
  }

  // Masks are only a concern for server writers.
  private val maskKey: ByteArray? = if (isClient) null else ByteArray(4)
  private val maskCursor: Buffer.UnsafeCursor? = if (isClient) null else Buffer.UnsafeCursor()

  interface FrameCallback {
    @Throws(IOException::class)
    fun onReadMessage(text: String)

    @Throws(IOException::class)
    fun onReadMessage(bytes: ByteString)