@doc """
  Sends a frame through the WebSocket.
  If the connection is either connecting or closing then this will return an
  error tuple with a `WebSockex.NotConnectedError` exception struct as the
  second element.
  If a connection failure is discovered while sending then it will return an
  error tuple with a `WebSockex.ConnError` exception struct as the second
  element.
  """
  @spec send_frame(client, frame) ::
          :ok
          | {:error,
             %WebSockex.FrameEncodeError{}
             | %WebSockex.ConnError{}
             | %WebSockex.NotConnectedError{}
             | %WebSockex.InvalidFrameError{}}
          | none
  def send_frame(client, _) when client == self() do
    raise %WebSockex.CallingSelfError{function: :send_frame}
  end

  def send_frame(client, frame) do
    try do
      {:ok, res} = :gen.call(client, :"$websockex_send", frame)
      res
    catch
      _, reason ->
        exit({reason, {__MODULE__, :call, [client, frame]}})
    end
  end