live_view_session_plug.ex

defmodule MyAppWeb.LiveView.Session do
  @moduledoc """

  Joe Noon on 9/3/2019

  LiveView does not have a direct way to change the session, but does provide a way to set flash.
  The application can use `put_flash(socket, :session, %{...})`, and this plug will merge that
  map when present into the persisted session.

  """

  @behaviour Plug

  def init(opts), do: opts

  def call(conn, _) do
    conn
    |> handle_session()
    |> handle_reset_session()
  end

  def handle_session(
        %{
          private: %{
            plug_session: %{"phoenix_flash" => %{"session" => %{} = session}} = plug_session
          }
        } = conn
      ) do
    conn
    |> Plug.Conn.put_private(:plug_session, Map.merge(plug_session, session))
  end

  def handle_session(conn), do: conn

  def handle_reset_session(
        %{
          private: %{
            plug_session: %{"phoenix_flash" => %{"reset_session" => %{}} = phoenix_flash}
          }
        } = conn
      ) do
    conn
    |> Plug.Conn.put_private(:plug_session, %{"phoenix_flash" => phoenix_flash})
  end

  def handle_reset_session(conn), do: conn
end