gen_statem.ex

defmodule Testing do
  @behaviour :gen_statem

  def test() do
    {:ok, pid} = start_link()

    :gen_statem.call(pid, :get_state) |> IO.inspect()

    to_party(pid)

    :gen_statem.call(pid, :get_state) |> IO.inspect()

    :ok
  end

  def callback_mode() do
    :handle_event_function
  end

  def start_link do
    :gen_statem.start_link(__MODULE__, nil, [])
  end

  def init(nil = data) do
    {:ok, :idle, data}
  end

  def to_party(pid) do
    :gen_statem.call(pid, :to_party)
  end

  def handle_event(:enter, _old_state, :idle, data) do
    IO.inspect("Entering idle state")

    {:keep_state, data}
  end

  def handle_event(:enter, _old_state, :to_party, data) do
    IO.inspect("Entering to_party state")

    {:keep_state, data}
  end

  def handle_event({:call, from}, :to_party, :idle, data) do
    {:next_state, :party, data, [{:reply, from, :ok}]}
  end

  def handle_event({:call, from}, :get_state, state, data) do
    {:keep_state, data, [{:reply, from, state}]}
  end
end