Helpers and assertions to make Finitomata implementation easily testable.

Testing with Finitomata.ExUnit

There are several steps needed to enable extended testing with Finitomata.ExUnit.

In the first place, mox dependency should be included in your mix.exs project file

{:mox, "~> 1.0", only: [:test]}

Then, the Finitomata declaration should include a listener. If you already have the listener, it should be changed to Mox in :test environment, and the respecive Mox should be defined somewhere in test/support or like

@listener (if Mix.env() == :test, do: MyFSM.Mox, else: MyFSM.Listener)
use Finitomata, fsm: @fsm, listener: @listener

If you don’t have an actual listener, the special :mox value for listener would do everything, including an actual Mox declaration in test environment.

use Finitomata, fsm: @fsm, listener: :mox

The last thing you need, import Mox into your test file which also does import Finitomata.ExUnit. That’s it, now your code is ready to use Finitomata.ExUnit fancy testing.

Example

Consider the following simple FSM

defmodule Turnstile do
  @fsm ~S[
    ready --> |on!| closed
    opened --> |walk_in| closed
    closed --> |coin_in| opened
    closed --> |switch_off| switched_off
  ]
  use Finitomata, fsm: @fsm, auto_terminate: true

  @impl Finitomata
  def on_transition(:opened, :walk_in, _payload, state) do
    {:ok, :closed, update_in(state, [:data, :passengers], & &1 + 1)}
  end
  def on_transition(:closed, :coin_in, _payload, state) do
    {:ok, :opened, state}
  end
  def on_transition(:closed, :off, _payload, state) do
    {:ok, :switched_off, state}
  end
end

Of course, in the real life, one would not only collect the total number of passengers passed in the state, but also validate the coin value to let in or fail a transition, but for the demonstration purposes this one is already good enough.

We now want to test it works as expected. Without Finitomata.ExUnit, one would write the test like below

# somewhere else → Mox.defmock(Turnstile.Mox, for: Finitomata.Listener)
test "standard approach" do
  start_supervised(Finitomata.Supervisor)

  fini_name = "Turnstile_1"
  fsm_name = {:via, Registry, {Finitomata.Registry, fini_name}}

  Finitomata.start_fsm(Turnstile, fini_name, %{data: %{passengers: 0}})

  Finitomata.transition(fini_name, :coin_in)
  assert %{data: %{passengers: 0}}} = Finitomata.state(Turnstile, "Turnstile_1", :payload)

  Finitomata.transition(fini_name, :walk_in)
  assert %{data: %{passengers: 1}}} = Finitomata.state(Turnstile, "Turnstile_1", :payload)

  Finitomata.transition(fini_name, :switch_off)

  Process.sleep(200)
  refute Finitomata.alive?(Turnstile, "Turnstile_1")
end

At the first glance, there is nothing wrong with this approach, but it requires an enormous boilerplate, it cannot check it’s gone without using Process.sleep/1, but most importantly, it does not allow testing intermediate states.

If the FSM has instant transitions (named with a trailing bang, like foo!) which are invoked automatically by Finitomata itself, there is no way to test intermediate states with the approach above.

OK, let’s use Mox then (assuming Turnstile.Mox has been declared and added as a listener in test environment to use Finitomata)

# somewhere else → Mox.defmock(Turnstile.Mox, for: Finitomata.Listener)
test "standard approach" do
  start_supervised(Finitomata.Supervisor)

  fini_name = "Turnstile_1"
  fsm_name = {:via, Registry, {Finitomata.Registry, fini_name}}
  parent = self()

  Turnstile.Mox
  |> allow(parent, fn -> GenServer.whereis(fsm_name) end)
  |> expect(:after_transition, 4, fn id, state, payload ->
    parent |> send({:on_transition, id, state, payload}) |> then(fn _ -> :ok end)
  end)

  Finitomata.start_fsm(Turnstile, fini_name, %{data: %{passengers: 0}})

  Finitomata.transition(fini_name, :coin_in)
  assert_receive {:on_transition, ^fsm_name, :opened, %{data: %{passengers: 0}}}
  # assert %{data: %{passengers: 0}}} = Finitomata.state(Turnstile, "Turnstile_1", :payload)

  Finitomata.transition(fini_name, :walk_in)
  assert_receive {:on_transition, ^fsm_name, :closed, %{data: %{passengers: 1}}}
  # assert %{data: %{passengers: 1}}} = Finitomata.state(Turnstile, "Turnstile_1", :payload)

  Finitomata.transition(fini_name, :switch_off)
  assert_receive {:on_transition, ^fsm_name, :switched_off, %{data: %{passengers: 1}}}

  Process.sleep(200)
  refute Finitomata.alive?(Turnstile, "Turnstile_1")
end

That looks better, but there is still too much of boilerplate. Let’s see how it’d look like with Finitomata.ExUnit.

describe "Turnstile" do
  setup_finitomata do
    parent = self()
    initial_passengers = 42

    [
      fsm: [implementation: Turnstile, payload: %{data: %{passengers: initial_passengers}})],
      context: [passengers: initial_passengers]
    ]
  end

  test_path "respectful passenger", %{passengers: initial_passengers} do
    :coin_in ->
      assert_state :opened do
        assert_payload do
          data.passengers ~> ^initial_passengers
        end
      end

    :walk_in ->
      assert_state :closed do
        assert_payload do
          data.passengers ~> one_more when one_more == 1 + initial_passengers
        end
      end

    :switch_off ->
      assert_state :switched_off
      assert_state :*
  end

With this approach, one could test the payload in the intermediate states, and validate messages received from the FSM with assert_receive/3.

No other code besides assert_state/2, assert_payload/1, and ExUnit.Assertions.assert_receive/3 is permitted to fully isolate the FSM execution from side effects.