A dependency-free Finitomata.Listener implementation for tests.

Declaring listener: Finitomata.ExUnit.Listener on an FSM lets Finitomata.ExUnit drive and assert transitions without Mox. On every successful transition it forwards a {:on_transition, fsm_name, state, payload} message — the exact shape the Finitomata.ExUnit assertions expect — to the test process that registered itself for that FSM. A failed Finitomata.on_fork/2 resolution is forwarded the same way as {:on_fork_failure, fsm_name, fork_state, error}.

The test process is looked up through the per-id Finitomata Registry that setup_finitomata/1 (via init_finitomata/6) already starts, so no extra process or global state is involved, and the registration is dropped automatically when the test process exits.

Usage

defmodule MyFSM do
  use Finitomata, fsm: @fsm, listener: Finitomata.ExUnit.Listener
end

In :test/:dev only, pick the listener conditionally to keep production lean:

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

Then write the test exactly as with the :mox listener, but without import Mox:

import Finitomata.ExUnit

describe "MyFSM" do
  setup_finitomata do
    [fsm: [implementation: MyFSM, payload: %{}]]
  end

  test_path "happy path", _ctx do
    :go -> assert_state :done
  end
end