# `Finitomata.ExUnit.Listener`
[🔗](https://github.com/am-kantox/finitomata/blob/v0.41.0/lib/finitomata/test/listener.ex#L1)

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 `c: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

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

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

```elixir
@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`:

```elixir
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
```

# `register`

```elixir
@spec register(Finitomata.fsm_name()) :: :ok
```

Registers the calling (test) process to receive `{:on_transition, fsm_name, state, payload}`
  notifications for the _FSM_ identified by `fsm_name` (its fully-qualified `:via` tuple).

---

*Consult [api-reference.md](api-reference.md) for complete listing*