# `Finitomata.Listener`
[🔗](https://github.com/am-kantox/finitomata/blob/v1.0.0-rc.0/lib/finitomata/listener.ex#L1)

The behaviour to be implemented and passed to `use Finitomata` to receive
  all the state transitions notifications.

## Modules and processes

The `listener:` option accepts either an implementation of this behaviour, or anything
  `send/2` accepts (a `t:pid/0`, a `t:port/0`, a registered name, or a
  `{:via, module, term}`/`{name, node}` tuple.) The two are told apart by whether the
  given term is a module exporting `c:after_transition/3`; a process receives
  `{:finitomata, {:transition, state, payload}}` (and
  `{:finitomata, {:fork_error, state, error}}`) messages instead of a callback invocation.

Because a registered name is an atom too, the distinction is only meaningful for a
  _loaded_ module: `function_exported?/3` answers `false` for a module that has never
  been reached, which is the normal state of affairs in a freshly booted release or at
  the very beginning of a test run. `Finitomata` therefore ensures the module is loaded
  before deciding, so a lazily-loaded listener module is never mistaken for a process
  name (which would raise inside the transition and take the _FSM_ down with it.)

No explicit `Code.ensure_loaded!/1` call on the listener is needed on the consumer side.

# `after_fork_failure`
*since 0.41.0* *optional* 

```elixir
@callback after_fork_failure(
  id :: Finitomata.fsm_name(),
  state :: Finitomata.Transition.state(),
  error :: any()
) :: :ok
```

To be called when a `c:Finitomata.on_fork/2` resolution fails.

Optional callback; implement it to be notified when the FSM reaches a fork state but the
  fork cannot be resolved (an unknown/ambiguous/missing fork target, a bad return, or a
  raise). `state` is the fork state and `error` is the resolution failure reason.

# `after_transition`

```elixir
@callback after_transition(
  id :: Finitomata.fsm_name(),
  state :: Finitomata.Transition.state(),
  payload :: Finitomata.State.payload()
) :: :ok
```

To be called after a successful transition

---

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