# POE::NFA - an event-driven state machine (nondeterministic finite... - perldoc - [phpMan]

## NAME
    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) - an event-driven state machine (nondeterministic finite automaton)

## SYNOPSIS
      use [POE::Kernel](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3AKernel/markdown);
      use [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown);
      use [POE::Wheel::ReadLine](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3AWheel%3A%3AReadLine/markdown);

      # Spawn an NFA and enter its initial state.
      [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown)->spawn(
        inline_states => {
          initial => {
            setup => \&setup_stuff,
          },
          state_login => {
            on_entry => \&login_prompt,
            on_input => \&save_login,
          },
          state_password => {
            on_entry => \&password_prompt,
            on_input => \&check_password,
          },
          state_cmd => {
            on_entry => \&command_prompt,
            on_input => \&handle_command,
          },
        },
      )->goto_state(initial => "setup");

      [POE::Kernel](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3AKernel/markdown)->run();
      exit;

      sub setup_stuff {
        $_[RUNSTATE]{io} = [POE::Wheel::ReadLine](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3AWheel%3A%3AReadLine/markdown)->new(
          InputEvent => 'on_input',
        );
        $_[MACHINE]->goto_state(state_login => "on_entry");
      }

      sub login_prompt { $_[RUNSTATE]{io}->get('Login: '); }

      sub save_login {
        $_[RUNSTATE]{login} = $_[ARG0];
        $_[MACHINE]->goto_state(state_password => "on_entry");
      }

      sub password_prompt { $_[RUNSTATE]{io}->get('Password: '); }

      sub check_password {
        if ($_[RUNSTATE]{login} eq $_[ARG0]) {
          $_[MACHINE]->goto_state(state_cmd => "on_entry");
        }
        else {
          $_[MACHINE]->goto_state(state_login => "on_entry");
        }
      }

      sub command_prompt { $_[RUNSTATE]{io}->get('Cmd: '); }

      sub handle_command {
        $_[RUNSTATE]{io}->put("  <<$_[ARG0]>>");
        if ($_[ARG0] =~ /^(?:quit|stop|exit|halt|bye)$/i) {
          $_[RUNSTATE]{io}->put('Bye!');
          $_[MACHINE]->stop();
        }
        else {
          $_[MACHINE]->goto_state(state_cmd => "on_entry");
        }
      }

## DESCRIPTION
    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) implements a different kind of POE session: A non-deterministic finite automaton. Let's
    break that down.

    A finite automaton is a state machine with a bounded number of states and transitions.
    Technically, [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) objects may modify themselves at run time, so they aren't really "finite".
    Run-time modification isn't currently supported by the API, so plausible deniability is
    maintained!

    Deterministic state machines are ones where all possible transitions are known at compile time.
    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) is "non-deterministic" because transitions may change based on run-time conditions.

    But more simply, [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) is like [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown) but with banks of event handlers that may be
    swapped according to the session's run-time state. Consider the SYNOPSIS example, which has
    "on_entry" and "on_input" handlers that do different things depending on the run-time state.
    [POE::Wheel::ReadLine](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3AWheel%3A%3AReadLine/markdown) throws "on_input", but different things happen depending whether the
    session is in its "login", "password" or "command" state.

    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) borrows heavily from [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown), so this document will only discuss the differences.
    Please see [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown) for things which are similar.

## PUBLIC METHODS
    This document mainly focuses on the differences from [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown).

  get_current_state
    Each machine state has a name. get_current_state() returns the name of the machine's current
    state. get_current_state() is mainly used to retrieve the state of some other machine. It's
    easier (and faster) to use $_[STATE] in a machine's own event handlers.

  get_runstate
### get_runstate
    HEAPs, so this method does pretty much the same as [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)'s get_heap(). It's easier (and
    faster) to use $_[RUNSTATE] in a machine's own event handlers, however.

  spawn STATE_NAME => HANDLERS_HASHREF[, ...]
### spawn
    spawned like threads or processes rather than instantiated like objects.

    The machine itself is defined as a list of state names and hashes that map events to handlers
    within each state.

      my %states = (
        state_1 => {
          event_1 => \&handler_1,
          event_2 => \&handler_2,
        },
        state_2 => {
          event_1 => \&handler_3,
          event_2 => \&handler_4,
        },
      );

    A single event may be handled by many states. The proper handler will be called depending on the
    machine's current state. For example, if "event_1" is dispatched while the machine is in
    "state_2", then handler_3() will be called to handle the event. The state -> event -> handler
    map looks like this:

      $machine{state_2}{event_1} = \&handler_3;

    Instead of "inline_states", "object_states" or "package_states" may be used. These map the
    events of a state to an object or package method respectively.

      object_states => {
        state_1 => [
          $object_1 => [qw(event_1 event_2)],
        ],
        state_2 => [
          $object_2 => {
            event_1 => method_1,
            event_2 => method_2,
          }
        ]
      }

    In the example above, in the case of "event_1" coming in while the machine is in "state_1",
    method "event_1" will be called on $object_1. If the machine is in "state_2", method "method_1"
    will be called on $object_2.

    "package_states" is very similar, but instead of using an $object, you pass in a "[Package::Name](https://www.chedong.com/phpMan.php/perldoc/Package%3A%3AName/markdown)"

    The "runstate" parameter allows "RUNSTATE" to be initialized differently at instantiation time.
    "RUNSTATE", like heaps, are usually anonymous hashrefs, but "runstate" may set them to be array
    references or even objects.

    State transitions are not necessarily executed immediately by default. Rather, they are placed
    in POEs event queue behind any currently pending events. Enabling the "immediate" option causes
    state transitions to occur immediately, regardless of any queued events.

  goto_state NEW_STATE[, ENTRY_EVENT[, EVENT_ARGS]]
### goto_state
    will be dispatched after the machine enters the new state. EVENT_ARGS, if included, will be
    passed to the entry event's handler via "ARG0..$#_".

      # Switch to the next state.
      $_[MACHINE]->goto_state( 'next_state' );

      # Switch to the next state, and call a specific entry point.
      $_[MACHINE]->goto_state( 'next_state', 'entry_event' );

      # Switch to the next state; call an entry point with some values.
      $_[MACHINE]->goto_state( 'next_state', 'entry_event', @parameters );

  stop
### stop
    to do, just like [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown).

### stop
    in the machine's "RUNSTATE" are not POE's responsibility and may cause memory leaks.

      $_[MACHINE]->stop();

  call_state RETURN_EVENT, NEW_STATE[, ENTRY_EVENT[, EVENT_ARGS]]
### call_state
    later point, a handler can call return_state() to pop the call stack and return the machine to
    its old state. At that point, a "RETURN_EVENT" will be posted to notify the old state of the
    return.

      $machine->call_state( 'return_here', 'new_state', 'entry_event' );

    As with goto_state(), "ENTRY_EVENT" is the event that will be emitted once the machine enters
    its new state. "ENTRY_ARGS" are parameters passed to the "ENTRY_EVENT" handler via "ARG0..$#_".

  return_state [RETURN_ARGS]
### return_state
    preceding call_state() included a return event then its handler will be invoked along with some
    optional "RETURN_ARGS". The "RETURN_ARGS" will be passed to the return handler via "ARG0..$#_".

      $_[MACHINE]->return_state( 'success', @success_values );

### Methods that match [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)
    The following methods behave identically to the ones in [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown).

    ID
    option
    postback
    callback

  About new() and create()
    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown)'s constructor is spawn(), not new() or create().

## PREDEFINED EVENT FIELDS
    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown)'s predefined event fields are the same as [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)'s with the following three
    exceptions.

  MACHINE
    "MACHINE" is equivalent to Session's "SESSION" field. It holds a reference to the current state
    machine, and is useful for calling its methods.

    See [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)'s "SESSION" field for more information.

      $_[MACHINE]->goto_state( $next_state, $next_state_entry_event );

  RUNSTATE
    "RUNSTATE" is equivalent to Session's "HEAP" field. It holds an anonymous hash reference which
    POE is guaranteed not to touch. Data stored in "RUNSTATE" will persist between handler
    invocations.

  STATE
    "STATE" contains the name of the machine's current state. It is not equivalent to anything from
    [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown).

  EVENT
    "EVENT" is equivalent to Session's "STATE" field. It holds the name of the event which invoked
    the current handler. See [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)'s "STATE" field for more information.

## PREDEFINED EVENT NAMES
    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) defines four events of its own. These events are used internally and may not be
    overridden by application code.

    See [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)'s "PREDEFINED EVENT NAMES" section for more information about other predefined
    events.

    The events are: "poe_nfa_goto_state", "poe_nfa_push_state", "poe_nfa_pop_state", "poe_nfa_stop".

    Yes, all the internal events begin with "poe_nfa_". More may be forthcoming, but they will
    always begin the same way. Therefore please do not define events beginning with "poe_nfa_".

## SEE ALSO
    Many of [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown)'s features are taken directly from [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown). Please see [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown) for
    more information.

    The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.

## BUGS
    See [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown)'s documentation.

    [POE::NFA](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ANFA/markdown) is not as feature-complete as [POE::Session](https://www.chedong.com/phpMan.php/perldoc/POE%3A%3ASession/markdown). Your feedback is appreciated.

AUTHORS & COPYRIGHTS
    Please see POE for more information about authors and contributors.