POE::NFA - phpMan

NAME
    POE::NFA - an event-driven state machine (nondeterministic finite
    automaton)

SYNOPSIS
      use POE::Kernel;
      use POE::NFA;
      use POE::Wheel::ReadLine;

      # Spawn an NFA and enter its initial state.
      POE::NFA->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->run();
      exit;

      sub setup_stuff {
        $_[RUNSTATE]{io} = POE::Wheel::ReadLine->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 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 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 is "non-deterministic" because
    transitions may change based on run-time conditions.

    But more simply, POE::NFA is like POE::Session 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 throws "on_input", but different things happen
    depending whether the session is in its "login", "password" or "command"
    state.

    POE::NFA borrows heavily from POE::Session, so this document will only
    discuss the differences. Please see POE::Session for things which are
    similar.

PUBLIC METHODS
    This document mainly focuses on the differences from POE::Session.

  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() returns the machine's current runstate. Runstates are
    equivalent to POE::Session HEAPs, so this method does pretty much the
    same as POE::Session'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() is POE::NFA's constructor. The name reflects the idea that new
    state machines are 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"

    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() puts the machine into a new state. If an ENTRY_EVENT is
    specified, then that event 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() forces a machine to stop. The machine will also stop gracefully
    if it runs out of things to do, just like POE::Session.

    stop() is heavy-handed. It will force resources to be cleaned up.
    However, circular references 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() is similar to goto_state(), but it pushes the current state
    on a stack. At some 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() returns to the most recent state in which call_state()
    was invoked. If the 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
    The following methods behave identically to the ones in POE::Session.

    ID
    option
    postback
    callback

  About new() and create()
    POE::NFA's constructor is spawn(), not new() or create().

PREDEFINED EVENT FIELDS
    POE::NFA's predefined event fields are the same as POE::Session'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'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.

  EVENT
    "EVENT" is equivalent to Session's "STATE" field. It holds the name of
    the event which invoked the current handler. See POE::Session's "STATE"
    field for more information.

PREDEFINED EVENT NAMES
    POE::NFA defines four events of its own. These events are used
    internally and may not be overridden by application code.

    See POE::Session'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's features are taken directly from POE::Session. Please
    see POE::Session for more information.

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

BUGS
    See POE::Session's documentation.

    POE::NFA is not as feature-complete as POE::Session. Your feedback is
    appreciated.

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