Provided by: libpoe-perl_1.3670-1_all 
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.