POE::NFA(3) User Contributed Perl Documentation POE::NFA(3)NAMEPOE::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");
}
}
DESCRIPTIONPOE::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 $object, you
pass in a "Package::Name"
The "runstate" parameter allows "RUNSTATE" to be initialized
differently at instantiation time. "RUNSTATE", like heaps, are usually
anonymous hashrefs, but "runstate" may set them to be array references
or even objects.
goto_state NEW_STATE[, ENTRY_EVENT[, EVENT_ARGS]]
goto_state() puts the machine into a new state. If an ENTRY_EVENT is
specified, then that event will be dispatched after the machine enters
the new state. EVENT_ARGS, if included, will be passed to the entry
event's handler via "ARG0..$#_".
# 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 NAMESPOE::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.
perl v5.14.2 2011-12-15 POE::NFA(3)
