1POE::Session(3)       User Contributed Perl Documentation      POE::Session(3)
2
3
4

NAME

6       POE::Session - a generic event-driven task
7

SYNOPSIS

9         use POE; # auto-includes POE::Kernel and POE::Session
10
11         POE::Session->create(
12           inline_states => {
13             _start => sub { $_[KERNEL]->yield("next") },
14             next   => sub {
15               print "tick...\n";
16               $_[KERNEL]->delay(next => 1);
17             },
18           },
19         );
20
21         POE::Kernel->run();
22         exit;
23
24       POE::Session can also dispatch to object and class methods through
25       "object_states" and "package_states" callbacks.
26

DESCRIPTION

28       POE::Session and its subclasses translate events from POE::Kernel's
29       generic dispatcher into the particular calling conventions suitable for
30       application code.  In design pattern parlance, POE::Session classes are
31       adapters between POE::Kernel and application code.
32
33       The sessions that POE::Kernel manages are more like generic task
34       structures.  Unfortunately these two disparate concepts have virtually
35       identical names.
36
37   A note on nomenclature
38       This documentation will refer to event handlers as "states" in certain
39       unavoidable situations.  Sessions were originally meant to be event-
40       driven state machines, but their purposes evolved over time.  Some of
41       the legacy vocabulary lives on in the API for backward compatibility,
42       however.
43
44       Confusingly, POE::NFA is a class for implementing actual event-driven
45       state machines.  Its documentation uses "state" in the proper sense.
46

USING POE::Session

48       POE::Session has two main purposes.  First, it maps event names to the
49       code that will handle them.  Second, it maps a consistent event
50       dispatch interface to those handlers.
51
52       Consider the "SYNOPSIS" for example.  A POE::Session instance is
53       created with two "inline_states", each mapping an event name ("_start"
54       and "next") to an inline subroutine.  POE::Session ensures that
55       "$_[KERNEL]" and so on are meaningful within an event handler.
56
57       Event handlers may also be object or class methods, using
58       "object_states" and "package_states" respectively.  The create() syntax
59       is different than for "inline_states", but the calling convention is
60       nearly identical.
61
62       Notice that the created POE::Session object has not been saved to a
63       variable.  The new POE::Session object gives itself to POE::Kernel,
64       which then manages it and all the resources it uses.
65
66       It's possible to keep references to new POE::Session objects, but it's
67       not usually necessary.  If an application is not careful about cleaning
68       up these references you will create circular references, which will
69       leak memory when POE::Kernel would normally destroy the POE::Session
70       object.  It is recommended that you keep the session's ID instead.
71
72   POE::Session's Calling Convention
73       The biggest syntactical hurdle most people have with POE is
74       POE::Session's unconventional calling convention.  For example:
75
76         sub handle_event {
77           my ($kernel, $heap, $parameter) = @_[KERNEL, HEAP, ARG0];
78           ...;
79         }
80
81       Or the use of $_[KERNEL], $_[HEAP] and $_[ARG0] inline, as is done in
82       most examples.
83
84       What's going on here is rather basic.  Perl passes parameters into
85       subroutines or methods using the @_ array.  "KERNEL", "HEAP", "ARG0"
86       and others are constants exported by POE::Session (which is included
87       for free when a program uses POE).
88
89       So $_[KERNEL] is an event handler's KERNELth parameter.  @_[HEAP, ARG0]
90       is a slice of @_ containing the HEAPth and ARG0th parameters.
91
92       While this looks odd, it's perfectly plain and legal Perl syntax.  POE
93       uses it for a few reasons:
94
95       1.  In the common case, passing parameters in @_ is faster than passing
96           hash or array references and then dereferencing them in the
97           handler.
98
99       2.  Typos in hash-based parameter lists are either subtle run-time
100           errors or requires constant run-time checking.  Constants are
101           either known at compile time, or are clear compile-time errors.
102
103       3.  Referencing @_ offsets by constants allows parameters to move in
104           the future without breaking application code.
105
106       4.  Most event handlers don't need all of @_.  Slices allow handlers to
107           use only the parameters they're interested in.
108
109   POE::Session Parameters
110       Event handlers receive most of their run-time context in up to nine
111       callback parameters.  POE::Kernel provides many of them.
112
113       $_[OBJECT]
114
115       $_[OBJECT] is $self for event handlers that are an object method.  It
116       is the class (package) name for class-based event handlers.  It is
117       undef for plain coderef callbacks, which have no special $self-ish
118       value.
119
120       "OBJECT" is always zero, since $_[0] is always $self or $class in
121       object and class methods.  Coderef handlers are called with an "undef"
122       placeholder in $_[0] so that the other offsets remain valid.
123
124       It's often useful for method-based event handlers to call other methods
125       in the same object.  $_[OBJECT] helps this happen.
126
127         sub ui_update_everything {
128           my $self = $_[OBJECT];
129           $self->update_menu();
130           $self->update_main_window();
131           $self->update_status_line();
132         }
133
134       You may also use method inheritance.  Here we invoke
135       $self->a_method(@_).  Since Perl's "->" operator unshifts $self onto
136       the beginning of @_, we must first shift a copy off to maintain POE's
137       parameter offsets:
138
139         sub a_method {
140           my $self = shift;
141           $self->SUPER::a_method( @_ );
142           # ... more work ...
143         }
144
145       $_[SESSION]
146
147       $_[SESSION] is a reference to the current session object.  This lets
148       event handlers access their session's methods.  Programs may also
149       compare $_[SESSION] to $_[SENDER] to verify that intra-session events
150       did not come from other sessions.
151
152       $_[SESSION] may also be used as the destination for intra-session
153       post() and call().  yield() is marginally more convenient and efficient
154       than "post($_[SESSION], ...)" however.
155
156       It is bad form to access another session directly.  The recommended
157       approach is to manipulate a session through an event handler.
158
159         sub enable_trace {
160           my $previous_trace = $_[SESSION]->option( trace => 1 );
161           my $id = $_[SESSION]->ID;
162           if ($previous_trace) {
163             print "Session $id: dispatch trace is still on.\n";
164           }
165           else {
166             print "Session $id: dispatch trace has been enabled.\n";
167           }
168         }
169
170       $_[KERNEL]
171
172       The KERNELth parameter is always a reference to the application's
173       singleton POE::Kernel instance.  It is most often used to call
174       POE::Kernel methods from event handlers.
175
176         # Set a 10-second timer.
177         $_[KERNEL]->delay( time_is_up => 10 );
178
179       $_[HEAP]
180
181       Every POE::Session object contains its own variable namespace known as
182       the session's "HEAP".  It is modeled and named after process memory
183       heaps (not priority heaps).  Heaps are by default anonymous hash
184       references, but they may be initialized in create() to be almost
185       anything.  POE::Session itself never uses $_[HEAP], although some POE
186       components do.
187
188       Heaps do not overlap between sessions, although create()'s "heap"
189       parameter can be used to make this happen.
190
191       These two handlers time the lifespan of a session:
192
193         sub _start_handler {
194           $_[HEAP]{ts_start} = time();
195         }
196
197         sub _stop_handler {
198           my $time_elapsed = time() - $_[HEAP]{ts_start};
199           print "Session ", $_[SESSION]->ID, " elapsed seconds: $elapsed\n";
200         }
201
202       $_[STATE]
203
204       The STATEth handler parameter contains the name of the event being
205       dispatched in the current callback.  This can be important since the
206       event and handler names may significantly differ.  Also, a single
207       handler may be assigned to more than one event.
208
209         POE::Session->create(
210           inline_states => {
211             one => \&some_handler,
212             two => \&some_handler,
213             six => \&some_handler,
214             ten => \&some_handler,
215             _start => sub {
216               $_[KERNEL]->yield($_) for qw(one two six ten);
217             }
218           }
219         );
220
221         sub some_handler {
222           print(
223             "Session ", $_[SESSION]->ID,
224             ": some_handler() handled event $_[STATE]\n"
225           );
226         }
227
228       It should be noted however that having event names and handlers names
229       match will make your code easier to navigate.
230
231       $_[SENDER]
232
233       Events must come from somewhere.  $_[SENDER] contains the currently
234       dispatched event's source.
235
236       $_[SENDER] is commonly used as a return address for responses.  It may
237       also be compared against $_[KERNEL] to verify that timers and other
238       POE::Kernel-generated events were not spoofed.
239
240       This "echo_handler()" responds to the sender with an "echo" event that
241       contains all the parameters it received.  It avoids a feedback loop by
242       ensuring the sender session and event (STATE) are not identical to the
243       current ones.
244
245         sub echo_handler {
246           return if $_[SENDER] == $_[SESSION] and $_[STATE] eq "echo";
247           $_[KERNEL]->post( $_[SENDER], "echo", @_[ARG0..$#_] );
248         }
249
250       $_[CALLER_FILE], $_[CALLER_LINE] and $_[CALLER_STATE]
251
252       These parameters are a form of caller(), but they describe where the
253       currently dispatched event originated.  CALLER_FILE and CALLER_LINE are
254       fairly plain.  CALLER_STATE contains the name of the event that was
255       being handled when the event was created, or when the event watcher
256       that ultimately created the event was registered.
257
258       @_[ARG0..ARG9] or @_[ARG0..$#_]
259
260       Parameters $_[ARG0] through the end of @_ contain parameters provided
261       by application code, event watchers, or higher-level libraries.  These
262       parameters are guaranteed to be at the end of @_ so that @_[ARG0..$#_]
263       will always catch them all.
264
265       $#_ is the index of the last value in @_.  Blame Perl if it looks odd.
266       It's merely the $#array syntax where the array name is an underscore.
267
268       Consider
269
270         $_[KERNEL]->yield( ev_whatever => qw( zero one two three ) );
271
272       The handler for ev_whatever will be called with "zero" in $_[ARG0],
273       "one" in $_[ARG1], and so on.  @_[ARG0..$#_] will contain all four
274       words.
275
276         sub ev_whatever {
277           $_[OBJECT]->whatever( @_[ARG0..$#_] );
278         }
279
280   Using POE::Session With Objects
281       One session may handle events across many objects.  Or looking at it
282       the other way, multiple objects can be combined into one session.  And
283       what the heck---go ahead and mix in some inline code as well.
284
285         POE::Session->create(
286           object_states => [
287             $object_1 => { event_1a => "method_1a" },
288             $object_2 => { event_2a => "method_2a" },
289           ],
290           inline_states => {
291             event_3 => \&piece_of_code,
292           },
293         );
294
295       However only one handler may be assigned to a given event name.
296       Duplicates will overwrite earlier ones.
297
298       event_1a is handled by calling "$object_1->method_1a(...)".  $_[OBJECT]
299       is $object_1 in this case.  $_[HEAP] belongs to the session, which
300       means anything stored there will be available to any other event
301       handler regardless of the object.
302
303       event_2a is handled by calling "$object_2->method_2a(...)".  In this
304       case $_[OBJECT] is $object_2.  $_[HEAP] is the same anonymous hashref
305       that was passed to the event_1a handler, though.  The methods are
306       resolved when the event is handled (late-binding).
307
308       event_3 is handled by calling "piece_of_code(...)".  $_[OBJECT] is
309       "undef" here because there's no object.  And once again, $_[HEAP] is
310       the same shared hashref that the handlers for event_1a and event_2a
311       saw.
312
313       Interestingly, there's no technical reason that a single object can't
314       handle events from more than one session:
315
316         for (1..2) {
317           POE::Session->create(
318             object_states => [
319               $object_4 => { event_4 => "method_4" },
320             ]
321           );
322         }
323
324       Now "$object_4->method_4(...)" may be called to handle events from one
325       of two sessions.  In both cases, $_[OBJECT] will be $object_4, but
326       $_[HEAP] will hold data for a particular session.
327
328       The same goes for inline states.  One subroutine may handle events from
329       many sessions.  $_[SESSION] and $_[HEAP] can be used within the handler
330       to easily access the context of the session in which the event is being
331       handled.
332

PUBLIC METHODS

334       POE::Session has just a few public methods.
335
336   create LOTS_OF_STUFF
337       "create()" starts a new session running.  It returns a new POE::Session
338       object upon success, but most applications won't need to save it.
339
340       "create()" invokes the newly started session's _start event handler
341       before returning.
342
343       "create()" also passes the new POE::Session object to POE::Kernel.
344       POE's kernel holds onto the object in order to dispatch events to it.
345       POE::Kernel will release the object when it detects the object has
346       become moribund.  This should cause Perl to destroy the object if
347       application code has not saved a copy of it.
348
349       "create()" accepts several named parameters, most of which are
350       optional.  Note however that the parameters are not part of a hashref.
351
352       args => ARRAYREF
353
354       The "args" parameter accepts a reference to a list of parameters that
355       will be passed to the session's _start event handler in @_ positions
356       "ARG0" through $#_ (the end of @_).
357
358       This example would print "arg0 arg1 etc.":
359
360         POE::Session->create(
361           inline_states => {
362             _start => sub {
363               print "Session started with arguments: @_[ARG0..$#_]\n";
364             },
365           },
366           args => [ 'arg0', 'arg1', 'etc.' ],
367         );
368
369       heap => ANYTHING
370
371       The "heap" parameter allows a session's heap to be initialized
372       differently at instantiation time.  Heaps are usually anonymous
373       hashrefs, but "heap" may set them to be array references or even
374       objects.
375
376       This example prints "tree":
377
378         POE::Session->create(
379           inline_states => {
380             _start => sub {
381               print "Slot 0 = $_[HEAP][0]\n";
382             },
383           },
384           heap => [ 'tree', 'bear' ],
385         );
386
387       Be careful when initializing the heap to be something that doesn't
388       behave like a hashref.  Some libraries assume hashref heap semantics,
389       and they will fail if the heap doesn't work that way.
390
391       inline_states => HASHREF
392
393       "inline_states" maps events names to the subroutines that will handle
394       them.  Its value is a hashref that maps event names to the coderefs of
395       their corresponding handlers:
396
397         POE::Session->create(
398           inline_states => {
399             _start => sub {
400               print "arg0=$_[ARG0], arg1=$_[ARG1], etc.=$_[ARG2]\n";
401             },
402             _stop  => \&stop_handler,
403           },
404           args => [qw( arg0 arg1 etc. )],
405         );
406
407       The term "inline" comes from the fact that coderefs can be inlined
408       anonymous subroutines.
409
410       Be very careful with closures, however.  "Beware circular references".
411
412       object_states => ARRAYREF
413
414       "object_states" associates one or more objects to a session and maps
415       event names to the object methods that will handle them.  It's value is
416       an "ARRAYREF"; "HASHREFs" would stringify the objects, ruining them for
417       method invocation.
418
419       Here _start is handled by "$object->_session_start()" and _stop
420       triggers "$object->_session_stop()":
421
422         POE::Session->create(
423           object_states => [
424             $object => {
425               _start => '_session_start',
426               _stop  => '_session_stop',
427             }
428           ]
429         );
430
431       POE::Session also supports a short form where the event and method
432       names are identical.  Here _start invokes $object->_start(), and _stop
433       triggers $object->_stop():
434
435         POE::Session->create(
436           object_states => [
437             $object => [ '_start', '_stop' ],
438           ]
439         );
440
441       Methods are verified when the session is created, but also resolved
442       when the handler is called (late binding).  Most of the time, a method
443       won't change.  But in some circumstance, such as dynamic inheritance, a
444       method could resolve to a different subroutine.
445
446       options => HASHREF
447
448       POE::Session sessions support a small number of options, which may be
449       initially set with the "option" constructor parameter and changed at
450       run time with the "option()|/option" method.
451
452       "option" takes a hashref with option => value pairs:
453
454         POE::Session->create(
455           ... set up handlers ...,
456           options => { trace => 1, debug => 1 },
457         );
458
459       This is equivalent to the previous example:
460
461         POE::Session->create(
462           ... set up handlers ...,
463         )->option( trace => 1, debug => 1 );
464
465       The supported options and values are documented with the
466       "option()|/option" method.
467
468       package_states => ARRAYREF
469
470       "package_states" associates one or more classes to a session and maps
471       event names to the class methods that will handle them.  Its function
472       is analogous to "object_states", but package names are specified rather
473       than objects.
474
475       In fact, the following documentation is a copy of the "object_states"
476       description with some word substitutions.
477
478       The value for "package_states" is an ARRAYREF to be consistent with
479       "object_states", even though class names (also known as package names)
480       are already strings, so it's not necessary to avoid stringifying them.
481
482       Here _start is handled by "$class_name->_session_start()" and _stop
483       triggers "$class_name->_session_stop()":
484
485         POE::Session->create(
486           package_states => [
487             $class_name => {
488               _start => '_session_start',
489               _stop  => '_session_stop',
490             }
491           ]
492         );
493
494       POE::Session also supports a short form where the event and method
495       names are identical.  Here _start invokes "$class_name->_start()", and
496       _stop triggers "$class_name->_stop()":
497
498         POE::Session->create(
499           package_states => [
500             $class_name => [ '_start', '_stop' ],
501           ]
502         );
503
504   ID
505       "ID()" returns the session instance's unique identifier.  This is an
506       integer that starts at 1 and counts up forever, or until the number
507       wraps around.
508
509       It's theoretically possible that a session ID will not be unique, but
510       this requires at least 4.29 billion sessions to be created within a
511       program's lifespan.  POE guarantees that no two sessions will have the
512       same ID at the same time, however;  your computer doesn't have enough
513       memory to store 4.29 billion session objects.
514
515       A session's ID is unique within a running process, but multiple
516       processes are likely to have the same session IDs.  If a global ID is
517       required, it will need to include both "$_[KERNEL]->ID" and
518       "$_[SESSION]->ID".
519
520   option OPTION_NAME [, OPTION_VALUE [, OPTION_NAME, OPTION_VALUE]... ]
521       "option()" sets and/or retrieves the values of various session options.
522       The options in question are implemented by POE::Session and do not have
523       any special meaning anywhere else.
524
525       It may be called with a single OPTION_NAME to retrieve the value of
526       that option.
527
528         my $trace_value = $_[SESSION]->option('trace');
529
530       "option()" sets an option's value when called with a single
531       OPTION_NAME, OPTION_VALUE pair.  In this case, "option()" returns the
532       option's previous value.
533
534         my $previous_trace = $_[SESSION]->option(trace => 1);
535
536       "option()" may also be used to set the values of multiple options at
537       once.  In this case, "option()" returns all the specified options'
538       previous values in an anonymous hashref:
539
540         my $previous_values = $_[SESSION]->option(
541           trace => 1,
542           debug => 1,
543         );
544
545         print "Previous option values:\n";
546         while (my ($option, $old_value) = each %$previous_values) {
547           print "  $option = $old_value\n";
548         }
549
550       POE::Session currently supports three options:
551
552       The "debug" option.
553
554       The "debug" option is intended to enable additional warnings when
555       strange things are afoot within POE::Session.  At this time, there is
556       only one additional warning:
557
558       ยท   Redefining an event handler does not usually cause a warning, but
559           it will when the "debug" option is set.
560
561       The "default" option.
562
563       Enabling the "default" option causes unknown events to become warnings,
564       if there is no _default handler to catch them.
565
566       The class-level "POE::Session::ASSERT_STATES" flag is implemented by
567       enabling the "default" option on all new sessions.
568
569       The "trace" option.
570
571       Turn on the "trace" option to dump a log of all the events dispatched
572       to a particular session.  This is a session-specific trace option that
573       allows individual sessions to be debugged.
574
575       Session-level tracing also indicates when events are redirected to
576       _default.  This can be used to discover event naming errors.
577
578       User-defined options.
579
580       "option()" does not verify whether OPTION_NAMEs are known, so
581       "option()" may be used to store and retrieve user-defined information.
582
583       Choose option names with caution.  There is no established convention
584       to avoid namespace collisions between user-defined options and future
585       internal options.
586
587   postback EVENT_NAME, EVENT_PARAMETERS
588       "postback()" manufactures callbacks that post POE events.  It returns
589       an anonymous code reference that will post EVENT_NAME to the target
590       session, with optional EVENT_PARAMETERS in an array reference in ARG0.
591       Parameters passed to the callback will be sent in an array reference in
592       ARG1.
593
594       In other words, ARG0 allows the postback's creator to pass context
595       through the postback.  ARG1 allows the caller to return information.
596
597       This example creates a coderef that when called posts "ok_button" to
598       $some_session with ARG0 containing "[ 8, 6, 7 ]".
599
600         my $postback = $some_session->postback( "ok_button", 8, 6, 7 );
601
602       Here's an example event handler for "ok_button".
603
604         sub handle_ok_button {
605           my ($creation_args, $called_args) = @_[ARG0, ARG1];
606           print "Postback created with (@$creation_args).\n";
607           print "Postback called with (@$called_args).\n";
608         }
609
610       Calling $postback->(5, 3, 0, 9) would perform the equivalent of...
611
612         $poe_kernel->post(
613           $some_session, "ok_button",
614           [ 8, 6, 7 ],
615           [ 5, 3, 0, 9 ]
616         );
617
618       This would be displayed when "ok_button" was dispatched to
619       handle_ok_button():
620
621         Postback created with (8 6 7).
622         Postback called with (5 3 0 9).
623
624       Postbacks hold references to their target sessions.  Therefore sessions
625       with outstanding postbacks will remain active.  Under every event loop
626       except Tk, postbacks are blessed so that DESTROY may be called when
627       their users are done.  This triggers a decrement on their reference
628       counts, allowing sessions to stop.
629
630       Postbacks have one method, weaken(), which may be used to reduce their
631       reference counts upon demand.  weaken() returns the postback, so you
632       can do:
633
634         my $postback = $session->postback("foo")->weaken();
635
636       Postbacks were created as a thin adapter between callback libraries and
637       POE.  The problem at hand was how to turn callbacks from the Tk
638       graphical toolkit's widgets into POE events without subclassing several
639       Tk classes.  The solution was to provide Tk with plain old callbacks
640       that posted POE events.
641
642       Since "postback()" and "callback()" are Session methods, they may be
643       called on $_[SESSION] or $_[SENDER], depending on particular needs.
644       There are usually better ways to interact between sessions than abusing
645       postbacks, however.
646
647       Here's a brief example of attaching a Gtk2 button to a POE event
648       handler:
649
650         my $btn = Gtk2::Button->new("Clear");
651         $btn->signal_connect( "clicked", $_[SESSION]->postback("ev_clear") );
652
653       Points to remember: The session will remain alive as long as $btn
654       exists and holds a copy of $_[SESSION]'s postback.  Any parameters
655       passed by the Gtk2 button will be in ARG1.
656
657   callback EVENT_NAME, EVENT_PARAMETERS
658       callback() manufactures callbacks that use "$poe_kernel->call()" to
659       deliver POE events rather than "$poe_kernel->post()".  It is identical
660       to "postback()" in every other respect.
661
662       callback() was created to avoid race conditions that arise when
663       external libraries assume callbacks will execute synchronously.
664       File::Find is an obvious (but not necessarily appropriate) example.  It
665       provides a lot of information in local variables that stop being valid
666       after the callback.  The information would be unavailable by the time a
667       post()ed event was dispatched.
668
669   get_heap
670       "get_heap()" returns a reference to a session's heap.  This is the same
671       value as $_[HEAP] for the target session.  "get_heap()" is intended to
672       be used with $poe_kernel and POE::Kernel's "get_active_session()" so
673       that libraries do not need these three common values explicitly passed
674       to them.
675
676       That is, it prevents the need for:
677
678         sub some_helper_function {
679           my ($kernel, $session, $heap, @specific_parameters) = @_;
680           ...;
681         }
682
683       Rather, helper functions may use:
684
685         use POE::Kernel; # exports $poe_kernel
686         sub some_helper_function {
687           my (@specific_parameters) = @_;
688           my $session = $poe_kernel->get_active_session();
689           my $heap = $session->get_heap();
690         }
691
692       This isn't very convenient for people writing libraries, but it makes
693       the libraries much more convenient to use.
694
695       Using "get_heap()" to break another session's encapsulation is strongly
696       discouraged.
697
698   instantiate CREATE_PARAMETERS
699       "instantiate()" creates and returns an empty POE::Session object.  It
700       is called with the CREATE_PARAMETERS in a hash reference just before
701       "create()" processes them.  Modifications to the CREATE_PARAMETERS will
702       affect how "create()" initializes the new session.
703
704       Subclasses may override "instantiate()" to alter the underlying
705       session's structure.  They may extend "instantiate()" to add new
706       parameters to "create()".
707
708       Any parameters not recognized by "create()" must be removed from the
709       CREATE_PARAMETERS before "instantiate()" returns.  "create()" will
710       croak if it discovers unknown parameters.
711
712       Be sure to return $self from instantiate.
713
714         sub instantiate {
715           my ($class, $create_params) = @_;
716
717           # Have the base class instantiate the new session.
718           my $self = $class->SUPER::instantiate($create_parameters);
719
720           # Extend the parameters recognized by create().
721           my $new_option = delete $create_parameters->{new_option};
722           if (defined $new_option) {
723             # ... customize $self here ...
724           }
725
726           return $self;
727         }
728
729   try_alloc START_ARGS
730       "try_alloc()" calls POE::Kernel's "session_alloc()" to allocate a
731       session structure and begin managing the session within POE's kernel.
732       It is called at the end of POE::Session's "create()".  It returns
733       $self.
734
735       It is a subclassing hook for late session customization prior to
736       "create()" returning.  It may also affect the contents of @_[ARG0..$#_]
737       that are passed to the session's _start handler.
738
739         sub try_alloc {
740           my ($self, @start_args) = @_;
741
742           # Perform late initialization.
743           # ...
744
745           # Give $self to POE::Kernel.
746           return $self->SUPER::try_alloc(@args);
747         }
748

POE::Session's EVENTS

750       Please do not define new events that begin with a leading underscore.
751       POE claims /^_/ events as its own.
752
753       POE::Session only generates one event, _default.  All other internal
754       POE events are generated by (and documented in) POE::Kernel.
755
756   _default
757       _default is the "AUTOLOAD" of event handlers.  If POE::Session can't
758       find a handler at dispatch time, it attempts to redirect the event to
759       _default's handler instead.
760
761       If there's no _default handler, POE::Session will silently drop the
762       event unless the "default" option is set.
763
764       To preserve the original information, the original event is slightly
765       changed before being redirected to the _default handler:  The original
766       event parameters are moved to an array reference in ARG1, and the
767       original event name is passed to _default in ARG0.
768
769         sub handle_default {
770           my ($event, $args) = @_[ARG0, ARG1];
771           print(
772             "Session ", $_[SESSION]->ID,
773             " caught unhandled event $event with (@$args).\n"
774           );
775         }
776
777       _default is quite flexible.  It may be used for debugging, or to handle
778       dynamically generated event names without pre-defining their handlers.
779       In the latter sense, _default performs analogously to Perl's
780       "AUTOLOAD".
781
782       _default may also be used as the default or "otherwise" clause of a
783       switch statement.  Consider an input handler that throws events based
784       on a command name:
785
786         sub parse_command {
787           my ($command, @parameters) = split /\s+/, $_[ARG0];
788           $_[KERNEL]->post( "cmd_$command", @parameters );
789         }
790
791       A _default handler may be used to emit errors for unknown commands:
792
793         sub handle_default {
794           my $event = $_[ARG0];
795           return unless $event =~ /^cmd_(\S+)/;
796           warn "Unknown command: $1\n";
797         }
798
799       The _default behavior is implemented in POE::Session, so it may be
800       different for other session types.
801
802   POE::Session's Debugging Features
803       POE::Session contains one debugging assertion, for now.
804
805       ASSERT_STATES
806
807       Setting ASSERT_STATES to true causes every Session to warn when they
808       are asked to handle unknown events.  Session.pm implements the guts of
809       ASSERT_STATES by defaulting the "default" option to true instead of
810       false.  See the option() method earlier in this document for details
811       about the "default" option.
812

SEE ALSO

814       POE::Kernel.
815
816       The SEE ALSO section in POE contains a table of contents covering the
817       entire POE distribution.
818

BUGS

820       There is a chance that session IDs may collide after Perl's integer
821       value wraps.  This can occur after as few as 4.29 billion sessions.
822
823   Beware circular references
824       As you're probably aware, a circular reference is when a variable is
825       part of a reference chain that eventually refers back to itself.  Perl
826       will not reclaim the memory involved in such a reference chain until
827       the chain is manually broken.
828
829       Here a POE::Session is created that refers to itself via an external
830       scalar.  The event handlers import $session via closures which are in
831       turn stored within $session.  Even if this session stops, the circular
832       references will remain.
833
834         my $session;
835         $session = POE::Session->create(
836           inline_states => {
837             _start => sub {
838               $_[HEAP]->{todo} = [ qw( step1 step2 step2a ) ],
839               $_[KERNEL]->post( $session, 'next' );
840             },
841             next => sub {
842               my $next = shift @{ $_[HEAP]->{todo} };
843               return unless $next;
844               $_[KERNEL]->post( $session, $next );
845             }
846             # ....
847           }
848         );
849
850       Reduced to its essence:
851
852         my %event_handlers;
853         $event_handler{_start} = sub { \%event_handlers };
854
855       Note also that an anonymous sub creates a closure on all lexical
856       variables in the scope it was defined in, even if it doesn't reference
857       them.  $session is still being held in a circular reference here:
858
859         my $self = $package->new;
860         my $session;
861         $session = POE::Session->create(
862           inline_state => {
863             _start => sub { $self->_start( @_[ARG0..$#_] ) }
864           }
865         );
866
867       To avoid this, a session may set an alias for itself.  Other parts of
868       the program may then refer to it by alias.  In this case, one needn't
869       keep track of the session themselves (POE::Kernel will do it anyway).
870
871         POE::Session->create(
872           inline_states => {
873             _start => sub {
874               $_[HEAP]->{todo} = [ qw( step1 step2 step2a ) ],
875               $_[KERNEL]->alias_set('step_doer');
876               $_[KERNEL]->post( 'step_doer', 'next' );
877             },
878             next => sub {
879               my $next = shift @{ $_[HEAP]->{todo} };
880               return unless $next;
881               $_[KERNEL]->post( 'step_doer', $next );
882             }
883             # ....
884           }
885         );
886
887       Aliases aren't even needed in the previous example because the session
888       refers to itself.  One could instead use POE::Kernel's yield() method
889       to post the event back to the current session:
890
891         next => sub {
892           my $next = shift @{ $_[HEAP]->{todo} };
893           return unless $next;
894           $_[KERNEL]->yield( $next );
895         }
896
897       Or the "$_[SESSION]" parameter passed to every event handler, but
898       yield() is more efficient.
899
900         next => sub {
901           my $next = shift @{ $_[HEAP]->{todo} };
902           return unless $next;
903           $_[KERNEL]->post( $_[SESSION], $next );
904         }
905
906       Along the same lines as "$_[SESSION]", a session can respond back to
907       the sender of an event by posting to "$_[SENDER]".  This is great for
908       responding to requests.
909
910       If a program must hold onto some kind of dynamic session reference,
911       it's recommended to use the session's numeric ID rather than the object
912       itself.  A session ID may be converted back into its object, but post()
913       accepts session IDs as well as objects and aliases:
914
915         my $session_id;
916         $session_id = POE::Session->create(
917           inline_states => {
918             _start => sub {
919               $_[HEAP]->{todo} = [ qw( step1 step2 step2a ) ],
920               $_[KERNEL]->post( $session_id, 'next' );
921             },
922             # ....
923           }
924         )->ID;
925

AUTHORS & COPYRIGHTS

927       Please see POE for more information about authors and contributors.
928
929
930
931perl v5.32.0                      2020-07-28                   POE::Session(3)
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