1gen_statem(3) Erlang Module Definition gen_statem(3)
2
6 gen_statem - Generic state machine behavior.
7
9 gen_statem provides a generic state machine behaviour that for new code
10 replaces its predecessor gen_fsm since Erlang/OTP 20.0. The gen_fsm be‐
11 haviour remains in OTP "as is".
12 Note:
14 If you are new to gen_statem and want an overview of concepts and oper‐
15 ation the section gen_statem Behaviour located in the User's Guide
16 OTP Design Principles is recommended to read before this reference
17 manual, possibly after the Description section you are reading here.
18 This reference manual contains type descriptions generated from types
21 in the gen_statem source code, so they are correct. However, the gener‐
22 ated descriptions also reflect the type hierarchy, which sometimes
23 makes it hard to get a good overview. If so, see the section gen_statem
24 Behaviour in the OTP Design Principles User's Guide.
25 Note:
27 * This behavior appeared in Erlang/OTP 19.0.
29 *
31 In OTP 19.1 a backwards incompatible change of the return tuple
32 from Module:init/1 was made and the mandatory callback function
33 Module:callback_mode/0 was introduced.
34 *
36 In OTP 20.0 generic time-outs were added.
37 *
39 In OTP 22.1 time-out content update and explicit time-out cancel
40 were added.
41 *
43 In OTP 22.3 the possibility to change the callback module with
44 actions change_callback_module, push_callback_module and pop_call‐
45 back_module, was added.
46 gen_statem has got the same features that gen_fsm had and adds some
48 really useful:
49 * Co-located state code
51 * Arbitrary term state
53 * Event postponing
55 * Self-generated events
57 * State time-out
59 * Multiple generic named time-outs
61 * Absolute time-out time
63 * Automatic state enter calls
65 *
67 Reply from other state than the request, sys traceable
68 * Multiple sys traceable replies
70 * Changing the callback module
72 Two callback modes are supported:
74 * One for finite-state machines (gen_fsm like), which requires the
76 state to be an atom and uses that state as the name of the current
77 callback function.
78 * One that allows the state to be any term and that uses one callback
80 function for all states.
81 The callback model(s) for gen_statem differs from the one for gen_fsm,
83 but it is still fairly easy to rewrite from gen_fsm to gen_statem.
84 A generic state machine server process (gen_statem) implemented using
86 this module has a standard set of interface functions and includes
87 functionality for tracing and error reporting. It also fits into an OTP
88 supervision tree. For more information, see OTP Design Principles.
89 A gen_statem assumes all specific parts to be located in a callback
91 module exporting a predefined set of functions. The relationship
92 between the behavior functions and the callback functions is as fol‐
93 lows:
94 gen_statem module Callback module
96 ----------------- ---------------
97 gen_statem:start
98 gen_statem:start_monitor
99 gen_statem:start_link -----> Module:init/1
100 Server start or code change
102 -----> Module:callback_mode/0
103 gen_statem:stop -----> Module:terminate/3
105 gen_statem:call
107 gen_statem:cast
108 gen_statem:send_request
109 erlang:send
110 erlang:'!' -----> Module:StateName/3
111 Module:handle_event/4
112 - -----> Module:terminate/3
114 - -----> Module:code_change/4
116 Events are of different types, so the callback functions can know the
118 origin of an event and how to respond.
119 If a callback function fails or returns a bad value, the gen_statem
121 terminates, unless otherwise stated. However, an exception of class
122 throw is not regarded as an error but as a valid return from all call‐
123 back functions.
124 The state callback for a specific state in a gen_statem is the callback
126 function that is called for all events in this state. It is selected
127 depending on which callback mode that the callback module defines with
128 the callback function Module:callback_mode/0.
129 When the callback mode is state_functions, the state must be an atom
131 and is used as the state callback name; see Module:StateName/3. This
132 co-locates all code for a specific state in one function as the
133 gen_statem engine branches depending on state name. Note the fact that
134 the callback function Module:terminate/3 makes the state name terminate
135 unusable in this mode.
136 When the callback mode is handle_event_function, the state can be any
138 term and the state callback name is Module:handle_event/4. This makes
139 it easy to branch depending on state or event as you desire. Be careful
140 about which events you handle in which states so that you do not acci‐
141 dentally postpone an event forever creating an infinite busy loop.
142 When gen_statem receives a process message it is converted into an
144 event and the state callback is called with the event as two arguments:
145 type and content. When the state callback has processed the event it
146 returns to gen_statem which does a state transition. If this state
147 transition is to a different state, that is: NextState =/= State, it is
148 a state change.
149 The state callback may return transition actions for gen_statem to exe‐
151 cute during the state transition, for example to reply to a
152 gen_statem:call/2,3.
153 One of the possible transition actions is to postpone the current
155 event. Then it is not retried in the current state. The gen_statem
156 engine keeps a queue of events divided into the postponed events and
157 the events still to process. After a state change the queue restarts
158 with the postponed events.
159 The gen_statem event queue model is sufficient to emulate the normal
161 process message queue with selective receive. Postponing an event cor‐
162 responds to not matching it in a receive statement, and changing states
163 corresponds to entering a new receive statement.
164 The state callback can insert events using the transition actions
166 next_event and such an event is inserted in the event queue as the next
167 to call the state callback with. That is, as if it is the oldest incom‐
168 ing event. A dedicated event_type() internal can be used for such
169 events making them impossible to mistake for external events.
170 Inserting an event replaces the trick of calling your own state han‐
172 dling functions that you often would have to resort to in, for example,
173 gen_fsm to force processing an inserted event before others.
174 The gen_statem engine can automatically make a specialized call to the
176 state callback whenever a new state is entered; see state_enter(). This
177 is for writing code common to all state entries. Another way to do it
178 is to explicitly insert an event at the state transition, and/or to use
179 a dedicated state transition function, but that is something you will
180 have to remember at every state transition to the state(s) that need
181 it.
182 Note:
184 If you in gen_statem, for example, postpone an event in one state and
185 then call another state callback of yours, you have not done a state
186 change and hence the postponed event is not retried, which is logical
187 but can be confusing.
188 For the details of a state transition, see type transition_option().
191 A gen_statem handles system messages as described in sys. The sys mod‐
193 ule can be used for debugging a gen_statem.
194 Notice that a gen_statem does not trap exit signals automatically, this
196 must be explicitly initiated in the callback module (by calling
197 process_flag(trap_exit, true).
198 Unless otherwise stated, all functions in this module fail if the spec‐
200 ified gen_statem does not exist or if bad arguments are specified.
201 The gen_statem process can go into hibernation; see proc_lib:hiber‐
203 nate/3. It is done when a state callback or Module:init/1 specifies
204 hibernate in the returned Actions list. This feature can be useful to
205 reclaim process heap memory while the server is expected to be idle for
206 a long time. However, use this feature with care, as hibernation can be
207 too costly to use after every event; see erlang:hibernate/3.
208 There is also a server start option {hibernate_after, Timeout} for
210 start/3,4, start_monitor/3,4, start_link/3,4 or enter_loop/4,5,6, that
211 may be used to automatically hibernate the server.
212
214 The following example shows a simple pushbutton model for a toggling
215 pushbutton implemented with callback mode state_functions. You can push
216 the button and it replies if it went on or off, and you can ask for a
217 count of how many times it has been pushed to switch on.
218 The following is the complete callback module file pushbutton.erl:
220 -module(pushbutton).
222 -behaviour(gen_statem).
223 -export([start/0,push/0,get_count/0,stop/0]).
225 -export([terminate/3,code_change/4,init/1,callback_mode/0]).
226 -export([on/3,off/3]).
227 name() -> pushbutton_statem. % The registered server name
229 %% API. This example uses a registered name name()
231 %% and does not link to the caller.
232 start() ->
233 gen_statem:start({local,name()}, ?MODULE, [], []).
234 push() ->
235 gen_statem:call(name(), push).
236 get_count() ->
237 gen_statem:call(name(), get_count).
238 stop() ->
239 gen_statem:stop(name()).
240 %% Mandatory callback functions
242 terminate(_Reason, _State, _Data) ->
243 void.
244 code_change(_Vsn, State, Data, _Extra) ->
245 {ok,State,Data}.
246 init([]) ->
247 %% Set the initial state + data. Data is used only as a counter.
248 State = off, Data = 0,
249 {ok,State,Data}.
250 callback_mode() -> state_functions.
251 %%% state callback(s)
253 off({call,From}, push, Data) ->
255 %% Go to 'on', increment count and reply
256 %% that the resulting status is 'on'
257 {next_state,on,Data+1,[{reply,From,on}]};
258 off(EventType, EventContent, Data) ->
259 handle_event(EventType, EventContent, Data).
260 on({call,From}, push, Data) ->
262 %% Go to 'off' and reply that the resulting status is 'off'
263 {next_state,off,Data,[{reply,From,off}]};
264 on(EventType, EventContent, Data) ->
265 handle_event(EventType, EventContent, Data).
266 %% Handle events common to all states
268 handle_event({call,From}, get_count, Data) ->
269 %% Reply with the current count
270 {keep_state,Data,[{reply,From,Data}]};
271 handle_event(_, _, Data) ->
272 %% Ignore all other events
273 {keep_state,Data}.
274 The following is a shell session when running it:
277 1> pushbutton:start().
279 {ok,<0.36.0>}
280 2> pushbutton:get_count().
281 0
282 3> pushbutton:push().
283 on
284 4> pushbutton:get_count().
285 1
286 5> pushbutton:push().
287 off
288 6> pushbutton:get_count().
289 1
290 7> pushbutton:stop().
291 ok
292 8> pushbutton:push().
293 ** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
294 in function gen:do_for_proc/2 (gen.erl, line 261)
295 in call from gen_statem:call/3 (gen_statem.erl, line 386)
296 To compare styles, here follows the same example using callback mode
299 handle_event_function, or rather the code to replace after function
300 init/1 of the pushbutton.erl example file above:
301 callback_mode() -> handle_event_function.
303 %%% state callback(s)
305 handle_event({call,From}, push, off, Data) ->
307 %% Go to 'on', increment count and reply
308 %% that the resulting status is 'on'
309 {next_state,on,Data+1,[{reply,From,on}]};
310 handle_event({call,From}, push, on, Data) ->
311 %% Go to 'off' and reply that the resulting status is 'off'
312 {next_state,off,Data,[{reply,From,off}]};
313 %%
314 %% Event handling common to all states
315 handle_event({call,From}, get_count, State, Data) ->
316 %% Reply with the current count
317 {next_state,State,Data,[{reply,From,Data}]};
318 handle_event(_, _, State, Data) ->
319 %% Ignore all other events
320 {next_state,State,Data}.
321
324 server_name() =
325 {global, GlobalName :: term()} |
326 {via, RegMod :: module(), Name :: term()} |
327 {local, atom()}
328 Name specification to use when starting a gen_statem server. See
330 start_link/3 and server_ref() below.
331 server_ref() =
333 pid() |
334 (LocalName :: atom()) |
335 {Name :: atom(), Node :: atom()} |
336 {global, GlobalName :: term()} |
337 {via, RegMod :: module(), ViaName :: term()}
338 Server specification to use when addressing a gen_statem server.
340 See call/2 and server_name() above.
341 It can be:
343 pid() | LocalName:
345 The gen_statem is locally registered.
346 {Name,Node}:
348 The gen_statem is locally registered on another node.
349 {global,GlobalName}:
351 The gen_statem is globally registered in global.
352 {via,RegMod,ViaName}:
354 The gen_statem is registered in an alternative process reg‐
355 istry. The registry callback module RegMod is to export
356 functions register_name/2, unregister_name/1,
357 whereis_name/1, and send/2, which are to behave like the
358 corresponding functions in global. Thus, {via,global,Global‐
359 Name} is the same as {global,GlobalName}.
360 start_opt() =
362 {timeout, Time :: timeout()} |
363 {spawn_opt, [proc_lib:start_spawn_option()]} |
364 enter_loop_opt()
365 Options that can be used when starting a gen_statem server
367 through, for example, start_link/3.
368 start_ret() = {ok, pid()} | ignore | {error, term()}
370 Return value from the start() and start_link() functions, for
372 example, start_link/3.
373 start_mon_ret() =
375 {ok, {pid(), reference()}} | ignore | {error, term()}
376 Return value from the start_monitor() functions.
378 enter_loop_opt() =
380 {hibernate_after, HibernateAfterTimeout :: timeout()} |
381 {debug, Dbgs :: [sys:debug_option()]}
382 Options that can be used when starting a gen_statem server
384 through, enter_loop/4-6.
385 hibernate_after:
387 HibernateAfterTimeout specifies that the gen_statem process
388 awaits any message for HibernateAfterTimeout milliseconds
389 and if no message is received, the process goes into hiber‐
390 nation automatically (by calling proc_lib:hibernate/3).
391 debug:
393 For every entry in Dbgs, the corresponding function in sys
394 is called.
395 from() = {To :: pid(), Tag :: term()}
397 Destination to use when replying through, for example, the
399 action() {reply,From,Reply} to a process that has called the
400 gen_statem server using call/2.
401 state() = state_name() | term()
403 If the callback mode is handle_event_function, the state can be
405 any term. After a state change (NextState =/= State), all post‐
406 poned events are retried.
407 state_name() = atom()
409 If the callback mode is state_functions, the state must be an
411 atom. After a state change (NextState =/= State), all postponed
412 events are retried. Note that the state terminate is not possi‐
413 ble to use since it would collide with the optional callback
414 function Module:terminate/3.
415 data() = term()
417 A term in which the state machine implementation is to store any
419 server data it needs. The difference between this and the
420 state() itself is that a change in this data does not cause
421 postponed events to be retried. Hence, if a change in this data
422 would change the set of events that are handled, then that data
423 item is to be made a part of the state.
424 event_type() =
426 external_event_type() | timeout_event_type() | internal
427 There are 3 categories of events: external, timeout, and inter‐
429 nal.
430 internal events can only be generated by the state machine
432 itself through the transition action next_event.
433 external_event_type() = {call, From :: from()} | cast | info
435 External events are of 3 types: {call,From}, cast, or info. Type
437 call originates from the API functions call/2 and
438 send_request/2. For calls, the event contains whom to reply to.
439 Type cast originates from the API function cast/2. Type info
440 originates from regular process messages sent to the gen_statem.
441 timeout_event_type() =
443 timeout | {timeout, Name :: term()} | state_timeout
444 There are 3 types of time-out events that the state machine can
446 generate for itself with the corresponding timeout_action()s.
447 callback_mode_result() =
449 callback_mode() | [callback_mode() | state_enter()]
450 This is the return type from Module:callback_mode/0 and selects
452 callback mode and whether to do state enter calls, or not.
453 callback_mode() = state_functions | handle_event_function
455 The callback mode is selected with the return value from Mod‐
457 ule:callback_mode/0:
458 state_functions:
460 The state must be of type state_name() and one callback
461 function per state, that is, Module:StateName/3, is used.
462 handle_event_function:
464 The state can be any term and the callback function Mod‐
465 ule:handle_event/4 is used for all states.
466 The function Module:callback_mode/0 is called when starting the
468 gen_statem, after code change and after changing the callback
469 module with any of the actions change_callback_module,
470 push_callback_module or pop_callback_module. The result is
471 cached for subsequent calls to state callbacks.
472 state_enter() = state_enter
474 Whether the state machine should use state enter calls or not is
476 selected when starting the gen_statem and after code change
477 using the return value from Module:callback_mode/0.
478 If Module:callback_mode/0 returns a list containing state_enter,
480 the gen_statem engine will, at every state change, call the
481 state callback with arguments (enter, OldState, Data) or (enter,
482 OldState, State, Data), depending on the callback mode. This may
483 look like an event but is really a call performed after the pre‐
484 vious state callback returned and before any event is delivered
485 to the new state callback. See Module:StateName/3 and Mod‐
486 ule:handle_event/4. Such a call can be repeated by returning a
487 repeat_state or repeat_state_and_data tuple from the state call‐
488 back.
489 If Module:callback_mode/0 does not return such a list, no state
491 enter calls are done.
492 If Module:code_change/4 should transform the state, it is
494 regarded as a state rename and not a state change, which will
495 not cause a state enter call.
496 Note that a state enter call will be done right before entering
498 the initial state even though this actually is not a state
499 change. In this case OldState =:= State, which cannot happen for
500 a subsequent state change, but will happen when repeating the
501 state enter call.
502 transition_option() =
504 postpone() |
505 hibernate() |
506 event_timeout() |
507 generic_timeout() |
508 state_timeout()
509 Transition options can be set by actions and modify the state
511 transition. The state transition takes place when the state
512 callback has processed an event and returns. Here are the
513 sequence of steps for a state transition:
514 * All returned actions are processed in order of appearance.
516 In this step all replies generated by any reply_action() are
517 sent. Other actions set transition_option()s that come into
518 play in subsequent steps.
519 * If state enter calls are used, and either it is the initial
521 state or one of the callback results repeat_state_and_data
522 or repeat_state_and_data is used the gen_statem engine calls
523 the current state callback with arguments (enter, State,
524 Data) or (enter, State, State, Data) (depending on callback
525 mode) and when it returns starts again from the top of this
526 sequence.
527 If state enter calls are used, and the state changes the
529 gen_statem engine calls the new state callback with argu‐
530 ments (enter, OldState, Data) or (enter, OldState, State,
531 Data) (depending on callback mode) and when it returns
532 starts again from the top of this sequence.
533 * If postpone() is true, the current event is postponed.
535 * If this is a state change, the queue of incoming events is
537 reset to start with the oldest postponed.
538 * All events stored with action() next_event are inserted to
540 be processed before previously queued events.
541 * Time-out timers event_timeout(), generic_timeout() and
543 state_timeout() are handled. Time-outs with zero time are
544 guaranteed to be delivered to the state machine before any
545 external not yet received event so if there is such a time-
546 out requested, the corresponding time-out zero event is
547 enqueued as the newest received event; that is after already
548 queued events such as inserted and postponed events.
549 Any event cancels an event_timeout() so a zero time event
551 time-out is only generated if the event queue is empty.
552 A state change cancels a state_timeout() and any new transi‐
554 tion option of this type belongs to the new state, that is;
555 a state_timeout() applies to the state the state machine
556 enters.
557 * If there are enqueued events the state callback for the pos‐
559 sibly new state is called with the oldest enqueued event,
560 and we start again from the top of this sequence.
561 * Otherwise the gen_statem goes into receive or hibernation
563 (if hibernate() is true) to wait for the next message. In
564 hibernation the next non-system event awakens the
565 gen_statem, or rather the next incoming message awakens the
566 gen_statem, but if it is a system event it goes right back
567 into hibernation. When a new message arrives the state call‐
568 back is called with the corresponding event, and we start
569 again from the top of this sequence.
570 postpone() = boolean()
572 If true, postpones the current event and retries it after a
574 state change (NextState =/= State).
575 hibernate() = boolean()
577 If true, hibernates the gen_statem by calling proc_lib:hiber‐
579 nate/3 before going into receive to wait for a new external
580 event.
581 Note:
583 If there are enqueued events to process when hibrnation is
584 requested, this is optimized by not hibernating but instead
585 calling erlang:garbage_collect/0 to simulate that the gen_statem
586 entered hibernation and immediately got awakened by an enqueued
587 event.
588 event_timeout() = timeout() | integer()
591 Starts a timer set by enter_action() timeout. When the timer
593 expires an event of event_type() timeout will be generated. See
594 erlang:start_timer/4 for how Time and Options are interpreted.
595 Future erlang:start_timer/4 Options will not necessarily be sup‐
596 ported.
597 Any event that arrives cancels this time-out. Note that a
599 retried or inserted event counts as arrived. So does a state
600 time-out zero event, if it was generated before this time-out is
601 requested.
602 If Time is infinity, no timer is started, as it never would
604 expire anyway.
605 If Time is relative and 0 no timer is actually started, instead
607 the the time-out event is enqueued to ensure that it gets pro‐
608 cessed before any not yet received external event, but after
609 already queued events.
610 Note that it is not possible nor needed to cancel this time-out,
612 as it is cancelled automatically by any other event.
613 generic_timeout() = timeout() | integer()
615 Starts a timer set by enter_action() {timeout,Name}. When the
617 timer expires an event of event_type() {timeout,Name} will be
618 generated. See erlang:start_timer/4 for how Time and Options are
619 interpreted. Future erlang:start_timer/4 Options will not neces‐
620 sarily be supported.
621 If Time is infinity, no timer is started, as it never would
623 expire anyway.
624 If Time is relative and 0 no timer is actually started, instead
626 the the time-out event is enqueued to ensure that it gets pro‐
627 cessed before any not yet received external event.
628 Setting a timer with the same Name while it is running will
630 restart it with the new time-out value. Therefore it is possible
631 to cancel a specific time-out by setting it to infinity.
632 state_timeout() = timeout() | integer()
634 Starts a timer set by enter_action() state_timeout. When the
636 timer expires an event of event_type() state_timeout will be
637 generated. See erlang:start_timer/4 for how Time and Options are
638 interpreted. Future erlang:start_timer/4 Options will not neces‐
639 sarily be supported.
640 If Time is infinity, no timer is started, as it never would
642 expire anyway.
643 If Time is relative and 0 no timer is actually started, instead
645 the the time-out event is enqueued to ensure that it gets pro‐
646 cessed before any not yet received external event.
647 Setting this timer while it is running will restart it with the
649 new time-out value. Therefore it is possible to cancel this
650 time-out by setting it to infinity.
651 timeout_option() = {abs, Abs :: boolean()}
653 If Abs is true an absolute timer is started, and if it is false
655 a relative, which is the default. See erlang:start_timer/4 for
656 details.
657 action() =
659 postpone |
660 {postpone, Postpone :: postpone()} |
661 {next_event,
662 EventType :: event_type(),
663 EventContent :: term()} |
664 {change_callback_module, NewModule :: module()} |
665 {push_callback_module, NewModule :: module()} |
666 pop_callback_module |
667 enter_action()
668 These transition actions can be invoked by returning them from
670 the state callback when it is called with an event, from Mod‐
671 ule:init/1 or by giving them to enter_loop/5,6.
672 Actions are executed in the containing list order.
674 Actions that set transition options override any previous of
676 the same type, so the last in the containing list wins. For
677 example, the last postpone() overrides any previous postpone()
678 in the list.
679 postpone:
681 Sets the transition_option() postpone() for this state tran‐
682 sition. This action is ignored when returned from Mod‐
683 ule:init/1 or given to enter_loop/5,6, as there is no event
684 to postpone in those cases.
685 next_event:
687 This action does not set any transition_option() but instead
688 stores the specified EventType and EventContent for inser‐
689 tion after all actions have been executed.
690 The stored events are inserted in the queue as the next to
692 process before any already queued events. The order of these
693 stored events is preserved, so the first next_event in the
694 containing list becomes the first to process.
695 An event of type internal is to be used when you want to
697 reliably distinguish an event inserted this way from any
698 external event.
699 change_callback_module:
701 Changes the callback module to NewModule which will be used
702 when calling all subsequent state callbacks.
703 The gen_statem engine will find out the callback mode of
705 NewModule by calling NewModule:callback_mode/0 before the
706 next state callback.
707 Changing the callback module does not affect the state tran‐
709 sition in any way, it only changes which module that handles
710 the events. Be aware that all relevant callback functions in
711 NewModule such as the state callback, NewMod‐
712 ule:code_change/4, NewModule:format_status/2 and NewMod‐
713 ule:terminate/3 must be able to handle the state and data
714 from the old module.
715 push_callback_module:
717 Pushes the current callback module to the top of an internal
718 stack of callback modules and changes the callback module to
719 NewModule. Otherwise like {change_callback_module, NewMod‐
720 ule} above.
721 pop_callback_module:
723 Pops the top module from the internal stack of callback
724 modules and changes the callback module to be the popped
725 module. If the stack is empty the server fails. Otherwise
726 like {change_callback_module, NewModule} above.
727 enter_action() =
729 hibernate |
730 {hibernate, Hibernate :: hibernate()} |
731 timeout_action() |
732 reply_action()
733 These transition actions can be invoked by returning them from
735 the state callback, from Module:init/1 or by giving them to
736 enter_loop/5,6.
737 Actions are executed in the containing list order.
739 Actions that set transition options override any previous of the
741 same type, so the last in the containing list wins. For example,
742 the last event_timeout() overrides any previous event_timeout()
743 in the list.
744 hibernate:
746 Sets the transition_option() hibernate() for this state
747 transition.
748 timeout_action() =
750 (Time :: event_timeout()) |
751 {timeout, Time :: event_timeout(), EventContent :: term()} |
752 {timeout,
753 Time :: event_timeout(),
754 EventContent :: term(),
755 Options :: timeout_option() | [timeout_option()]} |
756 {{timeout, Name :: term()},
757 Time :: generic_timeout(),
758 EventContent :: term()} |
759 {{timeout, Name :: term()},
760 Time :: generic_timeout(),
761 EventContent :: term(),
762 Options :: timeout_option() | [timeout_option()]} |
763 {state_timeout,
764 Time :: state_timeout(),
765 EventContent :: term()} |
766 {state_timeout,
767 Time :: state_timeout(),
768 EventContent :: term(),
769 Options :: timeout_option() | [timeout_option()]} |
770 timeout_cancel_action() |
771 timeout_update_action()
772 These transition actions can be invoked by returning them from
774 the state callback, from Module:init/1 or by giving them to
775 enter_loop/5,6.
776 These time-out actions sets time-out transition options.
778 Time:
780 Short for {timeout,Time,Time}, that is, the time-out message
781 is the time-out time. This form exists to make the state
782 callback return value {next_state,NextState,NewData,Time}
783 allowed like for gen_fsm.
784 timeout:
786 Sets the transition_option() event_timeout() to Time with
787 EventContent and time-out options Options.
788 {timeout,Name}:
790 Sets the transition_option() generic_timeout() to Time for
791 Name with EventContent and time-out options Options.
792 state_timeout:
794 Sets the transition_option() state_timeout() to Time with
795 EventContent and time-out options Options.
796 timeout_cancel_action() =
798 {timeout, cancel} |
799 {{timeout, Name :: term()}, cancel} |
800 {state_timeout, cancel}
801 This is a shorter and clearer form of timeout_action() with
803 Time = infinity which cancels a time-out.
804 timeout_update_action() =
806 {timeout, update, EventContent :: term()} |
807 {{timeout, Name :: term()}, update, EventContent :: term()} |
808 {state_timeout, update, EventContent :: term()}
809 Updates a time-out with a new EventContent. See time‐
811 out_action() for how to start a time-out.
812 If no time-out of the same type is active instead insert the
814 time-out event just like when starting a time-out with relative
815 Time = 0.
816 reply_action() = {reply, From :: from(), Reply :: term()}
818 This transition action can be invoked by returning it from the
820 state callback, from Module:init/1 or by giving it to
821 enter_loop/5,6.
822 It does not set any transition_option() but instead replies to a
824 caller waiting for a reply in call/2. From must be the term from
825 argument {call,From} in a call to a state callback.
826 Note that using this action from Module:init/1 or enter_loop/5,6
828 would be weird on the border of witchcraft since there has been
829 no earlier call to a state callback in this server.
830 init_result(StateType) =
832 {ok, State :: StateType, Data :: data()} |
833 {ok,
834 State :: StateType,
835 Data :: data(),
836 Actions :: [action()] | action()} |
837 ignore |
838 {stop, Reason :: term()}
839 For a succesful initialization, State is the initial state() and
841 Data the initial server data() of the gen_statem.
842 The Actions are executed when entering the first state just as
844 for a state callback, except that the action postpone is forced
845 to false since there is no event to postpone.
846 For an unsuccesful initialization, {stop,Reason} or ignore
848 should be used; see start_link/3,4.
849 state_enter_result(State) =
851 {next_state, State, NewData :: data()} |
852 {next_state, State,
853 NewData :: data(),
854 Actions :: [enter_action()] | enter_action()} |
855 state_callback_result(enter_action())
856 State is the current state and it cannot be changed since the
858 state callback was called with a state enter call.
859 next_state:
861 The gen_statem does a state transition to State, which has
862 to be the current state, sets NewData, and executes all
863 Actions.
864 event_handler_result(StateType) =
866 {next_state, NextState :: StateType, NewData :: data()} |
867 {next_state,
868 NextState :: StateType,
869 NewData :: data(),
870 Actions :: [action()] | action()} |
871 state_callback_result(action())
872 StateType is state_name() if callback mode is state_functions,
874 or state() if callback mode is handle_event_function.
875 next_state:
877 The gen_statem does a state transition to NextState (which
878 can be the same as the current state), sets NewData, and
879 executes all Actions. If NextState =/= CurrentState the
880 state transition is a state change.
881 state_callback_result(ActionType) =
883 {keep_state, NewData :: data()} |
884 {keep_state,
885 NewData :: data(),
886 Actions :: [ActionType] | ActionType} |
887 keep_state_and_data |
888 {keep_state_and_data, Actions :: [ActionType] | ActionType} |
889 {repeat_state, NewData :: data()} |
890 {repeat_state,
891 NewData :: data(),
892 Actions :: [ActionType] | ActionType} |
893 repeat_state_and_data |
894 {repeat_state_and_data, Actions :: [ActionType] | ActionType} |
895 stop |
896 {stop, Reason :: term()} |
897 {stop, Reason :: term(), NewData :: data()} |
898 {stop_and_reply,
899 Reason :: term(),
900 Replies :: [reply_action()] | reply_action()} |
901 {stop_and_reply,
902 Reason :: term(),
903 Replies :: [reply_action()] | reply_action(),
904 NewData :: data()}
905 ActionType is enter_action() if the state callback was called
907 with a state enter call and action() if the state callback was
908 called with an event.
909 keep_state:
911 The same as {next_state,CurrentState,NewData,Actions}.
912 keep_state_and_data:
914 The same as {keep_state,CurrentData,Actions}.
915 repeat_state:
917 If the gen_statem runs with state enter calls, the state
918 enter call is repeated, see type transition_option(), other
919 than that repeat_state is the same as keep_state.
920 repeat_state_and_data:
922 The same as {repeat_state,CurrentData,Actions}.
923 stop:
925 Terminates the gen_statem by calling Module:terminate/3 with
926 Reason and NewData, if specified.
927 stop_and_reply:
929 Sends all Replies, then terminates the gen_statem by calling
930 Module:terminate/3 with Reason and NewData, if specified.
931 All these terms are tuples or atoms and this property will hold
933 in any future version of gen_statem.
934 request_id() = term()
936 A request handle, see send_request/2 for details.
938
940 call(ServerRef :: server_ref(), Request :: term()) ->
941 Reply :: term()
942 call(ServerRef :: server_ref(),
944 Request :: term(),
945 Timeout ::
946 timeout() |
947 {clean_timeout, T :: timeout()} |
948 {dirty_timeout, T :: timeout()}) ->
949 Reply :: term()
950 Makes a synchronous call to the gen_statem ServerRef by sending
952 a request and waiting until its reply arrives. The gen_statem
953 calls the state callback with event_type() {call,From} and event
954 content Request.
955 A Reply is generated when a state callback returns with
957 {reply,From,Reply} as one action(), and that Reply becomes the
958 return value of this function.
959 Timeout is an integer > 0, which specifies how many milliseconds
961 to wait for a reply, or the atom infinity to wait indefinitely,
962 which is the default. If no reply is received within the speci‐
963 fied time, the function call fails.
964 Note:
966 For Timeout < infinity, to avoid getting a late reply in the
967 caller's inbox if the caller should catch exceptions, this func‐
968 tion spawns a proxy process that does the call. A late reply
969 gets delivered to the dead proxy process, hence gets discarded.
970 This is less efficient than using Timeout == infinity.
971 Timeout can also be a tuple {clean_timeout,T} or {dirty_time‐
974 out,T}, where T is the time-out time. {clean_timeout,T} works
975 like just T described in the note above and uses a proxy process
976 while {dirty_timeout,T} bypasses the proxy process which is more
977 lightweight.
978 Note:
980 If you combine catching exceptions from this function with
981 {dirty_timeout,T} to avoid that the calling process dies when
982 the call times out, you will have to be prepared to handle a
983 late reply. Note that there is an odd chance to get a late reply
984 even with {dirty_timeout,infinity} or infinity for example in
985 the event of network problems. So why not just let the calling
986 process die by not catching the exception?
987 The call can also fail, for example, if the gen_statem dies
990 before or during this function call.
991 cast(ServerRef :: server_ref(), Msg :: term()) -> ok
993 Sends an asynchronous event to the gen_statem ServerRef and
995 returns ok immediately, ignoring if the destination node or
996 gen_statem does not exist. The gen_statem calls the state call‐
997 back with event_type() cast and event content Msg.
998 check_response(Msg :: term(), RequestId :: request_id()) ->
1000 {reply, Reply :: term()} |
1001 no_reply |
1002 {error, {term(), server_ref()}}
1003 This function is used to check if a previously received message,
1005 for example by receive or handle_info/2, is a result of a
1006 request made with send_request/2. If Msg is a reply to the han‐
1007 dle RequestId the result of the request is returned in Reply.
1008 Otherwise returns no_reply and no cleanup is done, and thus the
1009 function shall be invoked repeatedly until a reply is returned.
1010 The return value Reply is generated when a state callback
1012 returns with {reply,From,Reply} as one action(), and that Reply
1013 becomes the return value of this function.
1014 The function returns an error if the gen_statem dies before or
1016 during this request.
1017 enter_loop(Module :: module(),
1019 Opts :: [enter_loop_opt()],
1020 State :: state(),
1021 Data :: data()) ->
1022 no_return()
1023 The same as enter_loop/6 with Actions = [] except that no
1025 server_name() must have been registered. This creates an anony‐
1026 mous server.
1027 enter_loop(Module :: module(),
1029 Opts :: [enter_loop_opt()],
1030 State :: state(),
1031 Data :: data(),
1032 Server_or_Actions :: server_name() | pid() | [action()]) ->
1033 no_return()
1034 If Server_or_Actions is a list(), the same as enter_loop/6
1036 except that no server_name() must have been registered and
1037 Actions = Server_or_Actions. This creates an anonymous server.
1038 Otherwise the same as enter_loop/6 with Server =
1040 Server_or_Actions and Actions = [].
1041 enter_loop(Module :: module(),
1043 Opts :: [enter_loop_opt()],
1044 State :: state(),
1045 Data :: data(),
1046 Server :: server_name() | pid(),
1047 Actions :: [action()] | action()) ->
1048 no_return()
1049 Makes the calling process become a gen_statem. Does not return,
1051 instead the calling process enters the gen_statem receive loop
1052 and becomes a gen_statem server. The process must have been
1053 started using one of the start functions in proc_lib. The user
1054 is responsible for any initialization of the process, including
1055 registering a name for it.
1056 This function is useful when a more complex initialization pro‐
1058 cedure is needed than the gen_statem behavior provides.
1059 Module, Opts have the same meaning as when calling
1061 start[_link|_monitor]/3,4.
1062 If Server is self() an anonymous server is created just as when
1064 using start[_link|_monitor]/3. If Server is a server_name() a
1065 named server is created just as when using start[_link|_moni‐
1066 tor]/4. However, the server_name() name must have been regis‐
1067 tered accordingly before this function is called.
1068 State, Data, and Actions have the same meanings as in the return
1070 value of Module:init/1. Also, the callback module does not need
1071 to export a Module:init/1 function.
1072 The function fails if the calling process was not started by a
1074 proc_lib start function, or if it is not registered according to
1075 server_name().
1076 reply(Replies :: [reply_action()] | reply_action()) -> ok
1078 reply(From :: from(), Reply :: term()) -> ok
1080 This function can be used by a gen_statem to explicitly send a
1082 reply to a process that waits in call/2 when the reply cannot be
1083 defined in the return value of a state callback.
1084 From must be the term from argument {call,From} to the state
1086 callback. A reply or multiple replies canalso be sent using one
1087 or several reply_action()s from a state callback.
1088 Note:
1090 A reply sent with this function is not visible in sys debug out‐
1091 put.
1092 send_request(ServerRef :: server_ref(), Request :: term()) ->
1095 RequestId :: request_id()
1096 Sends a request to the gen_statem ServerRef and returns a handle
1098 RequestId.
1099 The return value RequestId shall later be used with
1101 wait_response/1,2 or check_response/2 to fetch the actual result
1102 of the request.
1103 The call
1105 gen_statem:wait_response(gen_statem:send_request(Server‐
1106 Ref,Request), Timeout) can be seen as equivalent to
1107 gen_statem:call(Server,Request,Timeout), ignoring the error han‐
1108 dling.
1109 The gen_statem calls the state callback with event_type()
1111 {call,From} and event content Request.
1112 A Reply is generated when a state callback returns with
1114 {reply,From,Reply} as one action(), and that Reply becomes the
1115 return value of wait_response/1,2 or check_response/2 function.
1116 start(Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
1118 start_ret()
1119 start(ServerName :: server_name(),
1121 Module :: module(),
1122 Args :: term(),
1123 Opts :: [start_opt()]) ->
1124 start_ret()
1125 Creates a standalone gen_statem process according to OTP design
1127 principles (using proc_lib primitives). As it does not get
1128 linked to the calling process, this start function cannot be
1129 used by a supervisor to start a child.
1130 For a description of arguments and return values, see
1132 start_link/3,4.
1133 start_link(Module :: module(),
1135 Args :: term(),
1136 Opts :: [start_opt()]) ->
1137 start_ret()
1138 start_link(ServerName :: server_name(),
1140 Module :: module(),
1141 Args :: term(),
1142 Opts :: [start_opt()]) ->
1143 start_ret()
1144 Creates a gen_statem process according to OTP design principles
1146 (using proc_lib primitives) that is linked to the calling
1147 process. This is essential when the gen_statem must be part of a
1148 supervision tree so it gets linked to its supervisor.
1149 The gen_statem process calls Module:init/1 to initialize the
1151 server. To ensure a synchronized startup procedure,
1152 start_link/3,4 does not return until Module:init/1 has returned.
1153 ServerName specifies the server_name() to register for the
1155 gen_statem. If the gen_statem is started with start_link/3, no
1156 ServerName is provided and the gen_statem is not registered.
1157 Module is the name of the callback module.
1159 Args is an arbitrary term that is passed as the argument to Mod‐
1161 ule:init/1.
1162 * If option {timeout,Time} is present in Opts, the gen_statem
1164 is allowed to spend Time milliseconds initializing or it
1165 terminates and the start function returns {error,timeout}.
1166 * If option {hibernate_after,HibernateAfterTimeout} is
1168 present, the gen_statem process awaits any message for
1169 HibernateAfterTimeout milliseconds and if no message is
1170 received, the process goes into hibernation automatically
1171 (by calling proc_lib:hibernate/3).
1172 * If option {debug,Dbgs} is present in Opts, debugging through
1174 sys is activated.
1175 * If option {spawn_opt,SpawnOpts} is present in Opts,
1177 SpawnOpts is passed as option list to erlang:spawn_opt/2,
1178 which is used to spawn the gen_statem process.
1179 Note:
1181 Using spawn option monitor is not allowed, it causes this func‐
1182 tion to fail with reason badarg.
1183 If the gen_statem is successfully created and initialized, this
1186 function returns {ok,Pid}, where Pid is the pid() of the
1187 gen_statem. If a process with the specified ServerName exists
1188 already, this function returns {error,{already_started,Pid}},
1189 where Pid is the pid() of that process.
1190 If Module:init/1 fails with Reason, this function returns
1192 {error,Reason}. If Module:init/1 returns {stop,Reason} or
1193 ignore, the process is terminated and this function returns
1194 {error,Reason} or ignore, respectively.
1195 start_monitor(Module :: module(),
1197 Args :: term(),
1198 Opts :: [start_opt()]) ->
1199 start_mon_ret()
1200 start_monitor(ServerName :: server_name(),
1202 Module :: module(),
1203 Args :: term(),
1204 Opts :: [start_opt()]) ->
1205 start_mon_ret()
1206 Creates a standalone gen_statem process according to OTP design
1208 principles (using proc_lib primitives) and atomically sets up a
1209 monitor to the newly created process. As it does not get linked
1210 to the calling process, this start function cannot be used by a
1211 supervisor to start a child.
1212 For a description of arguments and return values, see
1214 start_link/3,4. Note that the return value on successful start
1215 differs from start_link/3,4. start_monitor/3,4 will return
1216 {ok,{Pid,Mon}} where Pid is the process identifier of the
1217 process, and Mon is a reference to the monitor set up to monitor
1218 the process. If the start is not successful, the caller will be
1219 blocked until the DOWN message has been received and removed
1220 from the message queue.
1221 stop(ServerRef :: server_ref()) -> ok
1223 The same as stop(ServerRef, normal, infinity).
1225 stop(ServerRef :: server_ref(),
1227 Reason :: term(),
1228 Timeout :: timeout()) ->
1229 ok
1230 Orders the gen_statem ServerRef to exit with the specified Rea‐
1232 son and waits for it to terminate. The gen_statem calls Mod‐
1233 ule:terminate/3 before exiting.
1234 This function returns ok if the server terminates with the
1236 expected reason. Any other reason than normal, shutdown, or
1237 {shutdown,Term} causes an error report to be issued through log‐
1238 ger(3). The default Reason is normal.
1239 Timeout is an integer > 0, which specifies how many milliseconds
1241 to wait for the server to terminate, or the atom infinity to
1242 wait indefinitely. Defaults to infinity. If the server does not
1243 terminate within the specified time, a timeout exception is
1244 raised.
1245 If the process does not exist, a noproc exception is raised.
1247 wait_response(RequestId :: request_id()) ->
1249 {reply, Reply :: term()} |
1250 {error, {term(), server_ref()}}
1251 wait_response(RequestId :: request_id(), Timeout :: timeout()) ->
1253 {reply, Reply :: term()} |
1254 timeout |
1255 {error, {term(), server_ref()}}
1256 This function is used to wait for a reply of a request made with
1258 send_request/2 from the gen_statem process. This function must
1259 be called from the same process from which send_request/2 was
1260 made.
1261 Timeout is an integer greater then or equal to zero that speci‐
1263 fies how many milliseconds to wait for an reply, or the atom
1264 infinity to wait indefinitely. Defaults to infinity. If no reply
1265 is received within the specified time, the function returns
1266 timeout and no cleanup is done, and thus the function can be
1267 invoked repeatedly until a reply is returned.
1268 The return value Reply is generated when a state callback
1270 returns with {reply,From,Reply} as one action(), and that Reply
1271 becomes the return value of this function.
1272 The function returns an error if the gen_statem dies before or
1274 during this function call.
1275
1277 The following functions are to be exported from a gen_statem callback
1278 module.
1279
1281 Module:callback_mode() -> CallbackMode
1282 Types:
1284 CallbackMode = callback_mode() | [ callback_mode() |
1286 state_enter() ]
1287 This function is called by a gen_statem when it needs to find
1289 out the callback mode of the callback module. The value is
1290 cached by gen_statem for efficiency reasons, so this function is
1291 only called once after server start, after code change, and
1292 after changing the callback module, but before the first state
1293 callback in the current callback module's code version is
1294 called. More occasions may be added in future versions of
1295 gen_statem.
1296 Server start happens either when Module:init/1 returns or when
1298 enter_loop/4-6 is called. Code change happens when Mod‐
1299 ule:code_change/4 returns. A change of the callback module hap‐
1300 pens when a state callback returns any of the actions
1301 change_callback_module, push_callback_module or pop_call‐
1302 back_module.
1303 The CallbackMode is either just callback_mode() or a list con‐
1305 taining callback_mode() and possibly the atom state_enter.
1306 Note:
1308 If this function's body does not return an inline constant value
1309 the callback module is doing something strange.
1310 Module:code_change(OldVsn, OldState, OldData, Extra) -> Result
1313 Types:
1315 OldVsn = Vsn | {down,Vsn}
1317 Vsn = term()
1318 OldState = NewState = term()
1319 Extra = term()
1320 Result = {ok,NewState,NewData} | Reason
1321 OldState = NewState = state()
1322 OldData = NewData = data()
1323 Reason = term()
1324 Note:
1326 This callback is optional, so callback modules need not export
1327 it. If a release upgrade/downgrade with Change =
1328 {advanced,Extra} specified in the .appup file is made when
1329 code_change/4 is not implemented the process will crash with
1330 exit reason undef.
1331 This function is called by a gen_statem when it is to update its
1334 internal state during a release upgrade/downgrade, that is, when
1335 the instruction {update,Module,Change,...}, where Change =
1336 {advanced,Extra}, is specified in the appup file. For more
1337 information, see OTP Design Principles.
1338 For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is
1340 {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old
1341 version of the callback module Module. If no such attribute is
1342 defined, the version is the checksum of the Beam file.
1343 OldState and OldData is the internal state of the gen_statem.
1345 Extra is passed "as is" from the {advanced,Extra} part of the
1347 update instruction.
1348 If successful, the function must return the updated internal
1350 state in an {ok,NewState,NewData} tuple.
1351 If the function returns a failure Reason, the ongoing upgrade
1353 fails and rolls back to the old release. Note that Reason cannot
1354 be an {ok,_,_} tuple since that will be regarded as a {ok,New‐
1355 State,NewData} tuple, and that a tuple matching {ok,_} is an
1356 also invalid failure Reason. It is recommended to use an atom as
1357 Reason since it will be wrapped in an {error,Reason} tuple.
1358 Also note when upgrading a gen_statem, this function and hence
1360 the Change = {advanced,Extra} parameter in the appup file is not
1361 only needed to update the internal state or to act on the Extra
1362 argument. It is also needed if an upgrade or downgrade should
1363 change callback mode, or else the callback mode after the code
1364 change will not be honoured, most probably causing a server
1365 crash.
1366 If the server changes callback module using any of the actions
1368 change_callback_module, push_callback_module or pop_call‐
1369 back_module, be aware that it is always the current callback
1370 module that will get this callback call. That the current call‐
1371 back module handles the current state and data update should be
1372 no surprise, but it must be able to handle even parts of the
1373 state and data that it is not familiar with, somehow.
1374 In the supervisor child specification there is a list of modules
1376 which is recommended to contain only the callback module. For a
1377 gen_statem with multiple callback modules there is no real need
1378 to list all of them, it may not even be possible since the list
1379 could change after code upgrade. If this list would contain only
1380 the start callback module, as recommended, what is important is
1381 to upgrade that module whenever a synchronized code replacement
1382 is done. Then the release handler concludes that an upgrade that
1383 upgrades that module needs to suspend, code change, and resume
1384 any server whose child specification declares that it is using
1385 that module. And again; the current callback module will get the
1386 Module:code_change/4 call.
1387 Module:init(Args) -> Result(StateType)
1389 Types:
1391 Args = term()
1393 Result(StateType) = init_result(StateType)
1394 Whenever a gen_statem is started using start_link/3,4,
1396 start_monitor/3,4, or start/3,4, this function is called by the
1397 new process to initialize the implementation state and server
1398 data.
1399 Args is the Args argument provided to that start function.
1401 Note:
1403 Note that if the gen_statem is started through proc_lib and
1404 enter_loop/4-6, this callback will never be called. Since this
1405 callback is not optional it can in that case be implemented as:
1406 init(Args) -> erlang:error(not_implemented, [Args]).
1408 Module:format_status(Opt, [PDict,State,Data]) -> Status
1411 Types:
1413 Opt = normal | terminate
1415 PDict = [{Key, Value}]
1416 State = state()
1417 Data = data()
1418 Key = term()
1419 Value = term()
1420 Status = term()
1421 Note:
1423 This callback is optional, so a callback module does not need to
1424 export it. The gen_statem module provides a default implementa‐
1425 tion of this function that returns {State,Data}.
1426 If this callback is exported but fails, to hide possibly sensi‐
1428 tive data, the default function will instead return
1429 {State,Info}, where Info says nothing but the fact that for‐
1430 mat_status/2 has crashed.
1431 This function is called by a gen_statem process when any of the
1434 following apply:
1435 *
1437 One of sys:get_status/1,2 is invoked to get the gen_statem
1438 status. Opt is set to the atom normal for this case.
1439 *
1441 The gen_statem terminates abnormally and logs an error. Opt
1442 is set to the atom terminate for this case.
1443 This function is useful for changing the form and appearance of
1445 the gen_statem status for these cases. A callback module wishing
1446 to change the sys:get_status/1,2 return value and how its status
1447 appears in termination error logs exports an instance of for‐
1448 mat_status/2, which returns a term describing the current status
1449 of the gen_statem.
1450 PDict is the current value of the process dictionary of the
1452 gen_statem.
1453 State is the internal state of the gen_statem.
1455 Data is the internal server data of the gen_statem.
1457 The function is to return Status, a term that contains the
1459 appropriate details of the current state and status of the
1460 gen_statem. There are no restrictions on the form Status can
1461 take, but for the sys:get_status/1,2 case (when Opt is normal),
1462 the recommended form for the Status value is [{data, [{"State",
1463 Term}]}], where Term provides relevant details of the gen_statem
1464 state. Following this recommendation is not required, but it
1465 makes the callback module status consistent with the rest of the
1466 sys:get_status/1,2 return value.
1467 One use for this function is to return compact alternative state
1469 representations to avoid having large state terms printed in log
1470 files. Another use is to hide sensitive data from being written
1471 to the error log.
1472 Module:StateName(enter, OldState, Data) -> StateEnterResult(StateName)
1474 Module:StateName(EventType, EventContent, Data) -> StateFunctionResult
1475 Module:handle_event(enter, OldState, State, Data) -> StateEnterRe‐
1476 sult(State)
1477 Module:handle_event(EventType, EventContent, State, Data) -> Han‐
1478 dleEventResult
1479 Types:
1481 EventType = event_type()
1483 EventContent = term()
1484 State = state()
1485 Data = NewData = data()
1486 StateEnterResult(StateName) = state_enter_result(StateName)
1487 StateFunctionResult = event_handler_result(state_name())
1488 StateEnterResult(State) = state_enter_result(State)
1489 HandleEventResult = event_handler_result(state())
1490 Whenever a gen_statem receives an event from call/2, cast/2, or
1492 as a normal process message, one of these functions is called.
1493 If callback mode is state_functions, Module:StateName/3 is
1494 called, and if it is handle_event_function, Module:han‐
1495 dle_event/4 is called.
1496 If EventType is {call,From}, the caller waits for a reply. The
1498 reply can be sent from this or from any other state callback by
1499 returning with {reply,From,Reply} in Actions, in Replies, or by
1500 calling reply(From, Reply).
1501 If this function returns with a next state that does not match
1503 equal (=/=) to the current state, all postponed events are
1504 retried in the next state.
1505 The only difference between StateFunctionResult and HandleEven‐
1507 tResult is that for StateFunctionResult the next state must be
1508 an atom, but for HandleEventResult there is no restriction on
1509 the next state.
1510 For options that can be set and actions that can be done by
1512 gen_statem after returning from this function, see action().
1513 When the gen_statem runs with state enter calls, these functions
1515 are also called with arguments (enter, OldState, ...) during
1516 every state change. In this case there are some restrictions on
1517 the actions that may be returned: postpone() is not allowed
1518 since a state enter call is not an event so there is no event to
1519 postpone, and {next_event,_,_} is not allowed since using state
1520 enter calls should not affect how events are consumed and pro‐
1521 duced. You may also not change states from this call. Should you
1522 return {next_state,NextState, ...} with NextState =/= State the
1523 gen_statem crashes. Note that it is actually allowed to use
1524 {repeat_state, NewData, ...} although it makes little sense
1525 since you immediately will be called again with a new state
1526 enter call making this just a weird way of looping, and there
1527 are better ways to loop in Erlang. If you do not update NewData
1528 and have some loop termination condition, or if you use
1529 {repeat_state_and_data, _} or repeat_state_and_data you have an
1530 infinite loop! You are advised to use {keep_state,...},
1531 {keep_state_and_data,_} or keep_state_and_data since changing
1532 states from a state enter call is not possible anyway.
1533 Note the fact that you can use throw to return the result, which
1535 can be useful. For example to bail out with
1536 throw(keep_state_and_data) from deep within complex code that
1537 cannot return {next_state,State,Data} because State or Data is
1538 no longer in scope.
1539 Module:terminate(Reason, State, Data) -> Ignored
1541 Types:
1543 Reason = normal | shutdown | {shutdown,term()} | term()
1545 State = state()
1546 Data = data()
1547 Ignored = term()
1548 Note:
1550 This callback is optional, so callback modules need not export
1551 it. The gen_statem module provides a default implementation
1552 without cleanup.
1553 This function is called by a gen_statem when it is about to ter‐
1556 minate. It is to be the opposite of Module:init/1 and do any
1557 necessary cleaning up. When it returns, the gen_statem termi‐
1558 nates with Reason. The return value is ignored.
1559 Reason is a term denoting the stop reason and State is the
1561 internal state of the gen_statem.
1562 Reason depends on why the gen_statem is terminating. If it is
1564 because another callback function has returned, a stop tuple
1565 {stop,Reason} in Actions, Reason has the value specified in that
1566 tuple. If it is because of a failure, Reason is the error rea‐
1567 son.
1568 If the gen_statem is part of a supervision tree and is ordered
1570 by its supervisor to terminate, this function is called with
1571 Reason = shutdown if both the following conditions apply:
1572 * The gen_statem has been set to trap exit signals.
1574 * The shutdown strategy as defined in the supervisor's child
1576 specification is an integer time-out value, not brutal_kill.
1577 Even if the gen_statem is not part of a supervision tree, this
1579 function is called if it receives an 'EXIT' message from its
1580 parent. Reason is the same as in the 'EXIT' message.
1581 Otherwise, the gen_statem is immediately terminated.
1583 Notice that for any other reason than normal, shutdown, or
1585 {shutdown,Term}, the gen_statem is assumed to terminate because
1586 of an error and an error report is issued using logger(3).
1587
1589 gen_event(3), gen_fsm(3), gen_server(3), proc_lib(3), supervisor(3),
1590 sys(3).
1591Ericsson AB stdlib 3.14.1 gen_statem(3)