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gen_statem - Generic state machine behavior.
Contents
Callback Functions
The following functions are to be exported from a gen_statem callback module.
Data Types
server_name() =
{global, GlobalName :: term()} |
{via, RegMod :: module(), Name :: term()} |
{local, atom()}
Name specification to use when starting a gen_statem server. See start_link/3 and server_ref()
below.
server_ref() =
pid() |
(LocalName :: atom()) |
{Name :: atom(), Node :: atom()} |
{global, GlobalName :: term()} |
{via, RegMod :: module(), ViaName :: term()}
Server specification to use when addressing a gen_statem server. See call/2 and server_name()
above.
It can be:
pid()|LocalName:
The gen_statem is locally registered.
{Name,Node}:
The gen_statem is locally registered on another node.
{global,GlobalName}:
The gen_statem is globally registered in global.
{via,RegMod,ViaName}:
The gen_statem is registered in an alternative process registry. The registry callback module
RegMod is to export functions register_name/2, unregister_name/1, whereis_name/1, and send/2,
which are to behave like the corresponding functions in global. Thus, {via,global,GlobalName}
is the same as {global,GlobalName}.
start_opt() =
{timeout, Time :: timeout()} |
{spawn_opt, [proc_lib:start_spawn_option()]} |
enter_loop_opt()
Options that can be used when starting a gen_statem server through, for example, start_link/3.
start_ret() = {ok, pid()} | ignore | {error, term()}
Return value from the start() and start_link() functions, for example, start_link/3.
start_mon_ret() =
{ok, {pid(), reference()}} | ignore | {error, term()}
Return value from the start_monitor() functions.
enter_loop_opt() =
{hibernate_after, HibernateAfterTimeout :: timeout()} |
{debug, Dbgs :: [sys:debug_option()]}
Options that can be used when starting a gen_statem server through, enter_loop/4-6.
hibernate_after:
HibernateAfterTimeout specifies that the gen_statem process awaits any message for
HibernateAfterTimeout milliseconds and if no message is received, the process goes into
hibernation automatically (by calling proc_lib:hibernate/3).
debug:
For every entry in Dbgs, the corresponding function in sys is called.
from() = {To :: pid(), Tag :: term()}
Destination to use when replying through, for example, the action(){reply,From,Reply} to a
process that has called the gen_statem server using call/2.
state() = state_name() | term()
If the callbackmode is handle_event_function, the state can be any term. After a statechange
(NextState=/=State), all postponed events are retried.
state_name() = atom()
If the callbackmode is state_functions, the state must be an atom. After a statechange
(NextState=/=State), all postponed events are retried. Note that the state terminate is not
possible to use since it would collide with the optional callback function Module:terminate/3.
data() = term()
A term in which the state machine implementation is to store any server data it needs. The
difference between this and the state() itself is that a change in this data does not cause
postponed events to be retried. Hence, if a change in this data would change the set of events
that are handled, then that data item is to be made a part of the state.
event_type() =
external_event_type() | timeout_event_type() | internal
There are 3 categories of events: external, timeout, and internal.
internal events can only be generated by the state machine itself through the transitionactionnext_event.
external_event_type() = {call, From :: from()} | cast | info
External events are of 3 types: {call,From}, cast, or info. Type call originates from the API
functions call/2 and send_request/2. For calls, the event contains whom to reply to. Type cast
originates from the API function cast/2. Type info originates from regular process messages sent
to the gen_statem.
timeout_event_type() =
timeout | {timeout, Name :: term()} | state_timeout
There are 3 types of time-out events that the state machine can generate for itself with the
corresponding timeout_action()s.
callback_mode_result() =
callback_mode() | [callback_mode() | state_enter()]
This is the return type from Module:callback_mode/0 and selects callbackmode and whether to do
stateentercalls, or not.
callback_mode() = state_functions | handle_event_function
The callbackmode is selected with the return value from Module:callback_mode/0:
state_functions:
The state must be of type state_name() and one callback function per state, that is,
Module:StateName/3, is used.
handle_event_function:
The state can be any term and the callback function Module:handle_event/4 is used for all
states.
The function Module:callback_mode/0 is called when starting the gen_statem, after code change and
after changing the callback module with any of the actions change_callback_module,
push_callback_module or pop_callback_module. The result is cached for subsequent calls to state
callbacks.
state_enter() = state_enter
Whether the state machine should use stateentercalls or not is selected when starting the
gen_statem and after code change using the return value from Module:callback_mode/0.
If Module:callback_mode/0 returns a list containing state_enter, the gen_statem engine will, at
every statechange, call the state callback with arguments (enter,OldState,Data) or (enter,OldState,State,Data), depending on the callbackmode. This may look like an event but is really
a call performed after the previous statecallback returned and before any event is delivered to
the new statecallback. See Module:StateName/3 and Module:handle_event/4. Such a call can be
repeated by returning a repeat_state or repeat_state_and_data tuple from the statecallback.
If Module:callback_mode/0 does not return such a list, no stateentercalls are done.
If Module:code_change/4 should transform the state, it is regarded as a state rename and not a
statechange, which will not cause a stateentercall.
Note that a stateentercallwill be done right before entering the initial state even though this
actually is not a statechange. In this case OldState=:=State, which cannot happen for a
subsequent state change, but will happen when repeating the stateentercall.
transition_option() =
postpone() |
hibernate() |
event_timeout() |
generic_timeout() |
state_timeout()
Transition options can be set by actions and modify the statetransition. The statetransition
takes place when the statecallback has processed an event and returns. Here are the sequence of
steps for a statetransition:
* All returned actions are processed in order of appearance. In this step all replies generated
by any reply_action() are sent. Other actions set transition_option()s that come into play in
subsequent steps.
* If stateentercalls are used, and either it is the initial state or one of the callback
results repeat_state_and_data or repeat_state_and_data is used the gen_statem engine calls the
current state callback with arguments (enter,State,Data) or (enter,State,State,Data)
(depending on callbackmode) and when it returns starts again from the top of this sequence.
If stateentercalls are used, and the state changes the gen_statem engine calls the new state
callback with arguments (enter,OldState,Data) or (enter,OldState,State,Data) (depending
on callbackmode) and when it returns starts again from the top of this sequence.
* If postpone() is true, the current event is postponed.
* If this is a statechange, the queue of incoming events is reset to start with the oldest
postponed.
* All events stored with action()next_event are inserted to be processed before previously
queued events.
* Time-out timers event_timeout(), generic_timeout() and state_timeout() are handled. Time-outs
with zero time are guaranteed to be delivered to the state machine before any external not yet
received event so if there is such a time-out requested, the corresponding time-out zero event
is enqueued as the newest received event; that is after already queued events such as inserted
and postponed events.
Any event cancels an event_timeout() so a zero time event time-out is only generated if the
event queue is empty.
A statechange cancels a state_timeout() and any new transition option of this type belongs to
the new state, that is; a state_timeout() applies to the state the state machine enters.
* If there are enqueued events the statecallback for the possibly new state is called with the
oldest enqueued event, and we start again from the top of this sequence.
* Otherwise the gen_statem goes into receive or hibernation (if hibernate() is true) to wait for
the next message. In hibernation the next non-system event awakens the gen_statem, or rather
the next incoming message awakens the gen_statem, but if it is a system event it goes right
back into hibernation. When a new message arrives the statecallback is called with the
corresponding event, and we start again from the top of this sequence.
postpone() = boolean()
If true, postpones the current event and retries it after a statechange (NextState=/=State).
hibernate() = boolean()
If true, hibernates the gen_statem by calling proc_lib:hibernate/3 before going into receive to
wait for a new external event.
Note:
If there are enqueued events to process when hibrnation is requested, this is optimized by not
hibernating but instead calling erlang:garbage_collect/0 to simulate that the gen_statem entered
hibernation and immediately got awakened by an enqueued event.
event_timeout() = timeout() | integer()
Starts a timer set by enter_action()timeout. When the timer expires an event of event_type()timeout will be generated. See erlang:start_timer/4 for how Time and Options are interpreted.
Future erlang:start_timer/4Options will not necessarily be supported.
Any event that arrives cancels this time-out. Note that a retried or inserted event counts as
arrived. So does a state time-out zero event, if it was generated before this time-out is
requested.
If Time is infinity, no timer is started, as it never would expire anyway.
If Time is relative and 0 no timer is actually started, instead the the time-out event is enqueued
to ensure that it gets processed before any not yet received external event, but after already
queued events.
Note that it is not possible nor needed to cancel this time-out, as it is cancelled automatically
by any other event.
generic_timeout() = timeout() | integer()
Starts a timer set by enter_action(){timeout,Name}. When the timer expires an event of
event_type(){timeout,Name} will be generated. See erlang:start_timer/4 for how Time and Options
are interpreted. Future erlang:start_timer/4Options will not necessarily be supported.
If Time is infinity, no timer is started, as it never would expire anyway.
If Time is relative and 0 no timer is actually started, instead the the time-out event is enqueued
to ensure that it gets processed before any not yet received external event.
Setting a timer with the same Name while it is running will restart it with the new time-out
value. Therefore it is possible to cancel a specific time-out by setting it to infinity.
state_timeout() = timeout() | integer()
Starts a timer set by enter_action()state_timeout. When the timer expires an event of
event_type()state_timeout will be generated. See erlang:start_timer/4 for how Time and Options
are interpreted. Future erlang:start_timer/4Options will not necessarily be supported.
If Time is infinity, no timer is started, as it never would expire anyway.
If Time is relative and 0 no timer is actually started, instead the the time-out event is enqueued
to ensure that it gets processed before any not yet received external event.
Setting this timer while it is running will restart it with the new time-out value. Therefore it
is possible to cancel this time-out by setting it to infinity.
timeout_option() = {abs, Abs :: boolean()}
If Abs is true an absolute timer is started, and if it is false a relative, which is the default.
See erlang:start_timer/4 for details.
action() =
postpone |
{postpone, Postpone :: postpone()} |
{next_event,
EventType :: event_type(),
EventContent :: term()} |
{change_callback_module, NewModule :: module()} |
{push_callback_module, NewModule :: module()} |
pop_callback_module |
enter_action()
These transitionactions can be invoked by returning them from the statecallback when it is
called with an event, from Module:init/1 or by giving them to enter_loop/5,6.
Actions are executed in the containing list order.
Actions that set transition options override any previous of the same type, so the last in the
containing list wins. For example, the last postpone() overrides any previous postpone() in the
list.
postpone:
Sets the transition_option()postpone() for this statetransition. This action is ignored when
returned from Module:init/1 or given to enter_loop/5,6, as there is no event to postpone in
those cases.
next_event:
This action does not set any transition_option() but instead stores the specified EventType
and EventContent for insertion after all actions have been executed.
The stored events are inserted in the queue as the next to process before any already queued
events. The order of these stored events is preserved, so the first next_event in the
containing list becomes the first to process.
An event of type internal is to be used when you want to reliably distinguish an event
inserted this way from any external event.
change_callback_module:
Changes the callback module to NewModule which will be used when calling all subsequent state
callbacks.
The gen_statem engine will find out the callbackmode of NewModule by calling
NewModule:callback_mode/0 before the next state callback.
Changing the callback module does not affect the statetransition in any way, it only changes
which module that handles the events. Be aware that all relevant callback functions in
NewModule such as the state callback, NewModule:code_change/4, NewModule:format_status/2 and
NewModule:terminate/3 must be able to handle the state and data from the old module.
push_callback_module:
Pushes the current callback module to the top of an internal stack of callback modules and
changes the callback module to NewModule. Otherwise like {change_callback_module,NewModule}
above.
pop_callback_module:
Pops the top module from the internal stack of callback modules and changes the callback
module to be the popped module. If the stack is empty the server fails. Otherwise like
{change_callback_module,NewModule} above.
enter_action() =
hibernate |
{hibernate, Hibernate :: hibernate()} |
timeout_action() |
reply_action()
These transitionactions can be invoked by returning them from the statecallback, from
Module:init/1 or by giving them to enter_loop/5,6.
Actions are executed in the containing list order.
Actions that set transition options override any previous of the same type, so the last in the
containing list wins. For example, the last event_timeout() overrides any previous event_timeout()
in the list.
hibernate:
Sets the transition_option()hibernate() for this statetransition.
timeout_action() =
(Time :: event_timeout()) |
{timeout, Time :: event_timeout(), EventContent :: term()} |
{timeout,
Time :: event_timeout(),
EventContent :: term(),
Options :: timeout_option() | [timeout_option()]} |
{{timeout, Name :: term()},
Time :: generic_timeout(),
EventContent :: term()} |
{{timeout, Name :: term()},
Time :: generic_timeout(),
EventContent :: term(),
Options :: timeout_option() | [timeout_option()]} |
{state_timeout,
Time :: state_timeout(),
EventContent :: term()} |
{state_timeout,
Time :: state_timeout(),
EventContent :: term(),
Options :: timeout_option() | [timeout_option()]} |
timeout_cancel_action() |
timeout_update_action()
These transitionactions can be invoked by returning them from the statecallback, from
Module:init/1 or by giving them to enter_loop/5,6.
These time-out actions sets time-out transition options.
Time:
Short for {timeout,Time,Time}, that is, the time-out message is the time-out time. This form
exists to make the statecallback return value {next_state,NextState,NewData,Time} allowed
like for gen_fsm.
timeout:
Sets the transition_option()event_timeout() to Time with EventContent and time-out options
Options.
{timeout,Name}:
Sets the transition_option()generic_timeout() to Time for Name with EventContent and time-out
options Options.
state_timeout:
Sets the transition_option()state_timeout() to Time with EventContent and time-out options
Options.
timeout_cancel_action() =
{timeout, cancel} |
{{timeout, Name :: term()}, cancel} |
{state_timeout, cancel}
This is a shorter and clearer form of timeout_action() with Time=infinity which cancels a
time-out.
timeout_update_action() =
{timeout, update, EventContent :: term()} |
{{timeout, Name :: term()}, update, EventContent :: term()} |
{state_timeout, update, EventContent :: term()}
Updates a time-out with a new EventContent. See timeout_action() for how to start a time-out.
If no time-out of the same type is active instead insert the time-out event just like when
starting a time-out with relative Time=0.
reply_action() = {reply, From :: from(), Reply :: term()}
This transitionaction can be invoked by returning it from the statecallback, from Module:init/1
or by giving it to enter_loop/5,6.
It does not set any transition_option() but instead replies to a caller waiting for a reply in
call/2. From must be the term from argument {call,From} in a call to a statecallback.
Note that using this action from Module:init/1 or enter_loop/5,6 would be weird on the border of
witchcraft since there has been no earlier call to a statecallback in this server.
init_result(StateType) = init_result(StateType, term())
init_result(StateType,DataType) =
{ok, State :: StateType, Data :: DataType} |
{ok,
State :: StateType,
Data :: DataType,
Actions :: [action()] | action()} |
ignore |
{stop, Reason :: term()}
For a succesful initialization, State is the initial state() and Data the initial server data() of
the gen_statem.
The Actions are executed when entering the first state just as for a statecallback, except that
the action postpone is forced to false since there is no event to postpone.
For an unsuccesful initialization, {stop,Reason} or ignore should be used; see start_link/3,4.
state_enter_result(State) = state_enter_result(State, term())
state_enter_result(State,DataType) =
{next_state, State, NewData :: DataType} |
{next_state, State,
NewData :: DataType,
Actions :: [enter_action()] | enter_action()} |
state_callback_result(enter_action())
State is the current state and it cannot be changed since the state callback was called with a
stateentercall.
next_state:
The gen_statem does a state transition to State, which has to be the current state, sets
NewData, and executes all Actions.
event_handler_result(StateType) =
event_handler_result(StateType, term())
event_handler_result(StateType,DataType) =
{next_state, NextState :: StateType, NewData :: DataType} |
{next_state,
NextState :: StateType,
NewData :: DataType,
Actions :: [action()] | action()} |
state_callback_result(action())
StateType is state_name() if callbackmode is state_functions, or state() if callbackmode is
handle_event_function.
next_state:
The gen_statem does a statetransition to NextState (which can be the same as the current
state), sets NewData, and executes all Actions. If NextState=/=CurrentState the statetransition is a statechange.
state_callback_result(ActionType) =
state_callback_result(ActionType, term())
state_callback_result(ActionType,DataType) =
{keep_state, NewData :: DataType} |
{keep_state,
NewData :: DataType,
Actions :: [ActionType] | ActionType} |
keep_state_and_data |
{keep_state_and_data, Actions :: [ActionType] | ActionType} |
{repeat_state, NewData :: DataType} |
{repeat_state,
NewData :: DataType,
Actions :: [ActionType] | ActionType} |
repeat_state_and_data |
{repeat_state_and_data, Actions :: [ActionType] | ActionType} |
stop |
{stop, Reason :: term()} |
{stop, Reason :: term(), NewData :: DataType} |
{stop_and_reply,
Reason :: term(),
Replies :: [reply_action()] | reply_action()} |
{stop_and_reply,
Reason :: term(),
Replies :: [reply_action()] | reply_action(),
NewData :: DataType}
ActionType is enter_action() if the state callback was called with a stateentercall and action()
if the state callback was called with an event.
keep_state:
The same as {next_state,CurrentState,NewData,Actions}.
keep_state_and_data:
The same as {keep_state,CurrentData,Actions}.
repeat_state:
If the gen_statem runs with stateentercalls, the stateentercall is repeated, see type
transition_option(), other than that repeat_state is the same as keep_state.
repeat_state_and_data:
The same as {repeat_state,CurrentData,Actions}.
stop:
Terminates the gen_statem by calling Module:terminate/3 with Reason and NewData, if specified.
An exit signal with this reason is sent to linked processes and ports. The default Reason is
normal.
stop_and_reply:
Sends all Replies, then terminates the gen_statem by calling Module:terminate/3 with Reason
and NewData, if specified. An exit signal with this reason is sent to linked processes and
ports.
All these terms are tuples or atoms and this property will hold in any future version of
gen_statem.
request_id() = term()
A request handle, see send_request/2 for details.
Description
gen_statem provides a generic state machine behaviour that for new code replaces its predecessor gen_fsm
since Erlang/OTP 20.0. The gen_fsm behaviour remains in OTP "as is".
Note:
If you are new to gen_statem and want an overview of concepts and operation the section gen_statem
Behaviour located in the User's Guide OTP Design Principles is recommended to read before this
reference manual, possibly after the Description section you are reading here.
This reference manual contains type descriptions generated from types in the gen_statem source code, so
they are correct. However, the generated descriptions also reflect the type hierarchy, which sometimes
makes it hard to get a good overview. If so, see the section gen_statem Behaviour in the OTP Design
Principles User's Guide.
Note:
* This behavior appeared in Erlang/OTP 19.0.
*
In OTP 19.1 a backwards incompatible change of the return tuple from Module:init/1 was made and the
mandatory callback function Module:callback_mode/0 was introduced.
*
In OTP 20.0 generic time-outs were added.
*
In OTP 22.1 time-out content update and explicit time-out cancel were added.
*
In OTP 22.3 the possibility to change the callback module with actions change_callback_module,
push_callback_module and pop_callback_module, was added.
gen_statem has got the same features that gen_fsm had and adds some really useful:
* Co-located state code
* Arbitrary term state
* Event postponing
* Self-generated events
* State time-out
* Multiple generic named time-outs
* Absolute time-out time
* Automatic state enter calls
*
Reply from other state than the request, sys traceable
* Multiple sys traceable replies
* Changing the callback module
Two callbackmodes are supported:
* One for finite-state machines (gen_fsm like), which requires the state to be an atom and uses that
state as the name of the current callback function.
* One that allows the state to be any term and that uses one callback function for all states.
The callback model(s) for gen_statem differs from the one for gen_fsm, but it is still fairly easy to
rewrite from gen_fsm to gen_statem.
A generic state machine server process (gen_statem) implemented using this module has a standard set of
interface functions and includes functionality for tracing and error reporting. It also fits into an OTP
supervision tree. For more information, see OTP Design Principles.
A gen_statem assumes all specific parts to be located in a callback module exporting a predefined set of
functions. The relationship between the behavior functions and the callback functions is as follows:
gen_statem module Callback module
----------------- ---------------
gen_statem:start
gen_statem:start_monitor
gen_statem:start_link -----> Module:init/1
Server start or code change
-----> Module:callback_mode/0
gen_statem:stop -----> Module:terminate/3
gen_statem:call
gen_statem:cast
gen_statem:send_request
erlang:send
erlang:'!' -----> Module:StateName/3
Module:handle_event/4
- -----> Module:terminate/3
- -----> Module:code_change/4
Events are of different types, so the callback functions can know the origin of an event and how to
respond.
If a callback function fails or returns a bad value, the gen_statem terminates, unless otherwise stated.
However, an exception of class throw is not regarded as an error but as a valid return from all callback
functions.
The statecallback for a specific state in a gen_statem is the callback function that is called for all
events in this state. It is selected depending on which callbackmode that the callback module defines
with the callback function Module:callback_mode/0.
When the callbackmode is state_functions, the state must be an atom and is used as the statecallback
name; see Module:StateName/3. This co-locates all code for a specific state in one function as the
gen_statem engine branches depending on state name. Note the fact that the callback function
Module:terminate/3 makes the state name terminate unusable in this mode.
When the callbackmode is handle_event_function, the state can be any term and the statecallback name is
Module:handle_event/4. This makes it easy to branch depending on state or event as you desire. Be careful
about which events you handle in which states so that you do not accidentally postpone an event forever
creating an infinite busy loop.
When gen_statem receives a process message it is converted into an event and the statecallback is called
with the event as two arguments: type and content. When the statecallback has processed the event it
returns to gen_statem which does a statetransition. If this statetransition is to a different state,
that is: NextState=/=State, it is a statechange.
The statecallback may return transitionactions for gen_statem to execute during the statetransition,
for example to reply to a gen_statem:call/2,3.
One of the possible transitionactions is to postpone the current event. Then it is not retried in the
current state. The gen_statem engine keeps a queue of events divided into the postponed events and the
events still to process. After a statechange the queue restarts with the postponed events.
The gen_statem event queue model is sufficient to emulate the normal process message queue with selective
receive. Postponing an event corresponds to not matching it in a receive statement, and changing states
corresponds to entering a new receive statement.
The statecallback can insert events using the transitionactionsnext_event and such an event is
inserted in the event queue as the next to call the statecallback with. That is, as if it is the oldest
incoming event. A dedicated event_type()internal can be used for such events making them impossible to
mistake for external events.
Inserting an event replaces the trick of calling your own state handling functions that you often would
have to resort to in, for example, gen_fsm to force processing an inserted event before others.
The gen_statem engine can automatically make a specialized call to the statecallback whenever a new
state is entered; see state_enter(). This is for writing code common to all state entries. Another way to
do it is to explicitly insert an event at the statetransition, and/or to use a dedicated statetransition function, but that is something you will have to remember at every statetransition to the
state(s) that need it.
Note:
If you in gen_statem, for example, postpone an event in one state and then call another statecallback of
yours, you have not done a statechange and hence the postponed event is not retried, which is logical
but can be confusing.
For the details of a statetransition, see type transition_option().
A gen_statem handles system messages as described in sys. The sys module can be used for debugging a
gen_statem.
Notice that a gen_statem does not trap exit signals automatically, this must be explicitly initiated in
the callback module (by calling process_flag(trap_exit,true).
Unless otherwise stated, all functions in this module fail if the specified gen_statem does not exist or
if bad arguments are specified.
The gen_statem process can go into hibernation; see proc_lib:hibernate/3. It is done when a statecallback or Module:init/1 specifies hibernate in the returned Actions list. This feature can be useful to
reclaim process heap memory while the server is expected to be idle for a long time. However, use this
feature with care, as hibernation can be too costly to use after every event; see erlang:hibernate/3.
There is also a server start option {hibernate_after,Timeout} for start/3,4, start_monitor/3,4,
start_link/3,4 or enter_loop/4,5,6, that may be used to automatically hibernate the server.
If the gen_statem process terminates, e.g. as a result of a function in the callback module returning
{stop,Reason}, an exit signal with this Reason is sent to linked processes and ports. See Processes in
the Reference Manual for details regarding error handling using exit signals.
Example
The following example shows a simple pushbutton model for a toggling pushbutton implemented with callbackmodestate_functions. You can push the button and it replies if it went on or off, and you can ask for a
count of how many times it has been pushed to switch on.
The following is the complete callback module file pushbutton.erl:
-module(pushbutton).
-behaviour(gen_statem).
-export([start/0,push/0,get_count/0,stop/0]).
-export([terminate/3,code_change/4,init/1,callback_mode/0]).
-export([on/3,off/3]).
name() -> pushbutton_statem. % The registered server name
%% API. This example uses a registered name name()
%% and does not link to the caller.
start() ->
gen_statem:start({local,name()}, ?MODULE, [], []).
push() ->
gen_statem:call(name(), push).
get_count() ->
gen_statem:call(name(), get_count).
stop() ->
gen_statem:stop(name()).
%% Mandatory callback functions
terminate(_Reason, _State, _Data) ->
void.
code_change(_Vsn, State, Data, _Extra) ->
{ok,State,Data}.
init([]) ->
%% Set the initial state + data. Data is used only as a counter.
State = off, Data = 0,
{ok,State,Data}.
callback_mode() -> state_functions.
%%% state callback(s)
off({call,From}, push, Data) ->
%% Go to 'on', increment count and reply
%% that the resulting status is 'on'
{next_state,on,Data+1,[{reply,From,on}]};
off(EventType, EventContent, Data) ->
handle_event(EventType, EventContent, Data).
on({call,From}, push, Data) ->
%% Go to 'off' and reply that the resulting status is 'off'
{next_state,off,Data,[{reply,From,off}]};
on(EventType, EventContent, Data) ->
handle_event(EventType, EventContent, Data).
%% Handle events common to all states
handle_event({call,From}, get_count, Data) ->
%% Reply with the current count
{keep_state,Data,[{reply,From,Data}]};
handle_event(_, _, Data) ->
%% Ignore all other events
{keep_state,Data}.
The following is a shell session when running it:
1> pushbutton:start().
{ok,<0.36.0>}
2> pushbutton:get_count().
0
3> pushbutton:push().
on
4> pushbutton:get_count().
1
5> pushbutton:push().
off
6> pushbutton:get_count().
1
7> pushbutton:stop().
ok
8> pushbutton:push().
** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
in function gen:do_for_proc/2 (gen.erl, line 261)
in call from gen_statem:call/3 (gen_statem.erl, line 386)
To compare styles, here follows the same example using callbackmodehandle_event_function, or rather the
code to replace after function init/1 of the pushbutton.erl example file above:
callback_mode() -> handle_event_function.
%%% state callback(s)
handle_event({call,From}, push, off, Data) ->
%% Go to 'on', increment count and reply
%% that the resulting status is 'on'
{next_state,on,Data+1,[{reply,From,on}]};
handle_event({call,From}, push, on, Data) ->
%% Go to 'off' and reply that the resulting status is 'off'
{next_state,off,Data,[{reply,From,off}]};
%%
%% Event handling common to all states
handle_event({call,From}, get_count, State, Data) ->
%% Reply with the current count
{next_state,State,Data,[{reply,From,Data}]};
handle_event(_, _, State, Data) ->
%% Ignore all other events
{next_state,State,Data}.
Exports
Module:callback_mode()->CallbackMode
Types:
CallbackMode = callback_mode() | [ callback_mode() | state_enter() ]
This function is called by a gen_statem when it needs to find out the callbackmode of the
callback module. The value is cached by gen_statem for efficiency reasons, so this function is
only called once after server start, after code change, and after changing the callback module,
but before the first statecallback in the current callback module's code version is called. More
occasions may be added in future versions of gen_statem.
Server start happens either when Module:init/1 returns or when enter_loop/4-6 is called. Code
change happens when Module:code_change/4 returns. A change of the callback module happens when a
statecallback returns any of the actions change_callback_module, push_callback_module or
pop_callback_module.
The CallbackMode is either just callback_mode() or a list containing callback_mode() and possibly
the atom state_enter.
Note:
If this function's body does not return an inline constant value the callback module is doing
something strange.
Module:code_change(OldVsn,OldState,OldData,Extra)->Result
Types:
OldVsn = Vsn | {down,Vsn}
Vsn = term()
OldState = NewState = term()
Extra = term()
Result = {ok,NewState,NewData} | Reason
OldState = NewState = state()
OldData = NewData = data()
Reason = term()
Note:
This callback is optional, so callback modules need not export it. If a release upgrade/downgrade
with Change={advanced,Extra} specified in the .appup file is made when code_change/4 is not
implemented the process will crash with exit reason undef.
This function is called by a gen_statem when it is to update its internal state during a release
upgrade/downgrade, that is, when the instruction {update,Module,Change,...}, where Change={advanced,Extra}, is specified in the appup file. For more information, see OTP Design Principles.
For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is {down,Vsn}. Vsn is defined by the
vsn attribute(s) of the old version of the callback module Module. If no such attribute is
defined, the version is the checksum of the Beam file.
OldState and OldData is the internal state of the gen_statem.
Extra is passed "as is" from the {advanced,Extra} part of the update instruction.
If successful, the function must return the updated internal state in an {ok,NewState,NewData}
tuple.
If the function returns a failure Reason, the ongoing upgrade fails and rolls back to the old
release. Note that Reason cannot be an {ok,_,_} tuple since that will be regarded as a
{ok,NewState,NewData} tuple, and that a tuple matching {ok,_} is an also invalid failure Reason.
It is recommended to use an atom as Reason since it will be wrapped in an {error,Reason} tuple.
Also note when upgrading a gen_statem, this function and hence the Change={advanced,Extra}
parameter in the appup file is not only needed to update the internal state or to act on the Extra
argument. It is also needed if an upgrade or downgrade should change callbackmode, or else the
callbackmode after the code change will not be honoured, most probably causing a server crash.
If the server changes callback module using any of the actions change_callback_module,
push_callback_module or pop_callback_module, be aware that it is always the current callback
module that will get this callback call. That the current callback module handles the current
state and data update should be no surprise, but it must be able to handle even parts of the state
and data that it is not familiar with, somehow.
In the supervisor child specification there is a list of modules which is recommended to contain
only the callback module. For a gen_statem with multiple callback modules there is no real need to
list all of them, it may not even be possible since the list could change after code upgrade. If
this list would contain only the start callback module, as recommended, what is important is to
upgrade that module whenever a synchronizedcodereplacement is done. Then the release handler
concludes that an upgrade that upgrades that module needs to suspend, code change, and resume any
server whose child specification declares that it is using that module. And again; the current
callback module will get the Module:code_change/4 call.
Module:init(Args)->Result(StateType)
Types:
Args = term()
Result(StateType) = init_result(StateType)
Whenever a gen_statem is started using start_link/3,4, start_monitor/3,4, or start/3,4, this
function is called by the new process to initialize the implementation state and server data.
Args is the Args argument provided to that start function.
Note:
Note that if the gen_statem is started through proc_lib and enter_loop/4-6, this callback will
never be called. Since this callback is not optional it can in that case be implemented as:
-spec init(_) -> no_return().
init(Args) -> erlang:error(not_implemented, [Args]).
Module:format_status(Opt,[PDict,State,Data])->Status
Types:
Opt = normal | terminate
PDict = [{Key, Value}]
State = state()
Data = data()
Key = term()
Value = term()
Status = term()
Note:
This callback is optional, so a callback module does not need to export it. The gen_statem module
provides a default implementation of this function that returns {State,Data}.
If this callback is exported but fails, to hide possibly sensitive data, the default function will
instead return {State,Info}, where Info says nothing but the fact that format_status/2 has
crashed.
This function is called by a gen_statem process when any of the following apply:
*
One of sys:get_status/1,2 is invoked to get the gen_statem status. Opt is set to the atom
normal for this case.
*
The gen_statem terminates abnormally and logs an error. Opt is set to the atom terminate for
this case.
This function is useful for changing the form and appearance of the gen_statem status for these
cases. A callback module wishing to change the sys:get_status/1,2 return value and how its status
appears in termination error logs exports an instance of format_status/2, which returns a term
describing the current status of the gen_statem.
PDict is the current value of the process dictionary of the gen_statem.
State is the internal state of the gen_statem.
Data is the internal server data of the gen_statem.
The function is to return Status, a term that contains the appropriate details of the current
state and status of the gen_statem. There are no restrictions on the form Status can take, but for
the sys:get_status/1,2 case (when Opt is normal), the recommended form for the Status value is
[{data,[{"State",Term}]}], where Term provides relevant details of the gen_statem state.
Following this recommendation is not required, but it makes the callback module status consistent
with the rest of the sys:get_status/1,2 return value.
One use for this function is to return compact alternative state representations to avoid having
large state terms printed in log files. Another use is to hide sensitive data from being written
to the error log.
Module:StateName(enter,OldState,Data)->StateEnterResult(StateName)Module:StateName(EventType,EventContent,Data)->StateFunctionResultModule:handle_event(enter,OldState,State,Data)->StateEnterResult(State)Module:handle_event(EventType,EventContent,State,Data)->HandleEventResult
Types:
EventType = event_type()
EventContent = term()
State = state()
Data = NewData = data()
StateEnterResult(StateName) = state_enter_result(StateName)
StateFunctionResult = event_handler_result(state_name())
StateEnterResult(State) = state_enter_result(State)
HandleEventResult = event_handler_result(state())
Whenever a gen_statem receives an event from call/2, cast/2, or as a normal process message, one
of these functions is called. If callbackmode is state_functions, Module:StateName/3 is called,
and if it is handle_event_function, Module:handle_event/4 is called.
If EventType is {call,From}, the caller waits for a reply. The reply can be sent from this or from
any other statecallback by returning with {reply,From,Reply} in Actions, in Replies, or by
calling reply(From,Reply).
If this function returns with a next state that does not match equal (=/=) to the current state,
all postponed events are retried in the next state.
The only difference between StateFunctionResult and HandleEventResult is that for
StateFunctionResult the next state must be an atom, but for HandleEventResult there is no
restriction on the next state.
For options that can be set and actions that can be done by gen_statem after returning from this
function, see action().
When the gen_statem runs with stateentercalls, these functions are also called with arguments
(enter,OldState,...) during every statechange. In this case there are some restrictions on the
actions that may be returned: postpone() is not allowed since a stateentercall is not an event
so there is no event to postpone, and {next_event,_,_} is not allowed since using stateentercalls should not affect how events are consumed and produced. You may also not change states from
this call. Should you return {next_state,NextState,...} with NextState=/=State the gen_statem
crashes. Note that it is actually allowed to use {repeat_state,NewData,...} although it makes
little sense since you immediately will be called again with a new stateentercall making this
just a weird way of looping, and there are better ways to loop in Erlang. If you do not update
NewData and have some loop termination condition, or if you use {repeat_state_and_data,_} or
repeat_state_and_data you have an infinite loop! You are advised to use {keep_state,...},
{keep_state_and_data,_} or keep_state_and_data since changing states from a stateentercall is
not possible anyway.
Note the fact that you can use throw to return the result, which can be useful. For example to
bail out with throw(keep_state_and_data) from deep within complex code that cannot return
{next_state,State,Data} because State or Data is no longer in scope.
Module:terminate(Reason,State,Data)->Ignored
Types:
Reason = normal | shutdown | {shutdown,term()} | term()
State = state()
Data = data()
Ignored = term()
Note:
This callback is optional, so callback modules need not export it. The gen_statem module provides
a default implementation without cleanup.
This function is called by a gen_statem when it is about to terminate. It is to be the opposite of
Module:init/1 and do any necessary cleaning up. When it returns, the gen_statem terminates with
Reason. The return value is ignored.
Reason is a term denoting the stop reason and State is the internal state of the gen_statem.
Reason depends on why the gen_statem is terminating. If it is because another callback function
has returned, a stop tuple {stop,Reason} in Actions, Reason has the value specified in that tuple.
If it is because of a failure, Reason is the error reason.
If the gen_statem is part of a supervision tree and is ordered by its supervisor to terminate,
this function is called with Reason=shutdown if both the following conditions apply:
* The gen_statem has been set to trap exit signals.
* The shutdown strategy as defined in the supervisor's child specification is an integer time-
out value, not brutal_kill.
Even if the gen_statem is not part of a supervision tree, this function is called if it receives
an 'EXIT' message from its parent. Reason is the same as in the 'EXIT' message.
Otherwise, the gen_statem is immediately terminated.
Notice that for any other reason than normal, shutdown, or {shutdown,Term}, the gen_statem is
assumed to terminate because of an error and an error report is issued using logger(3erl).
When the gen_statem process exits, an exit signal with the same reason is sent to linked processes
and ports.
Name
gen_statem - Generic state machine behavior.
See Also
gen_event(3erl), gen_fsm(3erl), gen_server(3erl), proc_lib(3erl), supervisor(3erl), sys(3erl). Ericsson AB stdlib 3.17 gen_statem(3erl)
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