<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
</legalnotice>
<title>gen_statem Behaviour</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>statem.xml</file>
</header>
<marker id="gen_statem behaviour"></marker>
<p>
This section is to be read with the
<seealso marker="stdlib:gen_statem"><c>gen_statem(3)</c></seealso>
manual page in STDLIB, where all interface functions and callback
functions are described in detail.
</p>
<!-- =================================================================== -->
<section>
<title>Event Driven State Machines</title>
<p>
Established Automata theory does not deal much with
how a state transition is triggered,
but in general assumes that the output is a function
of the input (and the state) and that they are
some kind of values.
</p>
<p>
For an Event Driven State Machine the input is an event
that triggers a state transition and the output
is actions executed during the state transition.
It can analogously to the mathematical model of a
Finite State Machine be described as
a set of relations of the form:
</p>
<pre>
State(S) x Event(E) -> Actions(A), State(S')</pre>
<p>These relations are interpreted as meaning:</p>
<p>
If we are in state <c>S</c> and event <c>E</c> occurs, we
are to perform actions <c>A</c> and make a transition to
state <c>S'</c>.
</p>
<p>
Note that <c>S'</c> may be equal to <c>S</c>.
</p>
<p>
Since <c>A</c> and <c>S'</c> depend only on
<c>S</c> and <c>E</c> the kind of state machine described
here is a Mealy Machine.
</p>
<p>
Like most <c>gen_</c> behaviours, <c>gen_statem</c> keeps
a server <c>Data</c> besides the state. This and the fact that
there is no restriction on the number of states
(assuming enough virtual machine memory)
or on the number of distinct input events actually makes
a state machine implemented with this behaviour Turing complete.
But it feels mostly like an Event Driven Mealy Machine.
</p>
<p>
The <c>gen_statem</c> behaviour supports two different
callback modes. In the mode <c>state_functions</c>,
the state transition rules are written as a number of Erlang
functions, which conform to the following convention:
</p>
<pre>
StateName(EventType, EventContent, Data) ->
.. code for actions here ...
{next_state, NewStateName, NewData}.</pre>
<p>
In the mode <c>handle_event_function</c> there is only one
Erlang function that implements all state transition rules:
</p>
<pre>
handle_event(EventType, EventContent, State, Data) ->
.. code for actions here ...
{next_state, State', Data'}</pre>
<p>
Both these modes allow other return tuples
that you can find in the
<seealso marker="stdlib:gen_statem#Module:StateName/3">
reference manual.
</seealso>
These other return tuples can for example stop the machine,
execute state transition actions on the machine engine itself
and send replies.
</p>
</section>
<!-- =================================================================== -->
<section>
<title>Example</title>
<p>
This is an example starting off as equivalent to the the example in the
<seealso marker="fsm"><c>gen_fsm</c> behaviour </seealso>
description. In later chapters additions and tweaks are made
using features in <c>gen_statem</c> that <c>gen_fsm</c> does not have.
At the end of this section you can find the example again
with all the added features.
</p>
<p>
A door with a code lock can be viewed as a state machine.
Initially, the door is locked. Anytime someone presses a button,
this generates an event.
Depending on what buttons have been pressed before,
the sequence so far can be correct, incomplete, or wrong.
</p>
<p>
If it is correct, the door is unlocked for 10 seconds (10000 ms).
If it is incomplete, we wait for another button to be pressed. If
it is is wrong, we start all over,
waiting for a new button sequence.
</p>
<image file="code_lock.png">
<icaption>Code lock state diagram</icaption>
</image>
<p>
We can implement such a code lock state machine using
<c>gen_statem</c> with the following callback module:
</p>
<marker id="ex"></marker>
<code type="erl"><![CDATA[
-module(code_lock).
-behaviour(gen_statem).
-export([start_link/1]).
-export([button/1]).
-export([init/1,terminate/3,code_change/4]).
-export([locked/3,open/3]).
start_link(Code) ->
gen_statem:start_link({local,code_lock}, ?MODULE, Code, []).
button(Digit) ->
gen_statem:cast(code_lock, {button,Digit}).
init(Code) ->
do_lock(),
Data = #{code => Code, remaining => Code},
{state_functions,locked,Data}.
locked(
cast, {button,Digit},
#{code := Code, remaining := Remaining} = Data) ->
case Remaining of
[Digit] ->
do_unlock(),
{next_state,open,Data#{remaining := Code},10000};
[Digit|Rest] -> % Incomplete
{next_state,locked,Data#{remaining := Rest}};
_Wrong ->
{next_state,locked,Data#{remaining := Code}}
end.
open(timeout, _, Data) ->
do_lock(),
{next_state,locked,Data};
open(cast, {button,_}, Data) ->
do_lock(),
{next_state,locked,Data}.
do_lock() ->
io:format("Lock~n", []).
do_unlock() ->
io:format("Unlock~n", []).
terminate(_Reason, State, _Data) ->
State =/= locked andalso do_lock(),
ok.
code_change(_Vsn, State, Data, _Extra) ->
{ok,State,Data}.
]]></code>
<p>The code is explained in the next sections.</p>
</section>
<!-- =================================================================== -->
<section>
<title>Starting gen_statem</title>
<p>
In the example in the previous section, the <c>gen_statem</c> is
started by calling <c>code_lock:start_link(Code)</c>:
</p>
<code type="erl"><![CDATA[
start_link(Code) ->
gen_statem:start_link({local,code_lock}, ?MODULE, Code, []).
]]></code>
<p>
<c>start_link</c> calls the function
<seealso marker="stdlib:gen_statem#start_link/4">
<c>gen_statem:start_link/4</c>
</seealso>
which spawns and links to a new process; a <c>gen_statem</c>.
</p>
<list type="bulleted">
<item>
<p>
The first argument, <c>{local,code_lock}</c>, specifies
the name. In this case, the <c>gen_statem</c> is locally
registered as <c>code_lock</c>.
</p>
<p>
If the name is omitted, the <c>gen_statem</c> is not registered.
Instead its pid must be used. The name can also be given
as <c>{global,Name}</c>, in which case the <c>gen_statem</c> is
registered using
<seealso marker="kernel:global#register_name/2">
<c>global:register_name/2</c>.
</seealso>
</p>
</item>
<item>
<p>
The second argument, <c>?MODULE</c>, is the name of
the callback module, that is; the module where the callback
functions are located, which is this module.
</p>
<p>
The interface functions (<c>start_link/1</c> and <c>button/1</c>)
are located in the same module as the callback functions
(<c>init/1</c>, <c>locked/3</c>, and <c>open/3</c>).
It is normally good programming practice to have the client
side and the server side code contained in one module.
</p>
</item>
<item>
<p>
The third argument, <c>Code</c>, is a list of digits that
is the correct unlock code which is passsed
to the callback function <c>init/1</c>.
</p>
</item>
<item>
<p>
The fourth argument, <c>[]</c>, is a list of options. See the
<seealso marker="stdlib:gen_statem#start_link/3">
<c>gen_statem:start_link/3</c>
</seealso>
manual page for available options.
</p>
</item>
</list>
<p>
If name registration succeeds, the new <c>gen_statem</c> process
calls the callback function <c>code_lock:init(Code)</c>.
This function is expected to return <c>{CallbackMode,State,Data}</c>,
where <c>CallbackMode</c> selects callback module state function
mode, in this case <c>state_functions</c> that is each state
has got its own handler function.
<c>State</c> is the initial state of the <c>gen_statem</c>,
in this case <c>locked</c>; assuming the door is locked to begin with.
<c>Data</c> is the internal server data of the <c>gen_statem</c>.
Here the server data is a
<seealso marker="stdlib:maps">
map
</seealso>
with the key <c>code</c> that stores
the correct button sequence and the key <c>remaining</c>
that stores the remaining correct button sequence
(the same as the <c>code</c> to begin with).
</p>
<code type="erl"><![CDATA[
init(Code) ->
do_lock(),
Data = #{code => Code, remaining => Code},
{state_functions,locked,Data}.
]]></code>
<p>
<seealso marker="stdlib:gen_statem#start_link/3">
<c>gen_statem:start_link</c>
</seealso>
is synchronous. It does not return until the <c>gen_statem</c>
has been initialized and is ready to receive events.
</p>
<p>
<seealso marker="stdlib:gen_statem#start_link/3">
<c>gen_statem:start_link</c>
</seealso>
must be used if the <c>gen_statem</c>
is part of a supervision tree, that is; started by a supervisor.
There is another function;
<seealso marker="stdlib:gen_statem#start/3">
<c>gen_statem:start</c>
</seealso>
to start a standalone <c>gen_statem</c>, that is;
a <c>gen_statem</c> that is not part of a supervision tree.
</p>
</section>
<!-- =================================================================== -->
<section>
<title>Events and Handling them</title>
<p>The function notifying the code lock about a button event is
implemented using
<seealso marker="stdlib:gen_statem#cast/2">
<c>gen_statem:cast/2</c>:
</seealso>
</p>
<code type="erl"><![CDATA[
button(Digit) ->
gen_statem:cast(code_lock, {button,Digit}).
]]></code>
<p>
<c>code_lock</c> is the name of the <c>gen_statem</c> and must
agree with the name used to start it.
<c>{button,Digit}</c> is the actual event content.
</p>
<p>
The event is made into a message and sent to the <c>gen_statem</c>.
When the event is received, the <c>gen_statem</c> calls
<c>StateName(cast, Event, Data)</c>, which is expected to
return a tuple <c>{next_state,NewStateName,NewData}</c>.
<c>StateName</c> is the name of the current state and
<c>NewStateName</c> is the name of the next state to go to.
<c>NewData</c> is a new value for the server data of
the <c>gen_statem</c>.
</p>
<code type="erl"><![CDATA[
locked(
cast, {button,Digit},
#{code := Code, remaining := Remaining} = Data) ->
case Remaining of
[Digit] -> % Complete
do_unlock(),
{next_state,open,Data#{remaining := Code},10000};
[Digit|Rest] -> % Incomplete
{next_state,locked,Data#{remaining := Rest}};
[_|_] -> % Wrong
{next_state,locked,Data#{remaining := Code}}
end.
open(timeout, _, Data) ->
do_lock(),
{next_state,locked,Data};
open(cast, {button,_}, Data) ->
do_lock(),
{next_state,locked,Data}.
]]></code>
<p>
If the door is locked and a button is pressed, the pressed
button is compared with the next correct button and,
depending on the result, the door is either unlocked
and the <c>gen_statem</c> goes to state <c>open</c>,
or the door remains in state <c>locked</c>.
</p>
<p>
If the pressed button is incorrect the server data
restarts from the start of the code sequence.
</p>
<p>
In state <c>open</c> any button locks the door since
any event cancels the event timer so we will not get
a timeout event after a button event.
</p>
</section>
<section>
<title>Event Time-Outs</title>
<p>
When a correct code has been given, the door is unlocked and
the following tuple is returned from <c>locked/2</c>:
</p>
<code type="erl"><![CDATA[
{next_state,open,Data#{remaining := Code},10000};
]]></code>
<p>
10000 is a time-out value in milliseconds.
After this time, that is; 10 seconds, a time-out occurs.
Then, <c>StateName(timeout, 10000, Data)</c> is called.
The time-out occurs when the door has been in state <c>open</c>
for 10 seconds. After that the door is locked again:
</p>
<code type="erl"><![CDATA[
open(timeout, _, Data) ->
do_lock(),
{next_state,locked,Data};
]]></code>
</section>
<!-- =================================================================== -->
<section>
<title>All State Events</title>
<p>
Sometimes an event can arrive in any state of the <c>gen_statem</c>.
It is convenient to handle these in a common state handler function
that all state functions call for events not specific to the state.
</p>
<p>
Let's introduce a <c>code_length/0</c> function that returns
the length of the correct code
(that should not be sensitive to reveal...).
We'll dispatch all events that are not state specific
to the common function <c>handle_event/3</c>.
</p>
<code type="erl"><![CDATA[
...
-export([button/1,code_length/0]).
...
code_length() ->
gen_statem:call(code_lock, code_length).
...
locked(...) -> ... ;
locked(EventType, EventContent, Data) ->
handle_event(EventType, EventContent, Data).
...
open(...) -> ... ;
open(EventType, EventContent, Data) ->
handle_event(EventType, EventContent, Data).
handle_event({call,From}, code_length, #{code := Code} = Data) ->
{keep_state,Data,[{reply,From,length(Code)}]}.
]]></code>
<p>
This example uses
<seealso marker="stdlib:gen_statem#call/2">
<c>gen_statem:call/2</c>
</seealso>
which waits for a reply from the server.
The reply is sent with a <c>{reply,From,Reply}</c> tuple
in an action list in the <c>{keep_state,...}</c> tuple
that retains the current state.
</p>
</section>
<!-- =================================================================== -->
<section>
<title>One Event Handler</title>
<p>
If you use the mode <c>handle_event_function</c>
all events are handled in <c>handle_event/4</c> and we
may (but do not have to) use an event-centered approach
where we dispatch on event first and then state:
</p>
<code type="erl"><![CDATA[
...
-export([handle_event/4]).
...
init(Code) ->
Data = #{code => Code, remaining => Code},
{handle_event_function,locked,Data}.
handle_event(cast, {button,Digit}, State, #{code := Code} = Data) ->
case State of
locked ->
case maps:get(remaining, Data) of
[Digit] -> % Complete
do_unlock(),
{next_state,open,Data#{remaining := Code},10000};
[Digit|Rest] -> % Incomplete
{keep_state,Data#{remaining := Rest}};
[_|_] -> % Wrong
{keep_state,Data#{remaining := Code}}
end;
open ->
do_lock(),
{next_state,locked,Data}
end;
handle_event(timeout, _, open, Data) ->
do_lock(),
{next_state,locked,Data}.
...
]]></code>
</section>
<!-- =================================================================== -->
<section>
<title>Stopping</title>
<section>
<title>In a Supervision Tree</title>
<p>
If the <c>gen_statem</c> is part of a supervision tree,
no stop function is needed.
The <c>gen_statem</c> is automatically terminated by its supervisor.
Exactly how this is done is defined by a
<seealso marker="sup_princ#shutdown">shutdown strategy</seealso>
set in the supervisor.
</p>
<p>
If it is necessary to clean up before termination, the shutdown
strategy must be a time-out value and the <c>gen_statem</c> must
in the <c>init/1</c> function set itself to trap exit signals
by calling
<seealso marker="erts:erlang#process_flag/2">
<c>process_flag(trap_exit, true)</c>.
</seealso>
When ordered to shutdown, the <c>gen_statem</c> then calls
the callback function
<c>terminate(shutdown, State, Data)</c>:
</p>
<code type="erl"><![CDATA[
init(Args) ->
process_flag(trap_exit, true),
...
{CallbackMode,State,Data}.
]]></code>
<p>
In this example we let the <c>terminate/3</c> function
lock the door if it is open so we do not accidentally leave the door
open when the supervision tree terminates.
</p>
<code type="erl"><![CDATA[
terminate(_Reason, State, _Data) ->
State =/= locked andalso do_lock(),
ok.
]]></code>
</section>
<section>
<title>Standalone gen_statem</title>
<p>
If the <c>gen_statem</c> is not part of a supervision tree,
it can be stopped using
<seealso marker="stdlib:gen_statem#stop/1">
<c>gen_statem:stop</c>,
</seealso>
preferably through an API function:
</p>
<code type="erl"><![CDATA[
...
-export([start_link/1,stop/0]).
...
stop() ->
gen_statem:stop(code_lock).
]]></code>
<p>
This makes the <c>gen_statem</c> call the <c>terminate/3</c>
callback function just like for a supervised server
and waits for the process to terminate.
</p>
</section>
</section>
<!-- =================================================================== -->
<section>
<title>Actions</title>
<p>
In the first chapters we mentioned actions as a part of
the general state machine model, and these actions
are implemented with the code the <c>gen_statem</c>
callback module executes in an event handling
callback function before returning
to the <c>gen_statem</c> engine.
</p>
<p>
There are more specific state transition actions
that a callback function can order the <c>gen_statem</c>
engine to do after the callback function return.
These are ordered by returning a list of
<seealso marker="stdlib:gen_statem#type-action">
actions
</seealso>
in the
<seealso marker="stdlib:gen_statem#type-state_function_result">
return tuple
</seealso>
from the
<seealso marker="stdlib:gen_statem#Module:StateName/3">
callback function.
</seealso>
These state transition actions affect the <c>gen_statem</c>
engine itself. They can:
</p>
<list type="bulleted">
<item>Postpone the current event.</item>
<item>Hibernate the <c>gen_statem</c>.</item>
<item>Start an event timeout.</item>
<item>Reply to a caller.</item>
<item>Generate the next event to handle.</item>
</list>
<p>
We have mentioned the event timeout
and replying to a caller in the example above.
An example of event postponing comes in later in this chapter.
See the
<seealso marker="stdlib:gen_statem#type-action">
reference manual
</seealso>
for details. You can for example actually reply to several callers
and generate multiple next events to handle.
</p>
</section>
<!-- =================================================================== -->
<section>
<title>Event Types</title>
<p>
So far we have mentioned a few
<seealso marker="stdlib:gen_statem#type-event_type">
event types.
</seealso>
Events of all types are handled in the same callback function,
for a given state, and the function gets
<c>EventType</c> and <c>EventContent</c> as arguments.
</p>
<p>
Here is the complete list of event types and where
they come from:
</p>
<taglist>
<tag><c>cast</c></tag>
<item>
Generated by
<seealso marker="stdlib:gen_statem#cast/2">
<c>gen_statem:cast</c>.
</seealso>
</item>
<tag><c>{call,From}</c></tag>
<item>
Generated by
<seealso marker="stdlib:gen_statem#call/2">
<c>gen_statem:call</c>
</seealso>
where <c>From</c> is the reply address to use
when replying either through the state transition action
<c>{reply,From,Msg}</c> or by calling
<seealso marker="stdlib:gen_statem#reply/1">
<c>gen_statem:reply</c>.
</seealso>
</item>
<tag><c>info</c></tag>
<item>
Generated by any regular process message sent to
the <c>gen_statem</c> process.
</item>
<tag><c>timeout</c></tag>
<item>
Generated by the state transition action
<c>{timeout,Time,EventContent}</c> (or its short form <c>Time</c>)
timer timing out.
</item>
<tag><c>internal</c></tag>
<item>
Generated by the state transition action
<c>{next_event,internal,EventContent}</c>.
In fact all event types above can be generated using
<c>{next_event,EventType,EventContent}</c>.
</item>
</taglist>
</section>
<!-- =================================================================== -->
<section>
<title>State Timeouts</title>
<p>
The timeout event generated by the state transition action
<c>{timeout,Time,EventContent}</c> is an event timeout,
that is; if an event arrives the timer is cancelled.
You get either an event or a timeout but not both.
</p>
<p>
Often you want a timer to not be cancelled by any event
or you want to start a timer in one state and respond
to the timeout in another. This can be accomplished
with a regular erlang timer:
<seealso marker="erts:erlang#start_timer/4">
<c>erlang:start_timer</c>.
</seealso>
</p>
<p>
Looking at the example in this chapter so far; using the
<c>gen_statem</c> event timer has the consequence that
if a button event is generated while in the <c>open</c> state,
the timeout is cancelled and the button event is delivered.
Therefore we chose to lock the door if this happended.
</p>
<p>
Suppose we do not want a button to lock the door,
instead we want to ignore button events in the <c>open</c> state.
Then we start a timer when entering the <c>open</c> state
and wait for it to expire while ignoring button events:
</p>
<code type="erl"><![CDATA[
...
locked(
cast, {button,Digit},
#{code := Code, remaining := Remaining} = Data) ->
case Remaining of
[Digit] ->
do_unlock(),
Tref = erlang:start_timer(10000, self(), lock),
{next_state,open,Data#{remaining := Code, timer := Tref}};
...
open(info, {timeout,Tref,lock}, #{timer := Tref} = Data) ->
do_lock(),
{next_state,locked,Data};
open(cast, {button,_}, Data) ->
{keep_state,Data};
...
]]></code>
<p>
If you need to cancel a timer due to some other event you can use
<seealso marker="erts:erlang#cancel_timer/2">
<c>erlang:cancel_timer(Tref)</c>.
</seealso>
Note that a timeout message can not arrive after this,
unless you have postponed it (see the next section) before,
so make sure you do not accidentally postpone such messages.
</p>
<p>
Another way to cancel a timer is to not cancel it,
but instead to ignore it if it arrives in a state
where it is known to be late.
</p>
</section>
<!-- =================================================================== -->
<section>
<title>Postponing Events</title>
<p>
If you want to ignore a particular event in the current state
and handle it in a future state, you can postpone the event.
A postponed event is retried after the state has
changed i.e <c>OldState =/= NewState</c>.
</p>
<p>
Postponing is ordered by the
<seealso marker="stdlib:gen_statem#type-action">
state transition action
</seealso>
<c>postpone</c>.
</p>
<p>
In this example, instead of ignoring button events
while in the <c>open</c> state we can postpone them
and they will be queued and later handled in the <c>locked</c> state:
</p>
<code type="erl"><![CDATA[
...
open(cast, {button,_}, Data) ->
{keep_state,Data,[postpone]};
...
]]></code>
<section>
<title>Fuzzy State Diagrams</title>
<p>
It is not uncommon that a state diagram does not specify
how to handle events that are not illustrated
in a particular state in the diagram.
Hopefully this is described in an associated text
or from the context.
</p>
<p>
Possible actions may be; ignore as in drop the event
(maybe log it) or deal with the event in some other state
as in postpone it.
</p>
</section>
<section>
<title>Selective Receive</title>
<p>
Erlang's selective receive statement is often used to
describe simple state machine examples in straightforward
Erlang code. Here is a possible implementation of
the first example:
</p>
<code type="erl"><![CDATA[
-module(code_lock).
-export([start_link/1,button/1]).
start_link(Code) ->
spawn(
fun () ->
true = register(code_lock, self()),
do_lock(),
locked(Code, Code)
end).
button(Digit) ->
code_lock ! {button,Digit}.
locked(Code, [Digit|Remaining]) ->
receive
{button,Digit} when Remaining =:= [] ->
do_unlock(),
open(Code);
{button,Digit} ->
locked(Code, Remaining);
{button,_} ->
locked(Code, Code)
end.
open(Code) ->
receive
after 10000 ->
do_lock(),
locked(Code, Code)
end.
do_lock() ->
io:format("Locked~n", []).
do_unlock() ->
io:format("Open~n", []).
]]></code>
<p>
The selective receive in this case causes <c>open</c>
to implicitly postpone any events to the <c>locked</c> state.
</p>
<p>
A selective receive can not be used from a <c>gen_statem</c>
behaviour just as for any <c>gen_*</c> behavior
since the receive statement is within the <c>gen_*</c> engine itself.
It has to be there because all
<seealso marker="stdlib:sys"><c>sys</c></seealso>
compatible behaviours must respond to system messages and therefore
do that in their engine receive loop,
passing non-system messages to the callback module.
</p>
<p>
The
<seealso marker="stdlib:gen_statem#type-action">
state transition action
</seealso>
<c>postpone</c> is designed to be able to model
selective receives. A selective receive implicitly postpones
any not received events, but the <c>postpone</c>
state transition action explicitly postpones one received event.
</p>
<p>
Other than that both mechanisms have got the same theoretical
time and memory complexity, while the selective receive
language construct has got smaller constant factors.
</p>
</section>
</section>
<!-- =================================================================== -->
<section>
<title>Self Generated Events</title>
<p>
It may be beneficial in some cases to be able to generate events
to your own state machine. This can be done with the
<seealso marker="stdlib:gen_statem#type-action">
state transition action
</seealso>
<c>{next_event,EventType,EventContent}</c>.
</p>
<p>
You can generate events of any existing
<seealso marker="stdlib:gen_statem#type-action">
type,
</seealso>
but the <c>internal</c> type can only be generated through the
<c>next_event</c> action and hence can not come from an external source,
so you can be certain that an <c>internal</c> event is an event
from your state machine to itself.
</p>
<p>
One example of using self generated events may be when you have
a state machine specification that uses state entry actions.
That you could code using a dedicated function
to do the state transition. But if you want that code to be
visible besides the other state logic you can insert
an <c>internal</c> event that does the entry actions.
This has the same unfortunate consequence as using
state transition functions that everywhere you go to
the state in question you will have to explicitly
insert the <c>internal</c> event
or use state transition function.
</p>
<p>
Here is an implementation of entry actions
using <c>internal</c> events with content <c>enter</c>
utilizing a helper function <c>enter/3</c> for state entry:
</p>
<code type="erl"><![CDATA[
init(Code) ->
Data = #{code => Code},
enter(state_functions, locked, Data).
...
locked(internal, enter, _Data) ->
do_lock(),
{keep_state,Data#{remaining => Code}};
locked(
cast, {button,Digit},
#{code := Code, remaining := Remaining} = Data) ->
case Remaining of
[Digit] ->
enter(next_state, open, Data);
...
open(internal, enter, _Data) ->
Tref = erlang:start_timer(10000, self(), lock),
do_unlock(),
{keep_state,Data#{timer => Tref}};
open(info, {timeout,Tref,lock}, #{timer := Tref} = Data) ->
enter(next_state, locked, Data);
...
enter(Tag, State, Data) ->
{Tag,State,Data,[{next_event,internal,enter}]}.
]]></code>
</section>
<!-- =================================================================== -->
<section>
<title>Example Revisited</title>
<p>
Here is the example after all mentioned modifications
and some more utilizing the entry actions,
which deserves a new state diagram:
</p>
<image file="code_lock_2.png">
<icaption>Code lock state diagram revisited</icaption>
</image>
<p>
Note that this state diagram does not specify how to handle
a button event in the state <c>open</c>, so you will have to
read some other place that is here that unspecified events
shall be ignored as in not consumed but handled in some other state.
Nor does it show that the <c>code_length/0</c> call shall be
handled in every state.
</p>
<section>
<title>Callback Mode: state_functions</title>
<p>
Using state functions:
</p>
<code type="erl"><![CDATA[
-module(code_lock).
-behaviour(gen_statem).
-export([start_link/1,stop/0]).
-export([button/1,code_length/0]).
-export([init/1,terminate/3,code_change/4]).
-export([locked/3,open/3]).
start_link(Code) ->
gen_statem:start_link({local,code_lock}, ?MODULE, Code, []).
stop() ->
gen_statem:stop(code_lock).
button(Digit) ->
gen_statem:cast(code_lock, {button,Digit}).
code_length() ->
gen_statem:call(code_lock, code_length).
init(Code) ->
Data = #{code => Code},
enter(state_functions, locked, Data).
locked(internal, enter, #{code := Code} = Data) ->
do_lock(),
{keep_state,Data#{remaining => Code}};
locked(
cast, {button,Digit},
#{code := Code, remaining := Remaining} = Data) ->
case Remaining of
[Digit] -> % Complete
enter(next_state, open, Data);
[Digit|Rest] -> % Incomplete
{keep_state,Data#{remaining := Rest}};
[_|_] -> % Wrong
{keep_state,Data#{remaining := Code}}
end;
locked(EventType, EventContent, Data) ->
handle_event(EventType, EventContent, Data).
open(internal, enter, Data) ->
Tref = erlang:start_timer(10000, self(), lock),
do_unlock(),
{keep_state,Data#{timer => Tref}};
open(info, {timeout,Tref,lock}, #{timer := Tref} = Data) ->
enter(next_state, locked, Data);
open(cast, {button,_}, _) ->
{keep_state_and_data,[postpone]};
open(EventType, EventContent, Data) ->
handle_event(EventType, EventContent, Data).
handle_event({call,From}, code_length, #{code := Code}) ->
{keep_state_and_data,[{reply,From,length(Code)}]}.
enter(Tag, State, Data) ->
{Tag,State,Data,[{next_event,internal,enter}]}.
do_lock() ->
io:format("Locked~n", []).
do_unlock() ->
io:format("Open~n", []).
terminate(_Reason, State, _Data) ->
State =/= locked andalso do_lock(),
ok.
code_change(_Vsn, State, Data, _Extra) ->
{ok,State,Data}.
]]></code>
</section>
<section>
<title>Callback Mode: handle_event_function</title>
<p>
What to change to use one <c>handle_event/4</c> function.
Here a clean first-dispatch-on-event approach
does not work that well due to the generated
entry actions:
</p>
<code type="erl"><![CDATA[
...
-export([handle_event/4]).
...
init(Code) ->
process_flag(trap_exit, true),
Data = #{code => Code},
enter(handle_event_function, locked, Data).
...
%% State: locked
handle_event(internal, enter, locked, #{code := Code} = Data) ->
do_lock(),
{keep_state,Data#{remaining => Code}};
handle_event(
cast, {button,Digit}, locked,
#{code := Code, remaining := Remaining} = Data) ->
case Remaining of
[Digit] -> % Complete
enter(next_state, open, Data, []);
[Digit|Rest] -> % Incomplete
{keep_state,Data#{remaining := Rest}};
[_|_] -> % Wrong
{keep_state,Data#{remaining := Code}}
end;
%%
%% State: open
handle_event(internal, enter, open, Data) ->
Tref = erlang:start_timer(10000, self(), lock),
do_unlock(),
{keep_state,Data#{timer => Tref}};
handle_event(info, {timeout,Tref,lock}, open, #{timer := Tref} = Data) ->
enter(next_state, locked, Data, []);
handle_event(cast, {button,_}, open, _) ->
{keep_state_and_data,[postpone]};
%%
%% Any state
handle_event({call,From}, code_length, _State, #{code := Code}) ->
{keep_state_and_data,[{reply,From,length(Code)}]}.
...
]]></code>
</section>
<p>
Note that postponing buttons from the <c>locked</c> state
to the <c>open</c> state feels like the wrong thing to do
for a code lock, but it illustrates event postponing.
</p>
</section>
</chapter>
