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<copyright>
<year>1996</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
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<legalnotice>
The contents of this file are subject to the Erlang Public License,
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<title>gen_event</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module>gen_event</module>
<modulesummary>Generic Event Handling Behaviour</modulesummary>
<description>
<p>A behaviour module for implementing event handling functionality.
The OTP event handling model consists of a generic event manager
process with an arbitrary number of event handlers which are added and
deleted dynamically.</p>
<p>An event manager implemented using this module will have a standard
set of interface functions and include functionality for tracing and
error reporting. It will also fit into an OTP supervision tree.
Refer to <em>OTP Design Principles</em> for more information.</p>
<p>Each event handler is implemented as a callback module exporting
a pre-defined set of functions. The relationship between the behaviour
functions and the callback functions can be illustrated as follows:</p>
<pre>
gen_event module Callback module
---------------- ---------------
gen_event:start_link -----> -
gen_event:add_handler
gen_event:add_sup_handler -----> Module:init/1
gen_event:notify
gen_event:sync_notify -----> Module:handle_event/2
gen_event:call -----> Module:handle_call/2
- -----> Module:handle_info/2
gen_event:delete_handler -----> Module:terminate/2
gen_event:swap_handler
gen_event:swap_sup_handler -----> Module1:terminate/2
Module2:init/1
gen_event:which_handlers -----> -
gen_event:stop -----> Module:terminate/2
- -----> Module:code_change/3</pre>
<p>Since each event handler is one callback module, an event manager
will have several callback modules which are added and deleted
dynamically. Therefore <c>gen_event</c> is more tolerant of callback
module errors than the other behaviours. If a callback function for
an installed event handler fails with <c>Reason</c>, or returns a
bad value <c>Term</c>, the event manager will not fail. It will delete
the event handler by calling the callback function
<c>Module:terminate/2</c> (see below), giving as argument
<c>{error,{'EXIT',Reason}}</c> or <c>{error,Term}</c>, respectively.
No other event handler will be affected.</p>
<p>A gen_event process handles system messages as documented in
<seealso marker="sys">sys(3)</seealso>. The <c>sys</c> module
can be used for debugging an event manager.</p>
<p>Note that an event manager <em>does</em> trap exit signals
automatically.</p>
<p>The gen_event process can go into hibernation
(see <seealso marker="erts:erlang#erlang:hibernate/3">erlang(3)</seealso>) if a callback
function in a handler module specifies <c>'hibernate'</c> in its return value.
This might be useful if the server is expected to be idle for a long
time. However this feature should be used with care as hibernation
implies at least two garbage collections (when hibernating and
shortly after waking up) and is not something you'd want to do
between each event handled by a busy event manager.</p>
<p>It's also worth noting that when multiple event handlers are
invoked, it's sufficient that one single event handler returns a
<c>'hibernate'</c> request for the whole event manager to go into
hibernation.</p>
<p>Unless otherwise stated, all functions in this module fail if
the specified event manager does not exist or if bad arguments are
given.</p>
</description>
<funcs>
<func>
<name>start_link() -> Result</name>
<name>start_link(EventMgrName) -> Result</name>
<fsummary>Create a generic event manager process in a supervision tree.</fsummary>
<type>
<v>EventMgrName = {local,Name} | {global,Name}</v>
<v> Name = atom()</v>
<v>Result = {ok,Pid} | {error,{already_started,Pid}}</v>
<v> Pid = pid()</v>
</type>
<desc>
<p>Creates an event manager process as part of a supervision
tree. The function should be called, directly or indirectly,
by the supervisor. It will, among other things, ensure that
the event manager is linked to the supervisor.</p>
<p>If <c>EventMgrName={local,Name}</c>, the event manager is
registered locally as <c>Name</c> using <c>register/2</c>.
If <c>EventMgrName={global,Name}</c>, the event manager is
registered globally as <c>Name</c> using
<c>global:register_name/2</c>. If no name is provided,
the event manager is not registered.</p>
<p>If the event manager is successfully created the function
returns <c>{ok,Pid}</c>, where <c>Pid</c> is the pid of
the event manager. If there already exists a process with
the specified <c>EventMgrName</c> the function returns
<c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is
the pid of that process.</p>
</desc>
</func>
<func>
<name>start() -> Result</name>
<name>start(EventMgrName) -> Result</name>
<fsummary>Create a stand-alone event manager process.</fsummary>
<type>
<v>EventMgrName = {local,Name} | {global,Name}</v>
<v> Name = atom()</v>
<v>Result = {ok,Pid} | {error,{already_started,Pid}}</v>
<v> Pid = pid()</v>
</type>
<desc>
<p>Creates a stand-alone event manager process, i.e. an event
manager which is not part of a supervision tree and thus has
no supervisor.</p>
<p>See <c>start_link/0,1</c> for a description of arguments and
return values.</p>
</desc>
</func>
<func>
<name>add_handler(EventMgrRef, Handler, Args) -> Result</name>
<fsummary>Add an event handler to a generic event manager.</fsummary>
<type>
<v>EventMgr = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
<v>Args = term()</v>
<v>Result = ok | {'EXIT',Reason} | term()</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Adds a new event handler to the event manager <c>EventMgrRef</c>.
The event manager will call <c>Module:init/1</c> to initiate
the event handler and its internal state.</p>
<p><c>EventMgrRef</c> can be:</p>
<list type="bulleted">
<item>the pid,</item>
<item><c>Name</c>, if the event manager is locally registered,</item>
<item><c>{Name,Node}</c>, if the event manager is locally
registered at another node, or</item>
<item><c>{global,Name}</c>, if the event manager is globally
registered.</item>
</list>
<p><c>Handler</c> is the name of the callback module <c>Module</c> or
a tuple <c>{Module,Id}</c>, where <c>Id</c> is any term.
The <c>{Module,Id}</c> representation makes it possible to
identify a specific event handler when there are several event
handlers using the same callback module.</p>
<p><c>Args</c> is an arbitrary term which is passed as the argument
to <c>Module:init/1</c>.</p>
<p>If <c>Module:init/1</c> returns a correct value, the event
manager adds the event handler and this function returns
<c>ok</c>. If <c>Module:init/1</c> fails with <c>Reason</c> or
returns an unexpected value <c>Term</c>, the event handler is
ignored and this function returns <c>{'EXIT',Reason}</c> or
<c>Term</c>, respectively.</p>
</desc>
</func>
<func>
<name>add_sup_handler(EventMgrRef, Handler, Args) -> Result</name>
<fsummary>Add a supervised event handler to a generic event manager.</fsummary>
<type>
<v>EventMgr = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
<v>Args = term()</v>
<v>Result = ok | {'EXIT',Reason} | term()</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Adds a new event handler in the same way as <c>add_handler/3</c>
but will also supervise the connection between the event handler
and the calling process.</p>
<list type="bulleted">
<item>If the calling process later terminates with <c>Reason</c>,
the event manager will delete the event handler by calling
<c>Module:terminate/2</c> with <c>{stop,Reason}</c> as argument.</item>
<item>
<p>If the event handler later is deleted, the event manager
sends a message<c>{gen_event_EXIT,Handler,Reason}</c> to
the calling process. <c>Reason</c> is one of the following:</p>
<list type="bulleted">
<item><c>normal</c>, if the event handler has been removed due to a
call to <c>delete_handler/3</c>, or <c>remove_handler</c>
has been returned by a callback function (see below).</item>
<item><c>shutdown</c>, if the event handler has been removed
because the event manager is terminating.</item>
<item><c>{swapped,NewHandler,Pid}</c>, if the process <c>Pid</c>
has replaced the event handler with another event handler
<c>NewHandler</c> using a call to <c>swap_handler/3</c> or
<c>swap_sup_handler/3</c>.</item>
<item>a term, if the event handler is removed due to an error.
Which term depends on the error.</item>
</list>
</item>
</list>
<p>See <c>add_handler/3</c> for a description of the arguments
and return values.</p>
</desc>
</func>
<func>
<name>notify(EventMgrRef, Event) -> ok</name>
<name>sync_notify(EventMgrRef, Event) -> ok</name>
<fsummary>Notify an event manager about an event.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Event = term()</v>
</type>
<desc>
<p>Sends an event notification to the event manager
<c>EventMgrRef</c>. The event manager will call
<c>Module:handle_event/2</c> for each installed event handler to
handle the event.</p>
<p><c>notify</c> is asynchronous and will return immediately after
the event notification has been sent. <c>sync_notify</c> is
synchronous in the sense that it will return <c>ok</c> after
the event has been handled by all event handlers.</p>
<p>See <c>add_handler/3</c> for a description of <c>EventMgrRef</c>.</p>
<p><c>Event</c> is an arbitrary term which is passed as one of
the arguments to <c>Module:handle_event/2</c>.</p>
<p><c>notify</c> will not fail even if the specified event manager
does not exist, unless it is specified as <c>Name</c>.</p>
</desc>
</func>
<func>
<name>call(EventMgrRef, Handler, Request) -> Result</name>
<name>call(EventMgrRef, Handler, Request, Timeout) -> Result</name>
<fsummary>Make a synchronous call to a generic event manager.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
<v>Request = term()</v>
<v>Timeout = int()>0 | infinity</v>
<v>Result = Reply | {error,Error}</v>
<v> Reply = term()</v>
<v> Error = bad_module | {'EXIT',Reason} | term()</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Makes a synchronous call to the event handler <c>Handler</c>
installed in the event manager <c>EventMgrRef</c> by sending a
request and waiting until a reply arrives or a timeout occurs.
The event manager will call <c>Module:handle_call/2</c> to handle
the request.</p>
<p>See <c>add_handler/3</c> for a description of <c>EventMgrRef</c>
and <c>Handler</c>.</p>
<p><c>Request</c> is an arbitrary term which is passed as one of
the arguments to <c>Module:handle_call/2</c>.</p>
<p><c>Timeout</c> is an integer greater than zero which specifies
how many milliseconds to wait for a reply, or the atom
<c>infinity</c> to wait indefinitely. Default value is 5000.
If no reply is received within the specified time, the function
call fails.</p>
<p>The return value <c>Reply</c> is defined in the return value of
<c>Module:handle_call/2</c>. If the specified event handler is not
installed, the function returns <c>{error,bad_module}</c>. If
the callback function fails with <c>Reason</c> or returns an
unexpected value <c>Term</c>, this function returns
<c>{error,{'EXIT',Reason}}</c> or <c>{error,Term}</c>,
respectively.</p>
</desc>
</func>
<func>
<name>delete_handler(EventMgrRef, Handler, Args) -> Result</name>
<fsummary>Delete an event handler from a generic event manager.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
<v>Args = term()</v>
<v>Result = term() | {error,module_not_found} | {'EXIT',Reason}</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Deletes an event handler from the event manager
<c>EventMgrRef</c>. The event manager will call
<c>Module:terminate/2</c> to terminate the event handler.</p>
<p>See <c>add_handler/3</c> for a description of <c>EventMgrRef</c>
and <c>Handler</c>.</p>
<p><c>Args</c> is an arbitrary term which is passed as one of
the arguments to <c>Module:terminate/2</c>.</p>
<p>The return value is the return value of <c>Module:terminate/2</c>.
If the specified event handler is not installed, the function
returns <c>{error,module_not_found}</c>. If the callback function
fails with <c>Reason</c>, the function returns
<c>{'EXIT',Reason}</c>.</p>
</desc>
</func>
<func>
<name>swap_handler(EventMgrRef, {Handler1,Args1}, {Handler2,Args2}) -> Result</name>
<fsummary>Replace an event handler in a generic event manager.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler1 = Handler2 = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
<v>Args1 = Args2 = term()</v>
<v>Result = ok | {error,Error}</v>
<v> Error = {'EXIT',Reason} | term()</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Replaces an old event handler with a new event handler in
the event manager <c>EventMgrRef</c>.</p>
<p>See <c>add_handler/3</c> for a description of the arguments.</p>
<p>First the old event handler <c>Handler1</c> is deleted.
The event manager calls <c>Module1:terminate(Args1, ...)</c>,
where <c>Module1</c> is the callback module of <c>Handler1</c>,
and collects the return value.</p>
<p>Then the new event handler <c>Handler2</c> is added and initiated
by calling <c>Module2:init({Args2,Term})</c>, where <c>Module2</c>
is the callback module of <c>Handler2</c> and <c>Term</c>
the return value of <c>Module1:terminate/2</c>. This makes it
possible to transfer information from <c>Handler1</c> to
<c>Handler2</c>.</p>
<p>The new handler will be added even if the the specified old event
handler is not installed in which case <c>Term=error</c>, or if
<c>Module1:terminate/2</c> fails with <c>Reason</c> in which case
<c>Term={'EXIT',Reason}</c>.
The old handler will be deleted even if <c>Module2:init/1</c>
fails.</p>
<p>If there was a supervised connection between <c>Handler1</c> and
a process <c>Pid</c>, there will be a supervised connection
between <c>Handler2</c> and <c>Pid</c> instead.</p>
<p>If <c>Module2:init/1</c> returns a correct value, this function
returns <c>ok</c>. If <c>Module2:init/1</c> fails with
<c>Reason</c> or returns an unexpected value <c>Term</c>, this
this function returns <c>{error,{'EXIT',Reason}}</c> or
<c>{error,Term}</c>, respectively.</p>
</desc>
</func>
<func>
<name>swap_sup_handler(EventMgrRef, {Handler1,Args1}, {Handler2,Args2}) -> Result</name>
<fsummary>Replace an event handler in a generic event manager.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler1 = Handler 2 = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
<v>Args1 = Args2 = term()</v>
<v>Result = ok | {error,Error}</v>
<v> Error = {'EXIT',Reason} | term()</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Replaces an event handler in the event manager <c>EventMgrRef</c>
in the same way as <c>swap_handler/3</c> but will also supervise
the connection between <c>Handler2</c> and the calling process.</p>
<p>See <c>swap_handler/3</c> for a description of the arguments
and return values.</p>
</desc>
</func>
<func>
<name>which_handlers(EventMgrRef) -> [Handler]</name>
<fsummary>Return all event handlers installed in a generic event manager.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v> Name = Node = atom()</v>
<v>Handler = Module | {Module,Id}</v>
<v> Module = atom()</v>
<v> Id = term()</v>
</type>
<desc>
<p>Returns a list of all event handlers installed in the event
manager <c>EventMgrRef</c>.</p>
<p>See <c>add_handler/3</c> for a description of <c>EventMgrRef</c>
and <c>Handler</c>.</p>
</desc>
</func>
<func>
<name>stop(EventMgrRef) -> ok</name>
<fsummary>Terminate a generic event manager.</fsummary>
<type>
<v>EventMgrRef = Name | {Name,Node} | {global,Name} | pid()</v>
<v>Name = Node = atom()</v>
</type>
<desc>
<p>Terminates the event manager <c>EventMgrRef</c>. Before
terminating, the event manager will call
<c>Module:terminate(stop,...)</c> for each installed event
handler.</p>
<p>See <c>add_handler/3</c> for a description of the argument.</p>
</desc>
</func>
</funcs>
<section>
<title>CALLBACK FUNCTIONS</title>
<p>The following functions should be exported from a <c>gen_event</c>
callback module.</p>
</section>
<funcs>
<func>
<name>Module:init(InitArgs) -> {ok,State} | {ok,State,hibernate}</name>
<fsummary>Initialize an event handler.</fsummary>
<type>
<v>InitArgs = Args | {Args,Term}</v>
<v> Args = Term = term()</v>
<v>State = term()</v>
</type>
<desc>
<p>Whenever a new event handler is added to an event manager,
this function is called to initialize the event handler.</p>
<p>If the event handler is added due to a call to
<c>gen_event:add_handler/3</c> or
<c>gen_event:add_sup_handler/3</c>, <c>InitArgs</c> is
the <c>Args</c> argument of these functions.</p>
<p>If the event handler is replacing another event handler due to
a call to <c>gen_event:swap_handler/3</c> or
<c>gen_event:swap_sup_handler/3</c>, or due to a <c>swap</c>
return tuple from one of the other callback functions,
<c>InitArgs</c> is a tuple <c>{Args,Term}</c> where <c>Args</c> is
the argument provided in the function call/return tuple and
<c>Term</c> is the result of terminating the old event handler,
see <c>gen_event:swap_handler/3</c>.</p>
<p>The function should return <c>{ok,State}</c> or <c>{ok,State, hibernate}</c>
where <c>State</c> is the initial internal state of the event handler.</p>
<p>If <c>{ok,State,hibernate}</c> is returned, the event
manager will go into hibernation (by calling <seealso
marker="proc_lib#hibernate/3">proc_lib:hibernate/3</seealso>),
waiting for the next event to occur.</p>
</desc>
</func>
<func>
<name>Module:handle_event(Event, State) -> Result</name>
<fsummary>Handle an event.</fsummary>
<type>
<v>Event = term()</v>
<v>State = term()</v>
<v>Result = {ok,NewState} | {ok,NewState,hibernate} </v>
<v> | {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler</v>
<v> NewState = term()</v>
<v> Args1 = Args2 = term()</v>
<v> Handler2 = Module2 | {Module2,Id}</v>
<v> Module2 = atom()</v>
<v> Id = term()</v>
</type>
<desc>
<p>Whenever an event manager receives an event sent using
<c>gen_event:notify/2</c> or <c>gen_event:sync_notify/2</c>, this
function is called for each installed event handler to handle
the event.</p>
<p><c>Event</c> is the <c>Event</c> argument of
<c>notify</c>/<c>sync_notify</c>.</p>
<p><c>State</c> is the internal state of the event handler.</p>
<p>If the function returns <c>{ok,NewState}</c> or <c>{ok,NewState,hibernate}</c>
the event handler
will remain in the event manager with the possible updated
internal state <c>NewState</c>.</p>
<p>If <c>{ok,NewState,hibernate}</c> is returned, the event
manager will also go into hibernation (by calling <seealso
marker="proc_lib#hibernate/3">proc_lib:hibernate/3</seealso>),
waiting for the next event to occur. It is sufficient that one of the event
handlers return <c>{ok,NewState,hibernate}</c> for the whole event manager
process to hibernate.</p>
<p>If the function returns
<c>{swap_handler,Args1,NewState,Handler2,Args2}</c> the event
handler will be replaced by <c>Handler2</c> by first calling
<c>Module:terminate(Args1,NewState)</c> and then
<c>Module2:init({Args2,Term})</c> where <c>Term</c> is the return
value of <c>Module:terminate/2</c>.
See <c>gen_event:swap_handler/3</c> for more information.</p>
<p>If the function returns <c>remove_handler</c> the event handler
will be deleted by calling
<c>Module:terminate(remove_handler,State)</c>.</p>
</desc>
</func>
<func>
<name>Module:handle_call(Request, State) -> Result</name>
<fsummary>Handle a synchronous request.</fsummary>
<type>
<v>Request = term()</v>
<v>State = term()</v>
<v>Result = {ok,Reply,NewState} | {ok,Reply,NewState,hibernate}</v>
<v> | {swap_handler,Reply,Args1,NewState,Handler2,Args2}</v>
<v> | {remove_handler, Reply}</v>
<v> Reply = term()</v>
<v> NewState = term()</v>
<v> Args1 = Args2 = term()</v>
<v> Handler2 = Module2 | {Module2,Id}</v>
<v> Module2 = atom()</v>
<v> Id = term()</v>
</type>
<desc>
<p>Whenever an event manager receives a request sent using
<c>gen_event:call/3,4</c>, this function is called for
the specified event handler to handle the request.</p>
<p><c>Request</c> is the <c>Request</c> argument of <c>call</c>.</p>
<p><c>State</c> is the internal state of the event handler.</p>
<p>The return values are the same as for <c>handle_event/2</c>
except they also contain a term <c>Reply</c> which is the reply
given back to the client as the return value of <c>call</c>.</p>
</desc>
</func>
<func>
<name>Module:handle_info(Info, State) -> Result</name>
<fsummary>Handle an incoming message.</fsummary>
<type>
<v>Info = term()</v>
<v>State = term()</v>
<v>Result = {ok,NewState} | {ok,NewState,hibernate}</v>
<v> | {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler</v>
<v> NewState = term()</v>
<v> Args1 = Args2 = term()</v>
<v> Handler2 = Module2 | {Module2,Id}</v>
<v> Module2 = atom()</v>
<v> Id = term()</v>
</type>
<desc>
<p>This function is called for each installed event handler when
an event manager receives any other message than an event or
a synchronous request (or a system message).</p>
<p><c>Info</c> is the received message.</p>
<p>See <c>Module:handle_event/2</c> for a description of State
and possible return values.</p>
</desc>
</func>
<func>
<name>Module:terminate(Arg, State) -> term()</name>
<fsummary>Clean up before deletion.</fsummary>
<type>
<v>Arg = Args | {stop,Reason} | stop | remove_handler</v>
<v> | {error,{'EXIT',Reason}} | {error,Term}</v>
<v> Args = Reason = Term = term()</v>
</type>
<desc>
<p>Whenever an event handler is deleted from an event manager,
this function is called. It should be the opposite of
<c>Module:init/1</c> and do any necessary cleaning up.</p>
<p>If the event handler is deleted due to a call to
<c>gen_event:delete_handler</c>, <c>gen_event:swap_handler/3</c>
or <c>gen_event:swap_sup_handler/3</c>, <c>Arg</c> is
the <c>Args</c> argument of this function call.</p>
<p><c>Arg={stop,Reason}</c> if the event handler has a supervised
connection to a process which has terminated with reason
<c>Reason</c>.</p>
<p><c>Arg=stop</c> if the event handler is deleted because
the event manager is terminating.</p>
<p>The event manager will terminate if it is part of a supervision
tree and it is ordered by its supervisor to terminate.
Even if it is <em>not</em> part of a supervision tree, it will
terminate if it receives an <c>'EXIT'</c> message from
its parent.</p>
<p><c>Arg=remove_handler</c> if the event handler is deleted because
another callback function has returned <c>remove_handler</c> or
<c>{remove_handler,Reply}</c>.</p>
<p><c>Arg={error,Term}</c> if the event handler is deleted because
a callback function returned an unexpected value <c>Term</c>,
or <c>Arg={error,{'EXIT',Reason}}</c> if a callback function
failed.</p>
<p><c>State</c> is the internal state of the event handler.</p>
<p>The function may return any term. If the event handler is
deleted due to a call to <c>gen_event:delete_handler</c>,
the return value of that function will be the return value of this
function. If the event handler is to be replaced with another event
handler due to a swap, the return value will be passed to
the <c>init</c> function of the new event handler. Otherwise
the return value is ignored.</p>
</desc>
</func>
<func>
<name>Module:code_change(OldVsn, State, Extra) -> {ok, NewState}</name>
<fsummary>Update the internal state during upgrade/downgrade.</fsummary>
<type>
<v>OldVsn = Vsn | {down, Vsn}</v>
<v> Vsn = term()</v>
<v>State = NewState = term()</v>
<v>Extra = term()</v>
</type>
<desc>
<p>This function is called for an installed event handler which
should update its internal state during a release
upgrade/downgrade, i.e. when the instruction
<c>{update,Module,Change,...}</c> where
<c>Change={advanced,Extra}</c> is given in the <c>.appup</c>
file. See <em>OTP Design Principles</em> for more
information.</p>
<p>In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
in the case of a downgrade, <c>OldVsn</c> is
<c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
attribute(s) of the old version of the callback module
<c>Module</c>. If no such attribute is defined, the version
is the checksum of the BEAM file.</p>
<p><c>State</c> is the internal state of the event handler.</p>
<p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
part of the update instruction.</p>
<p>The function should return the updated internal state.</p>
</desc>
</func>
</funcs>
<section>
<title>SEE ALSO</title>
<p><seealso marker="supervisor">supervisor(3)</seealso>,
<seealso marker="sys">sys(3)</seealso></p>
</section>
</erlref>
