path: root/lib/stdlib/doc/src/gen_event.xml

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  <header>
    <copyright>
      <year>1996</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
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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>
  <datatypes>
    <datatype>
      <name name="handler"/>
    </datatype>
    <datatype>
      <name name="handler_args"/>
    </datatype>
    <datatype>
      <name name="add_handler_ret"/>
    </datatype>
    <datatype>
      <name name="del_handler_ret"/>
    </datatype>
  </datatypes>
  <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,GlobalName}
	| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Result = {ok,Pid} | {error,{already_started,Pid}}</v>
        <v>&nbsp;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,GlobalName}</c>, the event manager is
          registered globally as <c>GlobalName</c> using
          <c>global:register_name/2</c>. If no name is provided,
          the event manager is not registered.
	If <c>EventMgrName={via,Module,ViaName}</c>, the event manager will
	register with the registry represented by <c>Module</c>.
	The <c>Module</c> callback should export the functions
	<c>register_name/2</c>, <c>unregister_name/1</c>,
	<c>whereis_name/1</c> and <c>send/2</c>, which should behave like the
	corresponding functions in <c>global</c>. Thus,
	<c>{via,global,GlobalName}</c> is a valid reference.</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,GlobalName}
	| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Result = {ok,Pid} | {error,{already_started,Pid}}</v>
        <v>&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;Id = term()</v>
        <v>Args = term()</v>
        <v>Result = ok | {'EXIT',Reason} | term()</v>
        <v>&nbsp;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,GlobalName}</c>, if the event manager is globally
           registered.</item>
	   <item><c>{via,Module,ViaName}</c>, if the event manager is registered
	   through an alternative process registry.</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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;Id = term()</v>
        <v>Args = term()</v>
        <v>Result = ok | {'EXIT',Reason} | term()</v>
        <v>&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;Id = term()</v>
        <v>Request = term()</v>
        <v>Timeout = int()>0 | infinity</v>
        <v>Result = Reply | {error,Error}</v>
        <v>&nbsp;Reply = term()</v>
        <v>&nbsp;Error = bad_module | {'EXIT',Reason} | term()</v>
        <v>&nbsp;&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;Id = term()</v>
        <v>Args = term()</v>
        <v>Result = term() | {error,module_not_found} | {'EXIT',Reason}</v>
        <v>&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler1 = Handler2 = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;Id = term()</v>
        <v>Args1 = Args2 = term()</v>
        <v>Result = ok | {error,Error}</v>
        <v>&nbsp;Error = {'EXIT',Reason} | term()</v>
        <v>&nbsp;&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler1 = Handler 2 = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;Id = term()</v>
        <v>Args1 = Args2 = term()</v>
        <v>Result = ok | {error,Error}</v>
        <v>&nbsp;Error = {'EXIT',Reason} | term()</v>
        <v>&nbsp;&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Handler = Module | {Module,Id}</v>
        <v>&nbsp;Module = atom()</v>
        <v>&nbsp;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,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>Name = Node = atom()</v>
        <v>GlobalName = ViaName = term()</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>&nbsp;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>&nbsp;&nbsp;| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Args1 = Args2 = term()</v>
        <v>&nbsp;Handler2 = Module2 | {Module2,Id}</v>
        <v>&nbsp;&nbsp;Module2 = atom()</v>
        <v>&nbsp;&nbsp;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>&nbsp;| {swap_handler,Reply,Args1,NewState,Handler2,Args2}</v>
        <v>&nbsp;| {remove_handler, Reply}</v>
        <v>&nbsp;Reply = term()</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Args1 = Args2 = term()</v>
        <v>&nbsp;Handler2 = Module2 | {Module2,Id}</v>
        <v>&nbsp;&nbsp;Module2 = atom()</v>
        <v>&nbsp;&nbsp;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>&nbsp;| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Args1 = Args2 = term()</v>
        <v>&nbsp;Handler2 = Module2 | {Module2,Id}</v>
        <v>&nbsp;&nbsp;Module2 = atom()</v>
        <v>&nbsp;&nbsp;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>&nbsp;| {error,{'EXIT',Reason}} | {error,Term}</v>
        <v>&nbsp;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>&nbsp;&nbsp;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>
    <func>
      <name>Module:format_status(Opt, [PDict, State]) -> Status</name>
      <fsummary>Optional function for providing a term describing the
        current event handler state.</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
        <v>State = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so event handler modules need
            not export it. If a handler does not export this function,
            the gen_event module uses the handler state directly for
            the purposes described below.</p>
        </note>
        <p>This function is called by a gen_event process when:</p>
        <list typed="bulleted">
          <item>One
            of <seealso marker="sys#get_status/1">sys:get_status/1,2</seealso>
            is invoked to get the gen_event status. <c>Opt</c> is set
            to the atom <c>normal</c> for this case.</item>
          <item>The event handler terminates abnormally and gen_event
            logs an error. <c>Opt</c> is set to the
            atom <c>terminate</c> for this case.</item>
        </list>
        <p>This function is useful for customising the form and
          appearance of the event handler state for these cases. An
          event handler callback module wishing to customise
          the <c>sys:get_status/1,2</c> return value as well as how
          its state appears in termination error logs exports an
          instance of <c>format_status/2</c> that returns a term
          describing the current state of the event handler.</p>
        <p><c>PDict</c> is the current value of the gen_event's
          process dictionary.</p>
        <p><c>State</c> is the internal state of the event
          handler.</p>
        <p>The function should return <c>Status</c>, a term that
          customises the details of the current state of the event
          handler. Any term is allowed for <c>Status</c>. The
          gen_event module uses <c>Status</c> as follows:</p>
        <list typed="bulleted">
          <item>When <c>sys:get_status/1,2</c> is called, gen_event
            ensures that its return value contains <c>Status</c> in
            place of the event handler's actual state term.</item>
          <item>When an event handler terminates abnormally, gen_event
            logs <c>Status</c> in place of the event handler's actual
            state term.</item>
        </list>
        <p>One use for this function is to return compact alternative
          state representations to avoid having large state terms
          printed in logfiles.</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>