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

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    <copyright>
      <year>1996</year><year>2018</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the 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 behavior.</modulesummary>
  <description>
    <p>This behavior module provides event handling functionality. It
      consists of a generic event manager process with any number of
      event handlers that are added and deleted dynamically.</p>

    <p>An event manager 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
      <seealso marker="doc/design_principles:events">OTP Design Principles</seealso>.
    </p>

    <p>Each event handler is implemented as a callback module exporting
      a predefined set of functions. The relationship between the behavior
      functions and the callback functions is as follows:</p>

    <pre>
gen_event module                   Callback module
----------------                   ---------------
gen_event:start
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>As each event handler is one callback module, an event manager
      has many callback modules that are added and deleted
      dynamically. <c>gen_event</c> is therefore more tolerant of callback
      module errors than the other behaviors. 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 does not fail. It deletes
      the event handler by calling callback function
      <seealso marker="#Module:terminate/2"><c>Module:terminate/2</c></seealso>,
      giving as argument
      <c>{error,{'EXIT',Reason}}</c> or <c>{error,Term}</c>, respectively.
      No other event handler is affected.</p>

    <p>A <c>gen_event</c> process handles system messages as described in
       <seealso marker="sys"><c>sys(3)</c></seealso>. The <c>sys</c> module
       can be used for debugging an event manager.</p>

    <p>Notice that an event manager <em>does</em> trap exit signals
      automatically.</p>

    <p>The <c>gen_event</c> process can go into hibernation 
      (see <seealso marker="erts:erlang#hibernate/3">
      <c>erlang:hibernate/3</c></seealso>) if a callback function in
      a handler module specifies <c>hibernate</c> in its return value.
      This can be useful if the server is expected to be idle for a long
      time. However, use this feature with care, as hibernation
      implies at least two garbage collections (when hibernating and
      shortly after waking up) and is not something you want to do
      between each event handled by a busy event manager.</p>

    <p>Notice that when multiple event handlers are
      invoked, it is 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
      specified.</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>add_handler(EventMgrRef, Handler, Args) -> Result</name>
      <fsummary>Add an event handler 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>Args = term()</v>
        <v>Result = ok | {'EXIT',Reason} | term()</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Adds a new event handler to event manager <c>EventMgrRef</c>.
          The event manager calls
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          to initiate the event handler and its internal state.</p>
        <p><c>EventMgrRef</c> can be any of the following:</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</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 many event handlers
          use the same callback module.</p>
        <p><c>Args</c> is any term that is passed as the argument
          to <c>Module:init/1</c>.</p>
        <p>If <c>Module:init/1</c> returns a correct value indicating
          successful completion, 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 <c>{error,Reason}</c>, the event handler is
          ignored and this function returns <c>{'EXIT',Reason}</c> or
          <c>{error,Reason}</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>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 = ok | {'EXIT',Reason} | term()</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Adds a new event handler in the same way as
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>,
          but also supervises 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 deletes the event handler by calling
           <seealso marker="#Module:terminate/2">
           <c>Module:terminate/2</c></seealso>
           with <c>{stop,Reason}</c> as argument.
          </item>
          <item>
            <p>If the event handler is deleted later, 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>
                <p><c>normal</c>, if the event handler has been removed
                  because of a
                  call to <c>delete_handler/3</c>, or <c>remove_handler</c>
                  has been returned by a callback function (see below).</p>
              </item>
              <item>
                <p><c>shutdown</c>, if the event handler has been removed
                  because the event manager is terminating.</p>
              </item>
              <item>
                <p><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
                  <seealso marker="#swap_handler/3">
                  <c>swap_handler/3</c></seealso> or
                  <seealso marker="#swap_sup_handler/3">
                  <c>swap_sup_handler/3</c></seealso>.</p>
              </item>
              <item>
                <p>A term, if the event handler is removed because of an error.
                  Which term depends on the error.</p></item>
            </list>
          </item>
        </list>
        <p>For a description of the arguments and return values, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</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 event handler <c>Handler</c>
          installed in event manager <c>EventMgrRef</c> by sending a
          request and waiting until a reply arrives or a time-out occurs.
          The event manager calls <seealso marker="#Module:handle_call/2">
          <c>Module:handle_call/2</c></seealso> to handle the request.</p>
        <p>For a description of <c>EventMgrRef</c> and <c>Handler</c>, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</p>
        <p><c>Request</c> is any term that 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 that specifies
          how many milliseconds to wait for a reply, or the atom
          <c>infinity</c> to wait indefinitely. Defaults to 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 event manager
          <c>EventMgrRef</c>. The event manager calls
          <seealso marker="#Module:terminate/2">
          <c>Module:terminate/2</c></seealso> to terminate the event
          handler.</p>
         <p>For a description of <c>EventMgrRef</c> and <c>Handler</c>, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</p>
        <p><c>Args</c> is any term that 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>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 event manager
          <c>EventMgrRef</c>. The event manager calls
          <seealso marker="#Module:handle_event/2">
          <c>Module:handle_event/2</c></seealso>
          for each installed event handler to handle the event.</p>
        <p><c>notify/2</c> is asynchronous and returns immediately after
          the event notification has been sent. <c>sync_notify/2</c> is
          synchronous in the sense that it returns <c>ok</c> after
          the event has been handled by all event handlers.</p>
        <p>For a description of <c>EventMgrRef</c>, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</p>
        <p><c>Event</c> is any term that is passed as one of
          the arguments to <seealso marker="#Module:handle_event/2">
          <c>Module:handle_event/2</c></seealso>.</p>
        <p><c>notify/1</c> does not fail even if the specified event manager
          does not exist, unless it is specified as <c>Name</c>.</p>
      </desc>
    </func>

    <func>
      <name>start() -> Result</name>
      <name>start(EventMgrName | Options) -> Result</name>
      <name>start(EventMgrName, Options) -> 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>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>&nbsp;&nbsp;SOpts = [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, that is, an event
          manager that is not part of a supervision tree and thus has
          no supervisor.</p>
        <p>For a description of the arguments and return values, see
          <seealso marker="#start_link/0"><c>start_link/0,1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>start_link() -> Result</name>
      <name>start_link(EventMgrName | Options) -> Result</name>
      <name>start_link(EventMgrName, Options) -> 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>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>&nbsp;&nbsp;SOpts = [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 is to be called, directly or indirectly,
          by the supervisor. For example, it ensures that
          the event manager is linked to the supervisor.</p>
        <list type="bulleted">
          <item>
            <p>If <c>EventMgrName={local,Name}</c>, the event manager is
              registered locally as <c>Name</c> using <c>register/2</c>.</p>
          </item>
          <item>
            <p>If <c>EventMgrName={global,GlobalName}</c>, the event manager is
              registered globally as <c>GlobalName</c> using
              <seealso marker="kernel:global#register_name/2">
              <c>global:register_name/2</c></seealso>.
              If no name is provided, the event manager is not registered.</p>
          </item>
          <item>
            <p>If <c>EventMgrName={via,Module,ViaName}</c>, the event manager
              registers with the registry represented by <c>Module</c>.
              The <c>Module</c> callback is to export the functions
              <c>register_name/2</c>, <c>unregister_name/1</c>,
              <c>whereis_name/1</c>, and <c>send/2</c>, which are to behave
              as the corresponding functions in
              <seealso marker="kernel:global"><c>global</c></seealso>.
              Thus, <c>{via,global,GlobalName}</c> is a valid reference.</p>
          </item>
          <item>
            <p>If option <c>{hibernate_after,HibernateAfterTimeout}</c> is present, the <c>gen_event</c>
              process awaits any message for <c>HibernateAfterTimeout</c> milliseconds and
              if no message is received, the process goes into hibernation automatically
              (by calling <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>).</p>
          </item>
        </list>
        <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 a process with the specified
          <c>EventMgrName</c> exists already, the function returns
          <c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is
          the pid of that process.</p>
      </desc>
    </func>

    <func>
      <name>stop(EventMgrRef) -> ok</name>
      <name>stop(EventMgrRef, Reason, Timeout) -> 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>
        <v>Reason = term()</v>
        <v>Timeout = int()>0 | infinity</v>
      </type>
      <desc>
        <p>Orders event manager <c>EventMgrRef</c> to exit with
          the specifies <c>Reason</c> and waits for it to
          terminate. Before terminating, <c>gen_event</c> calls
          <seealso marker="#Module:terminate/2">
          <c>Module:terminate(stop,...)</c></seealso>
          for each installed event handler.</p>
        <p>The function returns <c>ok</c> if the event manager terminates
          with the expected reason. Any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> causes an
          error report to be issued using
          <seealso marker="kernel:logger"><c>logger(3)</c></seealso>.
          The default <c>Reason</c> is <c>normal</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero that
          specifies how many milliseconds to wait for the event manager to
          terminate, or the atom <c>infinity</c> to wait
          indefinitely. Defaults to <c>infinity</c>. If the
          event manager has not terminated within the specified time, a
          <c>timeout</c> exception is raised.</p>
        <p>If the process does not exist, a <c>noproc</c> exception
          is raised.</p>
        <p>For a description of <c>EventMgrRef</c>, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</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
          event manager <c>EventMgrRef</c>.</p>
        <p>For a description of the arguments, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</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> is
          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 is 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 is 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 is 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 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 event manager <c>EventMgrRef</c>
          in the same way as <c>swap_handler/3</c>, but also supervises
          the connection between <c>Handler2</c> and the calling process.</p>
        <p>For a description of the arguments and return values, see 
          <seealso marker="#swap_handler/3"><c>swap_handler/3</c></seealso>.</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 event
          manager <c>EventMgrRef</c>.</p>
        <p>For a description of <c>EventMgrRef</c> and <c>Handler</c>, see
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Callback Functions</title>
    <p>The following functions are to be exported from a <c>gen_event</c>
      callback module.</p>
  </section>

  <funcs>
    <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>
        <note>
          <p>This callback is optional, so callback modules need not export it.
            If a release upgrade/downgrade with <c>Change={advanced,Extra}</c>
            specified in the <c>.appup</c> file is made when <c>code_change/3</c>
            isn't implemented the event handler will crash with an <c>undef</c> error
            reason.</p>
        </note>
        <p>This function is called for an installed event handler that
          is to update its internal state during a release
          upgrade/downgrade, that is, when the instruction
          <c>{update,Module,Change,...}</c>, where
          <c>Change={advanced,Extra}</c>, is specified in the <c>.appup</c>
          file. For more information, see <seealso
          marker="doc/design_principles:users_guide">OTP Design Principles</seealso>.</p>
        <p>For an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and for 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 is to 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 <c>gen_event</c> module uses the handler state directly for
            the purposes described below.</p>
        </note>
        <p>This function is called by a <c>gen_event</c> process in the
          following situations:</p>
        <list type="bulleted">
          <item>One of <seealso marker="sys#get_status/1">
            <c>sys:get_status/1,2</c></seealso>
            is invoked to get the <c>gen_event</c> status. <c>Opt</c> is set
            to the atom <c>normal</c> for this case.</item>
          <item>The event handler terminates abnormally and <c>gen_event</c>
            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 changing the form and
          appearance of the event handler state for these cases. An
          event handler callback module wishing to change the
          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
          process dictionary of <c>gen_event</c>.</p>
        <p><c>State</c> is the internal state of the event handler.</p>
        <p>The function is to return <c>Status</c>, a term that
          change the details of the current state of the event
          handler. Any term is allowed for <c>Status</c>. The
          <c>gen_event</c> module uses <c>Status</c> as follows:</p>
        <list type="bulleted">
          <item><p>When <c>sys:get_status/1,2</c> is called, <c>gen_event</c>
            ensures that its return value contains <c>Status</c> in
            place of the state term of the event handler.</p></item>
          <item><p>When an event handler terminates abnormally, <c>gen_event</c>
            logs <c>Status</c> in place of the state term of the
            event handler.</p></item>
        </list>
        <p>One use for this function is to return compact alternative
          state representations to avoid that large state terms
          are printed in log files.</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
          <seealso marker="#call/3"><c>call/3,4</c></seealso>,
          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/3,4</c>.</p>
        <p><c>State</c> is the internal state of the event handler.</p>
        <p>The return values are the same as for
          <seealso marker="#Module:handle_event/2">
          <c>Module:handle_event/2</c></seealso>
          except that they also contain a term <c>Reply</c>, which is the reply
          to the client as the return value of <c>call/3,4</c>.</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
          <seealso marker="#notify/2"><c>notify/2</c></seealso> or
          <seealso marker="#sync_notify/2"><c>sync_notify/2</c></seealso>,
          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/2</c>/<c>sync_notify/2</c>.</p>
        <p><c>State</c> is the internal state of the event handler.</p>
        <list type="bulleted">
          <item>
            <p>If <c>{ok,NewState}</c> or <c>{ok,NewState,hibernate}</c>
              is returned, the event handler
              remains in the event manager with the possible updated
              internal state <c>NewState</c>.</p>
          </item>
          <item>
            <p>If <c>{ok,NewState,hibernate}</c> is returned, the event
              manager also goes into hibernation (by calling
              <seealso marker="proc_lib#hibernate/3">
              <c>proc_lib:hibernate/3</c></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>
          </item>
          <item>
            <p>If <c>{swap_handler,Args1,NewState,Handler2,Args2}</c> is
              returned, the event handler is 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>. For more information, see
              <seealso marker="#swap_handler/3"><c>swap_handler/3</c></seealso>.
            </p>
          </item>
          <item>
            <p>If <c>remove_handler</c> is returned, the event handler is
              deleted by calling
              <c>Module:terminate(remove_handler,State)</c>.</p>
          </item>
        </list>
      </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>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_event</c> module provides a default
            implementation of this function that logs about the unexpected
            <c>Info</c> message, drops it and returns <c>{noreply, State}</c>.</p>
        </note>
        <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>For a description of <c>State</c> and possible return values, see
          <seealso marker="#Module:handle_event/2">
          <c>Module:handle_event/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:init(InitArgs) -> {ok,State} | {ok,State,hibernate} | {error,Reason}</name>
      <fsummary>Initialize an event handler.</fsummary>
      <type>
        <v>InitArgs = Args | {Args,Term}</v>
        <v>&nbsp;Args = Term = term()</v>
        <v>State = term()</v>
	<v>Reason = 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 because of a call to
          <seealso marker="#add_handler/3"><c>add_handler/3</c></seealso> or
          <seealso marker="#add_sup_handler/3">
          <c>add_sup_handler/3</c></seealso>, <c>InitArgs</c> is
          the <c>Args</c> argument of these functions.</p>
        <p>If the event handler replaces another event handler because of
          a call to
          <seealso marker="#swap_handler/3"><c>swap_handler/3</c></seealso> or
          <seealso marker="#swap_sup_handler/3">
          <c>swap_sup_handler/3</c></seealso>, or because of 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
          <seealso marker="#swap_handler/3"><c>swap_handler/3</c></seealso>.</p>
        <p>If successful, the function returns <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 goes into hibernation (by calling <seealso
          marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>),
          waiting for the next event to occur.</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>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_event</c> module provides a default
            implementation without cleanup.</p>
        </note>
        <p>Whenever an event handler is deleted from an event manager,
          this function is called. It is to be the opposite of
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          and do any necessary cleaning up.</p>
        <p>If the event handler is deleted because of a call to
          <seealso marker="#delete_handler/3"><c>delete_handler/3</c></seealso>,
          <seealso marker="#swap_handler/3"><c>swap_handler/3</c></seealso>, or
          <seealso marker="#swap_sup_handler/3">
          <c>swap_sup_handler/3</c></seealso>, <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 that 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 terminates 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
          terminates 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 can return any term. If the event handler is
          deleted because of a call to <c>gen_event:delete_handler/3</c>,
          the return value of that function becomes the return value of this
          function. If the event handler is to be replaced with another event
          handler because of a swap, the return value is passed to
          the <c>init</c> function of the new event handler. Otherwise
          the return value is ignored.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="supervisor"><c>supervisor(3)</c></seealso>,
      <seealso marker="sys"><c>sys(3)</c></seealso></p>
  </section>
</erlref>