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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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          http://www.apache.org/licenses/LICENSE-2.0

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      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
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    <title>gen_event Behaviour</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>events.xml</file>
  </header>
  <marker id="gen_event"></marker>
  <p>This section is to be read with the <c>gen_event(3)</c> manual
  page in STDLIB, where all interface functions and callback
  functions are described in detail.</p>

  <section>
    <title>Event Handling Principles</title>
    <p>In OTP, an <em>event manager</em> is a named object to which
      events can be sent. An <em>event</em> can be, for example,
      an error, an alarm, or some information that is to be logged.</p>
    <p>In the event manager, zero, one, or many <em>event handlers</em>
      are installed. When the event manager is notified
      about an event, the event is processed by all the installed
      event handlers. For example, an event manager for handling errors
      can by default have a handler installed, which writes error
      messages to the terminal. If the error messages during a certain
      period are to be saved to a file as well, the user adds another
      event handler that does this. When logging to the file is no
      longer necessary, this event handler is deleted.</p>
    <p>An event manager is implemented as a process and each event
      handler is implemented as a callback module.</p>
    <p>The event manager essentially maintains a list of
      <c>{Module, State}</c> pairs, where each <c>Module</c> is an
      event handler, and <c>State</c> is the internal state of that
      event handler.</p>
  </section>

  <section>
    <title>Example</title>
    <p>The callback module for the event handler writing error messages
      to the terminal can look as follows:</p>
    <code type="none">
-module(terminal_logger).
-behaviour(gen_event).

-export([init/1, handle_event/2, terminate/2]).

init(_Args) ->
    {ok, []}.

handle_event(ErrorMsg, State) ->
    io:format("***Error*** ~p~n", [ErrorMsg]),
    {ok, State}.

terminate(_Args, _State) ->
    ok.</code>
    <p>The callback module for the event handler writing error messages
      to a file can look as follows:</p>
    <code type="none">
-module(file_logger).
-behaviour(gen_event).

-export([init/1, handle_event/2, terminate/2]).

init(File) ->
    {ok, Fd} = file:open(File, read),
    {ok, Fd}.

handle_event(ErrorMsg, Fd) ->
    io:format(Fd, "***Error*** ~p~n", [ErrorMsg]),
    {ok, Fd}.

terminate(_Args, Fd) ->
    file:close(Fd).</code>
    <p>The code is explained in the next sections.</p>
  </section>

  <section>
    <marker id="mgr"></marker>
    <title>Starting an Event Manager</title>
    <p>To start an event manager for handling errors, as described in
      the previous example, call the following function:</p>
    <code type="none">
gen_event:start_link({local, error_man})</code>
    <p>This function spawns and links to a new process, an event
      manager.</p>
    <p>The argument, <c>{local, error_man}</c> specifies the name. The
    event manager is then locally registered as <c>error_man</c>.</p>
    <p>If the name is omitted, the event manager is not registered.
      Instead its pid must be used. The name can also be given
      as <c>{global, Name}</c>, in which case the event manager is
      registered using <c>global:register_name/2</c>.</p>
    <p><c>gen_event:start_link</c> must be used if the event manager is
      part of a supervision tree, that is, started by a supervisor.
      There is another function, <c>gen_event:start</c>, to start a
      standalone event manager, that is, an event manager that is not
      part of a supervision tree.</p>
  </section>

  <section>
    <title>Adding an Event Handler</title>
    <p>The following example shows how to start an event manager and
    add an event handler to it by using the shell:</p>
    <pre>
1> <input>gen_event:start({local, error_man}).</input>
{ok,&lt;0.31.0>}
2> <input>gen_event:add_handler(error_man, terminal_logger, []).</input>
ok</pre>
    <p>This function sends a message to the event manager registered as
      <c>error_man</c>, telling it to add the event handler
      <c>terminal_logger</c>. The event manager calls the callback
      function <c>terminal_logger:init([])</c>, where the argument
      <c>[]</c> is the third argument to <c>add_handler</c>. <c>init</c>
      is expected to return <c>{ok, State}</c>, where <c>State</c> is
      the internal state of the event handler.</p>
    <code type="none">
init(_Args) ->
    {ok, []}.</code>
    <p>Here, <c>init</c> does not need any input data and ignores its
      argument. For <c>terminal_logger</c>, the internal state is
      not used. For <c>file_logger</c>, the internal state is used
      to save the open file descriptor.</p>
    <code type="none">
init(File) ->
    {ok, Fd} = file:open(File, read),
    {ok, Fd}.</code>
  </section>

  <section>
    <title>Notifying about Events</title>
    <pre>
3> <input>gen_event:notify(error_man, no_reply).</input>
***Error*** no_reply
ok</pre>
    <p><c>error_man</c> is the name of the event manager and
      <c>no_reply</c> is the event.</p>
    <p>The event is made into a message and sent to the event manager.
      When the event is received, the event manager calls
      <c>handle_event(Event, State)</c> for each installed event
      handler, in the same order as they were added. The function is
      expected to return a tuple <c>{ok,State1}</c>, where
      <c>State1</c> is a new value for the state of the event handler.</p>
    <p>In <c>terminal_logger</c>:</p>
    <code type="none">
handle_event(ErrorMsg, State) ->
    io:format("***Error*** ~p~n", [ErrorMsg]),
    {ok, State}.</code>
    <p>In <c>file_logger</c>:</p>
    <code type="none">
handle_event(ErrorMsg, Fd) ->
    io:format(Fd, "***Error*** ~p~n", [ErrorMsg]),
    {ok, Fd}.</code>
  </section>

  <section>
    <title>Deleting an Event Handler</title>
    <pre>
4> <input>gen_event:delete_handler(error_man, terminal_logger, []).</input>
ok</pre>
    <p>This function sends a message to the event manager registered as
      <c>error_man</c>, telling it to delete the event handler
      <c>terminal_logger</c>. The event manager calls the callback
      function <c>terminal_logger:terminate([], State)</c>, where
      the argument <c>[]</c> is the third argument to <c>delete_handler</c>.
      <c>terminate</c> is to be the opposite of <c>init</c> and do any
      necessary cleaning up. Its return value is ignored.</p>
    <p>For <c>terminal_logger</c>, no cleaning up is necessary:</p>
    <code type="none">
terminate(_Args, _State) ->
    ok.</code>
    <p>For <c>file_logger</c>, the file descriptor opened in <c>init</c>
    must be closed:</p>
    <code type="none">
terminate(_Args, Fd) ->
    file:close(Fd).</code>
  </section>

  <section>
    <title>Stopping</title>
    <p>When an event manager is stopped, it gives each of
      the installed event handlers the chance to clean up by calling
      <c>terminate/2</c>, the same way as when deleting a handler.</p>

    <section>
      <title>In a Supervision Tree</title>
      <p>If the event manager is part of a supervision tree, no stop
        function is needed. The event manager is automatically
        terminated by its supervisor. Exactly how this is done is
        defined by a
        <seeguide marker="sup_princ#shutdown">shutdown strategy</seeguide>
        set in the supervisor.</p>
    </section>

    <section>
      <title>Standalone Event Managers</title>
      <p>An event manager can also be stopped by calling:</p>
      <pre>
> <input>gen_event:stop(error_man).</input>
ok</pre>
    </section>
  </section>
  <section>
    <title>Handling Other Messages</title>
    <p>If the <c>gen_event</c> is to be able to receive other messages
      than events, the callback function
      <c>handle_info(Info, State)</c>
      must be implemented to handle them. Examples of other
      messages are exit messages, if the <c>gen_event</c> is linked to
      other processes (than the supervisor) and trapping exit signals.</p>
    <code type="none">
handle_info({'EXIT', Pid, Reason}, State) ->
    ..code to handle exits here..
    {ok, NewState}.</code>
    <p>The <c>code_change</c> method must also be implemented.</p>
    <code type="none">
code_change(OldVsn, State, Extra) ->
    ..code to convert state (and more) during code change
    {ok, NewState}</code>
  </section>
</chapter>
