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<chapter>
<header>
<copyright>
<year>1997</year><year>2013</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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Software distributed under the License is distributed on an "AS IS"
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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 is 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,<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
<seealso marker="sup_princ#shutdown">shutdown strategy</seealso>
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, StateName, StateData)</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>
