<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> <year>1997</year><year>2013</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. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software 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 limitations under the License. </legalnotice> <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>