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# gen_event Behaviour

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It is recommended to read this section alongside `gen_event` in STDLIB.

## Event Handling Principles

In OTP, an event manager is a named object to which events can be sent. An event can be, for example, an error, an alarm, or some information that is to be logged.

In the event manager, zero, one, or many event handlers 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 that 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.

An event manager is implemented as a process and each event handler is implemented as a callback module.

The event manager essentially maintains a list of `{Module, State}` pairs, where each `Module` is an event handler, and `State` is the internal state of that event handler.

## Example

The callback module for the event handler writing error messages to the terminal can look as follows:

    -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.

The callback module for the event handler writing error messages to a file can look as follows:

    -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).

The code is explained in the next sections.

## Starting an Event Manager

To start an event manager for handling errors, as described in the previous example, call the following function:

    gen_event:start_link({local, error_man})

`gen_event:start_link/1` spawns and links to a new event manager process.

The argument, `{local, error_man}`, specifies the name under which the event manager should be locally registered. The name can also be given as `{global, Name}` to register the event manager globally using `global:register_name/2`.

If the name is omitted, the event manager is not registered. Instead its pid must be used.

`gen_event:start_link/1` must be used if the event manager is part of a supervision tree, meaning that it was started by a supervisor. There is another function, `gen_event:start/1`, to start a standalone event manager that is not part of a supervision tree.

## Adding an Event Handler

The following example shows how to start an event manager and add an event handler to it by using the shell:

    1> gen_event:start({local, error_man}).
    {ok,<0.31.0>}
    2> gen_event:add_handler(error_man, terminal_logger, []).
    ok

This function sends a message to the event manager registered as `error_man`, telling it to add the event handler `terminal_logger`. The event manager calls the callback function `terminal_logger:init([])`, where the argument `[]` is the third argument to `add_handler`. `init/1` is expected to return `{ok, State}`, where `State` is the internal state of the event handler.

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

Here, `init/1` does not need any input data and ignores its argument. For `terminal_logger`, the internal state is not used. For `file_logger`, the internal state is used to save the open file descriptor.

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

## Notifying about Events

    3> gen_event:notify(error_man, no_reply).
    ***Error*** no_reply
    ok

`error_man` is the name of the event manager and `no_reply` is the event.

The event is made into a message and sent to the event manager. When the event is received, the event manager calls `handle_event(Event, State)` for each installed event handler, in the same order as they were added. The function is expected to return a tuple `{ok,State1}`, where `State1` is a new value for the state of the event handler.

In `terminal_logger`:

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

In `file_logger`:

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

## Deleting an Event Handler

    4> gen_event:delete_handler(error_man, terminal_logger, []).
    ok

This function sends a message to the event manager registered as `error_man`, telling it to delete the event handler `terminal_logger`. The event manager calls the callback function `terminal_logger:terminate([], State)`, where the argument `[]` is the third argument to `delete_handler`. `terminate/2` is to be the opposite of `init/1` and do any necessary cleaning up. Its return value is ignored.

For `terminal_logger`, no cleaning up is necessary:

    terminate(_Args, _State) ->
        ok.

For `file_logger`, the file descriptor opened in `init` must be closed:

    terminate(_Args, Fd) ->
        file:close(Fd).

## Stopping

When an event manager is stopped, it gives each of the installed event handlers the chance to clean up by calling `terminate/2`, the same way as when deleting a handler.

### In a Supervision Tree

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 shutdown strategy set in the supervisor.

### Standalone Event Managers

An event manager can also be stopped by calling:

    1> gen_event:stop(error_man).
    ok

## Handling Other Messages

If the `gen_event` process is to be able to receive other messages than events, the callback function `handle_info(Info, State)` must be implemented to handle them. Examples of other messages are exit messages if the event manager is linked to other processes than the supervisor (for example via `gen_event:add_sup_handler/3`) and is trapping exit signals.

    handle_info({'EXIT', Pid, Reason}, State) ->
        %% Code to handle exits here.
        ...
        {noreply, State1}.

The final function to implement is `code_change/3`:

    code_change(OldVsn, State, Extra) ->
        %% Code to convert state (and more) during code change.
        ...
        {ok, NewState}.

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