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# gen_event

### MODULE

gen_event

### MODULE SUMMARY

Generic Event Handling Behaviour

### DESCRIPTION

A behaviour module for implementing event handling functionality. The OTP event handling model consists of a generic event manager process with an arbitrary number of event handlers which are added and deleted dynamically.

An event manager implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree. Refer to OTP Design Principles for more information.

Each event handler is implemented as a callback module exporting a pre-defined set of functions. The relationship between the behaviour functions and the callback functions can be illustrated as follows:

    gen_event module                   Callback module
    ----------------                   ---------------
    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

Since each event handler is one callback module, an event manager will have several callback modules which are added and deleted dynamically. Therefore gen_event is more tolerant of callback module errors than the other behaviours. If a callback function for an installed event handler fails with Reason, or returns a bad value Term, the event manager will not fail. It will delete the event handler by calling the callback function Module:terminate/2 (see below), giving as argument {error,{'EXIT',Reason}} or {error,Term}, respectively. No other event handler will be affected.

A gen_event process handles system messages as documented in sys(3). The sys module can be used for debugging an event manager.

Note that an event manager does trap exit signals automatically.

The gen_event process can go into hibernation (see erlang(3)) if a callback function in a handler module specifies 'hibernate' in its return value. This might be useful if the server is expected to be idle for a long time. However this feature should be used with care as hibernation implies at least two garbage collections (when hibernating and shortly after waking up) and is not something you'd want to do between each event handled by a busy event manager.

It's also worth noting that when multiple event handlers are invoked, it's sufficient that one single event handler returns a 'hibernate' request for the whole event manager to go into hibernation.

Unless otherwise stated, all functions in this module fail if the specified event manager does not exist or if bad arguments are given.

### EXPORTS

start_link() -> Result
start_link(EventMgrName) -> Result

Types:

EventMgrName = {local,Name} | {global,Name}
Name = atom()
Result = {ok,Pid} | {error,{already_started,Pid}}
Pid = pid()

Creates an event manager process as part of a supervision tree. The function should be called, directly or indirectly, by the supervisor. It will, among other things, ensure that the event manager is linked to the supervisor.

If EventMgrName={local,Name}, the event manager is registered locally as Name using register/2. If EventMgrName={global,Name}, the event manager is registered globally as Name using global:register_name/2. If no name is provided, the event manager is not registered.

If the event manager is successfully created the function returns {ok,Pid}, where Pid is the pid of the event manager. If there already exists a process with the specified EventMgrName the function returns {error,{already_started,Pid}}, where Pid is the pid of that process.

start() -> Result
start(EventMgrName) -> Result

Types:

EventMgrName = {local,Name} | {global,Name}
Name = atom()
Result = {ok,Pid} | {error,{already_started,Pid}}
Pid = pid()

Creates a stand-alone event manager process, i.e. an event manager which is not part of a supervision tree and thus has no supervisor.

See start_link/0,1 for a description of arguments and return values.

add_handler(EventMgrRef, Handler, Args) -> Result

Types:

EventMgr = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler = Module | {Module,Id}
Module = atom()
Id = term()
Args = term()
Result = ok | {'EXIT',Reason} | term()
Reason = term()

Adds a new event handler to the event manager EventMgrRef. The event manager will call Module:init/1 to initiate the event handler and its internal state.

EventMgrRef can be:

- the pid,
- Name, if the event manager is locally registered,
- {Name,Node}, if the event manager is locally registered at another node, or
- {global,Name}, if the event manager is globally registered.

Handler is the name of the callback module Module or a tuple {Module,Id}, where Id is any term. The {Module,Id} representation makes it possible to identify a specific event handler when there are several event handlers using the same callback module.

Args is an arbitrary term which is passed as the argument to Module:init/1.

If Module:init/1 returns a correct value, the event manager adds the event handler and this function returns ok. If Module:init/1 fails with Reason or returns an unexpected value Term, the event handler is ignored and this function returns {'EXIT',Reason} or Term, respectively.

add_sup_handler(EventMgrRef, Handler, Args) -> Result

Types:

EventMgr = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler = Module | {Module,Id}
Module = atom()
Id = term()
Args = term()
Result = ok | {'EXIT',Reason} | term()
Reason = term()

Adds a new event handler in the same way as add_handler/3 but will also supervise the connection between the event handler and the calling process.

- If the calling process later terminates with Reason, the event manager will delete the event handler by calling Module:terminate/2 with {stop,Reason} as argument.

- If the event handler later is deleted, the event manager sends a message{gen_event_EXIT,Handler,Reason} to the calling process. Reason is one of the following:

  - normal, if the event handler has been removed due to a call to delete_handler/3, or remove_handler has been returned by a callback function (see below).
  - shutdown, if the event handler has been removed because the event manager is terminating.
  - {swapped,NewHandler,Pid}, if the process Pid has replaced the event handler with another event handler NewHandler using a call to swap_handler/3 or swap_sup_handler/3.
  - a term, if the event handler is removed due to an error. Which term depends on the error.

See add_handler/3 for a description of the arguments and return values.

notify(EventMgrRef, Event) -> ok
sync_notify(EventMgrRef, Event) -> ok

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Event = term()

Sends an event notification to the event manager EventMgrRef. The event manager will call Module:handle_event/2 for each installed event handler to handle the event.

notify is asynchronous and will return immediately after the event notification has been sent. sync_notify is synchronous in the sense that it will return ok after the event has been handled by all event handlers.

See add_handler/3 for a description of EventMgrRef.

Event is an arbitrary term which is passed as one of the arguments to Module:handle_event/2.

notify will not fail even if the specified event manager does not exist, unless it is specified as Name.

call(EventMgrRef, Handler, Request) -> Result
call(EventMgrRef, Handler, Request, Timeout) -> Result

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler = Module | {Module,Id}
Module = atom()
Id = term()
Request = term()
Timeout = int()>0 | infinity
Result = Reply | {error,Error}
Reply = term()
Error = bad_module | {'EXIT',Reason} | term()
Reason = term()

Makes a synchronous call to the event handler Handler installed in the event manager EventMgrRef by sending a request and waiting until a reply arrives or a timeout occurs. The event manager will call Module:handle_call/2 to handle the request.

See add_handler/3 for a description of EventMgrRef and Handler.

Request is an arbitrary term which is passed as one of the arguments to Module:handle_call/2.

Timeout is an integer greater than zero which specifies how many milliseconds to wait for a reply, or the atom infinity to wait indefinitely. Default value is 5000. If no reply is received within the specified time, the function call fails.

The return value Reply is defined in the return value of Module:handle_call/2. If the specified event handler is not installed, the function returns {error,bad_module}. If the callback function fails with Reason or returns an unexpected value Term, this function returns {error,{'EXIT',Reason}} or {error,Term}, respectively.

delete_handler(EventMgrRef, Handler, Args) -> Result

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler = Module | {Module,Id}
Module = atom()
Id = term()
Args = term()
Result = term() | {error,module_not_found} | {'EXIT',Reason}
Reason = term()

Deletes an event handler from the event manager EventMgrRef. The event manager will call Module:terminate/2 to terminate the event handler.

See add_handler/3 for a description of EventMgrRef and Handler.

Args is an arbitrary term which is passed as one of the arguments to Module:terminate/2.

The return value is the return value of Module:terminate/2. If the specified event handler is not installed, the function returns {error,module_not_found}. If the callback function fails with Reason, the function returns {'EXIT',Reason}.

swap_handler(EventMgrRef, {Handler1,Args1}, {Handler2,Args2}) -> Result

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler1 = Handler2 = Module | {Module,Id}
Module = atom()
Id = term()
Args1 = Args2 = term()
Result = ok | {error,Error}
Error = {'EXIT',Reason} | term()
Reason = term()

Replaces an old event handler with a new event handler in the event manager EventMgrRef.

See add_handler/3 for a description of the arguments.

First the old event handler Handler1 is deleted. The event manager calls Module1:terminate(Args1, ...), where Module1 is the callback module of Handler1, and collects the return value.

Then the new event handler Handler2 is added and initiated by calling Module2:init({Args2,Term}), where Module2 is the callback module of Handler2 and Term the return value of Module1:terminate/2. This makes it possible to transfer information from Handler1 to Handler2.

The new handler will be added even if the the specified old event handler is not installed in which case Term=error, or if Module1:terminate/2 fails with Reason in which case Term={'EXIT',Reason}. The old handler will be deleted even if Module2:init/1 fails.

If there was a supervised connection between Handler1 and a process Pid, there will be a supervised connection between Handler2 and Pid instead.

If Module2:init/1 returns a correct value, this function returns ok. If Module2:init/1 fails with Reason or returns an unexpected value Term, this this function returns {error,{'EXIT',Reason}} or {error,Term}, respectively.

swap_sup_handler(EventMgrRef, {Handler1,Args1}, {Handler2,Args2}) -> Result

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler1 = Handler 2 = Module | {Module,Id}
Module = atom()
Id = term()
Args1 = Args2 = term()
Result = ok | {error,Error}
Error = {'EXIT',Reason} | term()
Reason = term()

Replaces an event handler in the event manager EventMgrRef in the same way as swap_handler/3 but will also supervise the connection between Handler2 and the calling process.

See swap_handler/3 for a description of the arguments and return values.

which_handlers(EventMgrRef) -> [Handler]

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()
Handler = Module | {Module,Id}
Module = atom()
Id = term()

Returns a list of all event handlers installed in the event manager EventMgrRef.

See add_handler/3 for a description of EventMgrRef and Handler.

stop(EventMgrRef) -> ok

Types:

EventMgrRef = Name | {Name,Node} | {global,Name} | pid()
Name = Node = atom()

Terminates the event manager EventMgrRef. Before terminating, the event manager will call Module:terminate(stop,...) for each installed event handler.

See add_handler/3 for a description of the argument.

### CALLBACK FUNCTIONS

The following functions should be exported from a gen_event callback module.

### EXPORTS

Module:init(InitArgs) -> {ok,State} | {ok,State,hibernate}

Types:

InitArgs = Args | {Args,Term}
Args = Term = term()
State = term()

Whenever a new event handler is added to an event manager, this function is called to initialize the event handler.

If the event handler is added due to a call to gen_event:add_handler/3 or gen_event:add_sup_handler/3, InitArgs is the Args argument of these functions.

If the event handler is replacing another event handler due to a call to gen_event:swap_handler/3 or gen_event:swap_sup_handler/3, or due to a swap return tuple from one of the other callback functions, InitArgs is a tuple {Args,Term} where Args is the argument provided in the function call/return tuple and Term is the result of terminating the old event handler, see gen_event:swap_handler/3.

The function should return {ok,State} or {ok,State, hibernate} where State is the initial internal state of the event handler.

If {ok,State,hibernate} is returned, the event manager will go into hibernation (by calling proc_lib:hibernate/3), waiting for the next event to occur.

Module:handle_event(Event, State) -> Result

Types:

Event = term()
State = term()
Result = {ok,NewState} | {ok,NewState,hibernate}
| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler
NewState = term()
Args1 = Args2 = term()
Handler2 = Module2 | {Module2,Id}
Module2 = atom()
Id = term()

Whenever an event manager receives an event sent using gen_event:notify/2 or gen_event:sync_notify/2, this function is called for each installed event handler to handle the event.

Event is the Event argument of notify/sync_notify.

State is the internal state of the event handler.

If the function returns {ok,NewState} or {ok,NewState,hibernate} the event handler will remain in the event manager with the possible updated internal state NewState.

If {ok,NewState,hibernate} is returned, the event manager will also go into hibernation (by calling proc_lib:hibernate/3), waiting for the next event to occur. It is sufficient that one of the event handlers return {ok,NewState,hibernate} for the whole event manager process to hibernate.

If the function returns {swap_handler,Args1,NewState,Handler2,Args2} the event handler will be replaced by Handler2 by first calling Module:terminate(Args1,NewState) and then Module2:init({Args2,Term}) where Term is the return value of Module:terminate/2. See gen_event:swap_handler/3 for more information.

If the function returns remove_handler the event handler will be deleted by calling Module:terminate(remove_handler,State).

Module:handle_call(Request, State) -> Result

Types:

Request = term()
State = term()
Result = {ok,Reply,NewState} | {ok,Reply,NewState,hibernate}
| {swap_handler,Reply,Args1,NewState,Handler2,Args2}
| {remove_handler, Reply}
Reply = term()
NewState = term()
Args1 = Args2 = term()
Handler2 = Module2 | {Module2,Id}
Module2 = atom()
Id = term()

Whenever an event manager receives a request sent using gen_event:call/3,4, this function is called for the specified event handler to handle the request.

Request is the Request argument of call.

State is the internal state of the event handler.

The return values are the same as for handle_event/2 except they also contain a term Reply which is the reply given back to the client as the return value of call.

Module:handle_info(Info, State) -> Result

Types:

Info = term()
State = term()
Result = {ok,NewState} | {ok,NewState,hibernate}
| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler
NewState = term()
Args1 = Args2 = term()
Handler2 = Module2 | {Module2,Id}
Module2 = atom()
Id = term()

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).

Info is the received message.

See Module:handle_event/2 for a description of State and possible return values.

Module:terminate(Arg, State) -> term()

Types:

Arg = Args | {stop,Reason} | stop | remove_handler
| {error,{'EXIT',Reason}} | {error,Term}
Args = Reason = Term = term()

Whenever an event handler is deleted from an event manager, this function is called. It should be the opposite of Module:init/1 and do any necessary cleaning up.

If the event handler is deleted due to a call to gen_event:delete_handler, gen_event:swap_handler/3 or gen_event:swap_sup_handler/3, Arg is the Args argument of this function call.

Arg={stop,Reason} if the event handler has a supervised connection to a process which has terminated with reason Reason.

Arg=stop if the event handler is deleted because the event manager is terminating.

The event manager will terminate if it is part of a supervision tree and it is ordered by its supervisor to terminate. Even if it is not part of a supervision tree, it will terminate if it receives an 'EXIT' message from its parent.

Arg=remove_handler if the event handler is deleted because another callback function has returned remove_handler or {remove_handler,Reply}.

Arg={error,Term} if the event handler is deleted because a callback function returned an unexpected value Term, or Arg={error,{'EXIT',Reason}} if a callback function failed.

State is the internal state of the event handler.

The function may return any term. If the event handler is deleted due to a call to gen_event:delete_handler, the return value of that function will be the return value of this function. If the event handler is to be replaced with another event handler due to a swap, the return value will be passed to the init function of the new event handler. Otherwise the return value is ignored.

Module:code_change(OldVsn, State, Extra) -> {ok, NewState}

Types:

OldVsn = Vsn | {down, Vsn}
Vsn = term()
State = NewState = term()
Extra = term()

This function is called for an installed event handler which should update its internal state during a release upgrade/downgrade, i.e. when the instruction {update,Module,Change,...} where Change={advanced,Extra} is given in the .appup file. See OTP Design Principles for more information.

In the case of an upgrade, OldVsn is Vsn, and in the case of a downgrade, OldVsn is {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old version of the callback module Module. If no such attribute is defined, the version is the checksum of the BEAM file.

State is the internal state of the event handler.

Extra is passed as-is from the {advanced,Extra} part of the update instruction.

The function should return the updated internal state.

Module:format_status(Opt, [PDict, State]) -> Status

Types:

Opt = normal | terminate
PDict = [{Key, Value}]
State = term()
Status = term()

Note

This callback is optional, so event handler modules need not export it. If a handler does not export this function, the gen_event module uses the handler state directly for the purposes described below.

This function is called by a gen_event process when:

- One of sys:get_status/1,2 is invoked to get the gen_event status. Opt is set to the atom normal for this case.
- The event handler terminates abnormally and gen_event logs an error. Opt is set to the atom terminate for this case.

This function is useful for customising the form and appearance of the event handler state for these cases. An event handler callback module wishing to customise the sys:get_status/1,2 return value as well as how its state appears in termination error logs exports an instance of format_status/2 that returns a term describing the current state of the event handler.

PDict is the current value of the gen_event's process dictionary.

State is the internal state of the event handler.

The function should return Status, a term that customises the details of the current state of the event handler. Any term is allowed for Status. The gen_event module uses Status as follows:

- When sys:get_status/1,2 is called, gen_event ensures that its return value contains Status in place of the event handler's actual state term.
- When an event handler terminates abnormally, gen_event logs Status in place of the event handler's actual state term.

One use for this function is to return compact alternative state representations to avoid having large state terms printed in logfiles.

### SEE ALSO

supervisor(3), sys(3)