gen_event(3)

NAME
gen_event - Generic event handling behavior.

DESCRIPTION

This behavior module provides event handling functionality. It consists of a generic event manager process with any number of event handlers that are added and deleted dynamically.

An event manager implemented using this module has a standard set of interface functions and includes functionality for tracing and error reporting. It also fits into an OTP supervision tree. For more information, see OTP Design Principles.

Each event handler is implemented as a callback module exporting a predefined set of functions. The relationship between the behavior functions and the callback functions is as follows:

gen_event module                   Callback module
----------------                   ---------------
gen_event:start
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

As each event handler is one callback module, an event manager has many callback modules that are added and deleted dynamically. gen_event is therefore more tolerant of callback module errors than the other behaviors. If a callback function for an installed event handler fails with Reason, or returns a bad value Term, the event manager does not fail. It deletes the event handler by calling callback function Module:terminate/2, giving as argument {error,{'EXIT',Reason}} or {error,Term}, respectively. No other event handler is affected.

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

Notice that an event manager does trap exit signals automatically.

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

Notice that when multiple event handlers are invoked, it is 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 specified.

DATA TYPES
handler() = atom() | {atom(), term()}
handler_args() = term()
add_handler_ret() = ok | term() | {'EXIT', term()}
del_handler_ret() = ok | term() | {'EXIT', term()}

EXPORTS

add_handler(EventMgrRef, Handler, Args) -> Result

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        Handler = Module | {Module,Id}
         Module = atom()
         Id = term()
        Args = term()
        Result = ok | {'EXIT',Reason} | term()
         Reason = term()

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

    EventMgrRef can be any of the following:

        *   The pid

        *   Name, if the event manager is locally registered

        *   {Name,Node}, if the event manager is locally registered at another node

        *   {global,GlobalName}, if the event manager is globally registered

        *   {via,Module,ViaName}, if the event manager is registered through an alternative process registry

    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 many event handlers use the same callback module.

    Args is any term that is passed as the argument to Module:init/1.

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

add_sup_handler(EventMgrRef, Handler, Args) -> Result

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        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 also supervises the connection between the event handler and the calling process.

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

        *   If the event handler is deleted later, 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 because of 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 because of an error. Which term depends on the error.

    For a description of the arguments and return values, see add_handler/3.

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

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        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 event handler Handler installed in event manager EventMgrRef by sending a request and waiting until a reply arrives or a time-out occurs. The event manager calls Module:handle_call/2 to handle the request.

    For a description of EventMgrRef and Handler, see add_handler/3.

    Request is any term that is passed as one of the arguments to Module:handle_call/2.

    Timeout is an integer greater than zero that specifies how many milliseconds to wait for a reply, or the atom infinity to wait indefinitely. Defaults to 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,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        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 event manager EventMgrRef. The event manager calls Module:terminate/2 to terminate the event handler.

    For a description of EventMgrRef and Handler, see add_handler/3.

    Args is any term that 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}.

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

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        Event = term()

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

    notify/2 is asynchronous and returns immediately after the event notification has been sent. sync_notify/2 is synchronous in the sense that it returns ok after the event has been handled by all event handlers.

    For a description of EventMgrRef, see add_handler/3.

    Event is any term that is passed as one of the arguments to Module:handle_event/2.

    notify/1 does not fail even if the specified event manager does not exist, unless it is specified as Name.

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

    Types:
        EventMgrName = {local,Name} | {global,GlobalName} | {via,Module,ViaName}
         Name = atom()
         GlobalName = ViaName = term()
        Result = {ok,Pid} | {error,{already_started,Pid}}
         Pid = pid()

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

    For a description of the arguments and return values, see start_link/0,1.

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

    Types:
        EventMgrName = {local,Name} | {global,GlobalName} | {via,Module,ViaName}
         Name = atom()
         GlobalName = ViaName = term()
        Result = {ok,Pid} | {error,{already_started,Pid}}
         Pid = pid()

    Creates an event manager process as part of a supervision tree. The function is to be called, directly or indirectly, by the supervisor. For example, it ensures 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,GlobalName}, the event manager is registered globally as GlobalName using global:register_name/2. If no name is provided, the event manager is not registered.

        *   If EventMgrName={via,Module,ViaName}, the event manager registers with the registry represented by Module. The Module callback is to export the functions register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to behave as the corresponding functions in global. Thus, {via,global,GlobalName} is a valid reference.

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

stop(EventMgrRef) -> ok
stop(EventMgrRef, Reason, Timeout) -> ok

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
        Name = Node = atom()
        GlobalName = ViaName = term()
        Reason = term()
        Timeout = int()>0 | infinity

    Orders event manager EventMgrRef to exit with the specifies Reason and waits for it to terminate. Before terminating, gen_event calls Module:terminate(stop,...) for each installed event handler.

    The function returns ok if the event manager terminates with the expected reason. Any other reason than normal, shutdown, or {shutdown,Term} causes an error report to be issued using error_logger:format/2. The default Reason is normal.

    Timeout is an integer greater than zero that specifies how many milliseconds to wait for the event manager to terminate, or the atom infinity to wait indefinitely. Defaults to infinity. If the event manager has not terminated within the specified time, a timeout exception is raised.

    If the process does not exist, a noproc exception is raised.

    For a description of EventMgrRef, see add_handler/3.

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

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        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 event manager EventMgrRef.

    For a description of the arguments, see add_handler/3.

    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 is the return value of Module1:terminate/2. This makes it possible to transfer information from Handler1 to Handler2.

    The new handler is 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 is deleted even if Module2:init/1 fails.

    If there was a supervised connection between Handler1 and a process Pid, there is 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 function returns {error,{'EXIT',Reason}} or {error,Term}, respectively.

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

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        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 event manager EventMgrRef in the same way as swap_handler/3, but also supervises the connection between Handler2 and the calling process.

    For a description of the arguments and return values, see swap_handler/3.

which_handlers(EventMgrRef) -> [Handler]

    Types:
        EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
         Name = Node = atom()
         GlobalName = ViaName = term()
        Handler = Module | {Module,Id}
         Module = atom()
         Id = term()

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

    For a description of EventMgrRef and Handler, see add_handler/3.

CALLBACK FUNCTIONS

The following functions are to be exported from a gen_event callback module.

EXPORTS

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 that is to update its internal state during a release upgrade/downgrade, that is, when the instruction {update,Module,Change,...}, where Change={advanced,Extra}, is specified in the .appup file. For more information, see OTP Design Principles.

    For an upgrade, OldVsn is Vsn, and for 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 is to 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 in the following situations:

        *   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 changing the form and appearance of the event handler state for these cases. An event handler callback module wishing to change the 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 process dictionary of gen_event.

    State is the internal state of the event handler.

    The function is to return Status, a term that change 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 state term of the event handler.

        *   When an event handler terminates abnormally, gen_event logs Status in place of the state term of the event handler.

    One use for this function is to return compact alternative state representations to avoid that large state terms are printed in log files.

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 call/3,4, this function is called for the specified event handler to handle the request.

    Request is the Request argument of call/3,4.

    State is the internal state of the event handler.

    The return values are the same as for Module:handle_event/2 except that they also contain a term Reply, which is the reply to the client as the return value of call/3,4.

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 notify/2 or sync_notify/2, this function is called for each installed event handler to handle the event.

    Event is the Event argument of notify/2/sync_notify/2.

    State is the internal state of the event handler.

        *   If {ok,NewState} or {ok,NewState,hibernate} is returned, the event handler remains in the event manager with the possible updated internal state NewState.

        *   If {ok,NewState,hibernate} is returned, the event manager also goes 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 {swap_handler,Args1,NewState,Handler2,Args2} is returned, the event handler is 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. For more information, see swap_handler/3.

        *   If remove_handler is returned, the event handler is deleted by calling Module:terminate(remove_handler,State).

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.

    For a description of State and possible return values, see Module:handle_event/2.

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

    Types:
        InitArgs = Args | {Args,Term}
         Args = Term = term()
        State = term()
        Reason = 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 because of a call to add_handler/3 or add_sup_handler/3, InitArgs is the Args argument of these functions.

    If the event handler replaces another event handler because of a call to swap_handler/3 or swap_sup_handler/3, or because of 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 swap_handler/3.

    If successful, the function returns {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 goes into hibernation (by calling proc_lib:hibernate/3), waiting for the next event to occur.

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 is to be the opposite of Module:init/1 and do any necessary cleaning up.

    If the event handler is deleted because of a call to delete_handler/3, swap_handler/3, or 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 that has terminated with reason Reason.

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

    The event manager terminates 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 terminates 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 can return any term. If the event handler is deleted because of a call to gen_event:delete_handler/3, the return value of that function becomes the return value of this function. If the event handler is to be replaced with another event handler because of a swap, the return value is passed to the init function of the new event handler. Otherwise the return value is ignored.

SEE ALSO

supervisor(3), sys(3)