Module:code_change(OldVsn,State,Extra)->{ok,NewState}
Types:
OldVsn = Vsn | {down, Vsn}
Vsn = term()
State = NewState = term()
Extra = term()
Note:
This callback is optional, so callback modules need not export it. If a release upgrade/downgrade
with Change={advanced,Extra} specified in the .appup file is made when code_change/3 isn't
implemented the event handler will crash with an undef error reason.
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()
Note:
This callback is optional, so callback modules need not export it. The gen_event module provides a
default implementation of this function that logs about the unexpected Info message, drops it and
returns {ok,State}.
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()
Note:
This callback is optional, so callback modules need not export it. The gen_event module provides a
default implementation without cleanup.
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.
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