Generic event handling behavior.
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.
Types
handler() = atom() | {atom(), term()}
handler_args() = term()
add_handler_ret() = ok | term() | {'EXIT', term()}
del_handler_ret() = ok | term() | {'EXIT', term()}
Functions
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.
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 callingModule: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 todelete_handler/3
, orremove_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 processPid
has replaced the event handler with another event handlerNewHandler
using a call toswap_handler/3
orswap_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
.
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.
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}
.
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
.
EventMgrName = {local,Name} | {global,GlobalName} | {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Options = [Option]
Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}
Dbgs = [Dbg]
Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}
SOpts = [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
.
EventMgrName = {local,Name} | {global,GlobalName} | {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Options = [Option]
Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}
Dbgs = [Dbg]
Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}
SOpts = [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 asName
usingregister/2
. -
If
EventMgrName={global,GlobalName}
, the event manager is registered globally asGlobalName
usingglobal: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 byModule
. TheModule
callback is to export the functionsregister_name/2
,unregister_name/1
,whereis_name/1
, andsend/2
, which are to behave as the corresponding functions inglobal
. Thus,{via,global,GlobalName}
is a valid reference. -
If option
{hibernate_after,HibernateAfterTimeout}
is present, thegen_event
process awaits any message forHibernateAfterTimeout
milliseconds and if no message is received, the process goes into hibernation automatically (by callingproc_lib:hibernate/3
).
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.
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
logger(3)
.
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
.
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.
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
.
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.
Functions
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.
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 thegen_event
status.Opt
is set to the atomnormal
for this case. - The event handler terminates abnormally and
gen_event
logs an error.Opt
is set to the atomterminate
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 containsStatus
in place of the state term of the event handler.When an event handler terminates abnormally,
gen_event
logsStatus
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.
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
.
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 stateNewState
. -
If
{ok,NewState,hibernate}
is returned, the event manager also goes into hibernation (by callingproc_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 byHandler2
by first callingModule:terminate(Args1,NewState)
and thenModule2:init({Args2,Term})
, whereTerm
is the return value ofModule:terminate/2
. For more information, seeswap_handler/3
. -
If
remove_handler
is returned, the event handler is deleted by callingModule:terminate(remove_handler,State)
.
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
.
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.
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.