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gen_event(3erl) | Erlang Module Definition | gen_event(3erl) |
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/3As 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(3erl). 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()
Name = Node = atom()
GlobalName = ViaName = term()
Module = atom()
Id = 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:
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
- *
- 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
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()
Name = Node = atom()
GlobalName = ViaName = term()
Module = atom()
Id = 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.
For a description of the arguments and return values, see
add_handler/3.
call(EventMgrRef, Handler, Request) -> Result
- *
- 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.
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()
Name = Node = atom()
GlobalName = ViaName = term()
Module = atom()
Id = term()
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()
Name = Node = atom()
GlobalName = ViaName = term()
Module = atom()
Id = term()
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
Types:
EventMgrRef = Name | {Name,Node} | {global,GlobalName} |
{via,Module,ViaName} | pid()
Name = Node = atom()
GlobalName = ViaName = term()
Event = term()
Name = Node = atom()
GlobalName = ViaName = 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
Types:
EventMgrName = {local,Name} | {global,GlobalName} |
{via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Result = {ok,Pid} | {error,{already_started,Pid}}
Pid = pid()
Name = atom()
GlobalName = ViaName = term()
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
Types:
EventMgrName = {local,Name} | {global,GlobalName} |
{via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Result = {ok,Pid} | {error,{already_started,Pid}}
Pid = pid()
Name = atom()
GlobalName = ViaName = term()
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 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
- *
- 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.
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()
Name = Node = atom()
GlobalName = ViaName = term()
Module = atom()
Id = term()
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()
Name = Node = atom()
GlobalName = ViaName = term()
Module = atom()
Id = term()
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()
Name = Node = atom()
GlobalName = ViaName = term()
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()
Vsn = 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.
- *
- 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.
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()
| {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()
| {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.
Module:handle_info(Info, State) -> Result
- *
- 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).
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()
| {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()
Args = Term = 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()
| {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(3erl), sys(3erl)stdlib 3.2 | Ericsson AB |