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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_suphandler -----> 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.
The sys module can be used for debugging an event manager.
Note that an event manager does trap exit signals automatically.
Unless otherwise stated, all functions in this module fail if the specified event manager does not exist or if bad arguments are given.
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:
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.
Reason,
the event manager will delete the event handler by calling
Module:terminate/2 with {stop,Reason} as argument.
{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.
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.
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.
The following functions should be exported from a gen_event
callback module.
Module:init(InitArgs) -> {ok,State}
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} where State
is the initial internal state of the event handler.
Module:handle_event(Event, State) -> Result
Types:
Event = term()
State = term()
Result = {ok,NewState}
| {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} the event handler
will remain in the event manager with the possible updated
internal state NewState.
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}
| {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}
| {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.
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.