From 68d53c01b0b8e9a007a6a30158c19e34b2d2a34e Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= Date: Wed, 18 May 2016 15:53:35 +0200 Subject: Update STDLIB documentation MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Language cleaned up by the technical writers xsipewe and tmanevik from Combitech. Proofreading and corrections by Björn Gustavsson and Hans Bolinder. --- lib/stdlib/doc/src/gen_event.xml | 887 +++++++++++++++++++++------------------ 1 file changed, 488 insertions(+), 399 deletions(-) (limited to 'lib/stdlib/doc/src/gen_event.xml') diff --git a/lib/stdlib/doc/src/gen_event.xml b/lib/stdlib/doc/src/gen_event.xml index b2c482d3ed..c24542002a 100644 --- a/lib/stdlib/doc/src/gen_event.xml +++ b/lib/stdlib/doc/src/gen_event.xml @@ -29,19 +29,23 @@ gen_event - Generic Event Handling Behaviour + Generic event handling behavior. -

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.

+

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 pre-defined set of functions. The relationship between the behaviour - functions and the callback functions can be illustrated as follows:

+ a predefined set of functions. The relationship between the behavior + functions and the callback functions is as follows:

+ 
 gen_event module                   Callback module
 ----------------                   ---------------
@@ -69,39 +73,46 @@ 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 + +

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 will not fail. It will delete - the event handler by calling the callback function - Module:terminate/2 (see below), giving as argument + 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 will be affected.

-

A gen_event process handles system messages as documented in - sys(3). The sys module + 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.

-

Note that an event manager does trap exit signals + +

Notice that an event manager does trap exit signals automatically.

-

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

- -

It's also worth noting that when multiple event handlers are - invoked, it's sufficient that one single event handler returns a - 'hibernate' request for the whole event manager to go into - hibernation.

+ +

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 - given.

+ specified.

 + @@ -116,65 +127,8 @@ gen_event:stop -----> Module:terminate/2 + - - start_link() -> Result - start_link(EventMgrName) -> Result - Create a generic event manager process in a supervision tree. - - 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 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,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 will - register with the registry represented by Module. - The Module callback should export the functions - register_name/2, unregister_name/1, - whereis_name/1 and send/2, which should behave like 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 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 - Create a stand-alone event manager process. - - 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, 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 Add an event handler to a generic event manager. @@ -191,26 +145,27 @@ gen_event:stop -----> Module:terminate/2  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:

+

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, + The pid + Name, if the event manager is locally registered {Name,Node}, if the event manager is locally - registered at another node, or + 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. + 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 there are several event - handlers using the same callback module.

-

Args is an arbitrary term which is passed as the argument + 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 @@ -221,9 +176,11 @@ gen_event:stop -----> Module:terminate/2 {error,Reason}, respectively.

 + add_sup_handler(EventMgrRef, Handler, Args) -> Result - Add a supervised event handler to a generic event manager. + Add a supervised event handler to a generic event manager. + EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid() @@ -237,63 +194,52 @@ gen_event:stop -----> Module:terminate/2  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 +

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 will delete the event handler by calling - Module:terminate/2 with {stop,Reason} as argument. + the event manager deletes the event handler by calling + + Module:terminate/2 + with {stop,Reason} as argument. + -

If the event handler later is deleted, the event manager +

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 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. - a term, if the event handler is removed due to an error. - Which term depends on the error. + +

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.

 -

See add_handler/3 for a description of the arguments - and return values.

- - - - notify(EventMgrRef, Event) -> ok - sync_notify(EventMgrRef, Event) -> ok - Notify an event manager about an event. - - EventMgrRef = Name | {Name,Node} | {global,GlobalName} - | {via,Module,ViaName} | pid() -  Name = Node = atom() -  GlobalName = ViaName = term() - 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.

+

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

 + call(EventMgrRef, Handler, Request) -> Result call(EventMgrRef, Handler, Request, Timeout) -> Result @@ -314,18 +260,18 @@ gen_event:stop -----> Module:terminate/2   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 +

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 which specifies +

Timeout is an integer greater than zero that specifies how many milliseconds to wait for a reply, or the atom - infinity to wait indefinitely. Default value is 5000. + 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 @@ -337,7 +283,8 @@ gen_event:stop -----> Module:terminate/2 respectively.

 - + + delete_handler(EventMgrRef, Handler, Args) -> Result Delete an event handler from a generic event manager. @@ -353,12 +300,14 @@ gen_event:stop -----> Module:terminate/2  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 +

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 @@ -367,6 +316,148 @@ gen_event:stop -----> Module:terminate/2 {'EXIT',Reason}.

 + + + notify(EventMgrRef, Event) -> ok + sync_notify(EventMgrRef, Event) -> ok + Notify an event manager about an event. + + 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 + Create a stand-alone event manager process. + + 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 + Create a generic event manager process in a supervision tree. + + + 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 + Terminate a generic event manager. + + 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 Replace an event handler in a generic event manager. @@ -385,34 +476,35 @@ gen_event:stop -----> Module:terminate/2

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.

+ 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 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 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.

+

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 will be a supervised connection + 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 + 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 Replace an event handler in a generic event manager. @@ -430,16 +522,18 @@ gen_event:stop -----> Module:terminate/2   Reason = term() -

Replaces an event handler in the event manager EventMgrRef - in the same way as swap_handler/3 but will also supervise +

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.

-

See swap_handler/3 for a description of the arguments - and return values.

+

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

 + which_handlers(EventMgrRef) -> [Handler] - Return all event handlers installed in a generic event manager. + Return all event handlers installed in a generic event manager. + EventMgrRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid() @@ -450,132 +544,106 @@ gen_event:stop -----> Module:terminate/2  Id = term() -

Returns a list of all event handlers installed in the event +

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

-

See add_handler/3 for a description of EventMgrRef - and Handler.

- - - - stop(EventMgrRef) -> ok - stop(EventMgrRef, Reason, Timeout) -> ok - Terminate a generic event manager. - - EventMgrRef = Name | {Name,Node} | {global,GlobalName} - | {via,Module,ViaName} | pid() - Name = Node = atom() - GlobalName = ViaName = term() - Reason = term() - Timeout = int()>0 | infinity - - -

Orders the event manager EventMgrRef to exit with - the given Reason and waits for it to - terminate. Before terminating, the gen_event will call - 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} will cause an - error report to be issued using - error_logger:format/2. - The default Reason is normal.

-

Timeout is an integer greater than zero which - specifies how many milliseconds to wait for the event manager to - terminate, or the atom infinity to wait - indefinitely. The default value is 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.

-

See add_handler/3 for a description of EventMgrRef.

+

For a description of EventMgrRef and Handler, see + add_handler/3.
- CALLBACK FUNCTIONS -

The following functions should be exported from a gen_event + Callback Functions +

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

+ - Module:init(InitArgs) -> {ok,State} | {ok,State,hibernate} | {error,Reason} - Initialize an event handler. + Module:code_change(OldVsn, State, Extra) -> {ok, NewState} + Update the internal state during upgrade/downgrade. - InitArgs = Args | {Args,Term} -  Args = Term = term() - State = term() - Reason = term() + OldVsn = Vsn | {down, Vsn} +   Vsn = term() + State = NewState = term() + Extra = 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.

-

If successful, the function should return {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 will go into hibernation (by calling proc_lib:hibernate/3), - waiting for the next event to occur.

+

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:handle_event(Event, State) -> Result - Handle an event. + Module:format_status(Opt, [PDict, State]) -> Status + Optional function for providing a term describing the + current event handler state. - Event = term() + Opt = normal | terminate + PDict = [{Key, Value}] 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() + Status = 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.

+ +

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.

-

If the function returns {ok,NewState} or {ok,NewState,hibernate} - the event handler - will remain in the event manager with the possible updated - internal state NewState.

-

If {ok,NewState,hibernate} is returned, the event - manager will also go 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 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).

+

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 Handle a synchronous request. @@ -594,15 +662,77 @@ gen_event:stop -----> Module:terminate/2

Whenever an event manager receives a request sent using - gen_event:call/3,4, this function is called for + call/3,4, + this function is called for the specified event handler to handle the request.

-

Request is the Request argument of call.

+

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 + Handle an event. + + 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.

-

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.

+ + +

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 Handle an incoming message. @@ -610,7 +740,8 @@ gen_event:stop -----> Module:terminate/2 Info = term() State = term() Result = {ok,NewState} | {ok,NewState,hibernate} -  | {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler +  | {swap_handler,Args1,NewState,Handler2,Args2} + | remove_handler  NewState = term()  Args1 = Args2 = term()  Handler2 = Module2 | {Module2,Id} @@ -622,10 +753,49 @@ gen_event:stop -----> Module:terminate/2 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.

+

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

+ + + + + Module:init(InitArgs) -> {ok,State} | {ok,State,hibernate} | {error,Reason} + Initialize an event handler. + + 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() Clean up before deletion. @@ -636,22 +806,25 @@ gen_event:stop -----> Module:terminate/2

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 + 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 which has terminated with reason + 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 will terminate 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 will - terminate if it receives an 'EXIT' message from - its parent.

+

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}.

@@ -660,104 +833,20 @@ gen_event:stop -----> Module:terminate/2 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 +

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 due to a swap, the return value will be passed to + 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.

 - - Module:code_change(OldVsn, State, Extra) -> {ok, NewState} - Update the internal state during upgrade/downgrade. - - 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.

- - - - Module:format_status(Opt, [PDict, State]) -> Status - Optional function for providing a term describing the - current event handler state. - - Opt = normal | terminate - PDict = [{Key, Value}] - State = term() - Status = term() - - - -

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 when:

- - 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 customising the form and - appearance of the event handler state for these cases. An - event handler callback module wishing to customise - 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 gen_event's - process dictionary.

-

State is the internal state of the event - handler.

-

The function should return Status, a term that - customises 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 event handler's actual state term. - When an event handler terminates abnormally, gen_event - logs Status in place of the event handler's actual - state term. - -

One use for this function is to return compact alternative - state representations to avoid having large state terms - printed in logfiles.

- -
- SEE ALSO -

supervisor(3), - sys(3)

+ See Also +

supervisor(3), + sys(3)

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