# gen_event

###
Module

gen_event

###
Module Summary

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_monitor
    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:send_request
    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.

Note

For some important information about distributed signals, see the Blocking Signaling Over Distribution section in the Processes chapter of the Erlang Reference Manual. Blocking signaling can, for example, cause call timeouts in gen_event to be significantly delayed.

###
Data Types

####
handler() = atom() | {atom(), term()}

####
handler_args() = term()

####
add_handler_ret() = ok | term() | {'EXIT', term()}

####
del_handler_ret() = ok | term() | {'EXIT', term()}

####
emgr_ref() =
atom() |
{atom(), atom()} |
{global, term()} |
{via, atom(), term()} |
pid()

####
request_id()

An opaque request identifier. See send_request/3 for details.

####
request_id_collection()

An opaque collection of request identifiers (request_id()) where each request identifier can be associated with a label chosen by the user. For more information see reqids_new/0.

####
response_timeout() = timeout() | {abs, integer()}

Used to set a time limit on how long to wait for a response using either receive_response/2, receive_response/3, wait_response/2, or wait_response/3. The time unit used is millisecond. Currently valid values:

0..4294967295

  Timeout relative to current time in milliseconds.

infinity

  Infinite timeout. That is, the operation will never time out.

{abs, Timeout}

  An absolute Erlang monotonic time timeout in milliseconds. That is, the operation will time out when erlang:monotonic_time(millisecond) returns a value larger than or equal to Timeout. Timeout is not allowed to identify a time further into the future than 4294967295 milliseconds. Identifying the timeout using an absolute timeout value is especially handy when you have a deadline for responses corresponding to a complete collection of requests (request_id_collection()) , since you do not have to recalculate the relative time until the deadline over and over again.

####
format_status() =
#{state => term(),
message => term(),
reason => term(),
log => [sys:system_event()]}

A map that describes the gen_event process status. The keys are:

state
  The internal state of the event handler.
message
  The message that caused the event handler to terminate.
reason
  The reason that caused the event handler to terminate.
log
  The sys log of the server.

New associations may be added into the status map without prior notice.

###
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()

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.

####
add_sup_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()

Adds a new event handler in the same way as add_handler/3, but also supervises the connection by linking the event handler and the calling process.

- If the calling process later terminates with Reason, the event manager deletes any supervised event handlers by calling Module:terminate/2, then calls Module:handle_info/2 for each remaining handler.

- 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.

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

####
call(EventMgrRef, Handler, Request) -> Result

####
call(EventMgrRef, Handler, Request, Timeout) -> 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()
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.

When this call fails it exits the calling process. The exit term is on the form {Reason, Location} where Location = {gen_event,call,ArgList}. See gen_server:call/3 that has a description of relevant values for the Reason in the exit term.

####
check_response(Msg, ReqId) -> Result
OTP 23.0

### Types

Msg = term()
ReqId = request_id()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), emgr_ref()}}
Result = Response | no_reply

Check if Msg is a response corresponding to the request identifier ReqId. The request must have been made by send_request/3.

If Msg is a response corresponding to ReqId the response is returned; otherwise, no_reply is returned and no cleanup is done, and thus the function must be invoked repeatedly until a response is returned.

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. If the event manager dies before or during the request this function returns {error,{Reason, EventMgrRef}}.

####
check_response(Msg, ReqIdCollection, Delete) -> Result
OTP 25.0

### Types

Msg = term()
ReqIdCollection = request_id_collection()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), emgr_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | no_reply

Check if Msg is a response corresponding to a request identifier saved in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/3 or send_request/5, and all request must have been made by the process calling this function.

The Label in the response equals the Label associated with the request identifier that the response corresponds to. The Label of a request identifier is associated when saving the request id in a request identifier collection, or when sending the request using send_request/5.

Compared to check_response/2, the returned result associated with a specific request identifier or an exception associated with a specific request identifier will be wrapped in a 3-tuple. The first element of this tuple equals the value that would have been produced by check_response/2, the second element equals the Label associated with the specific request identifier, and the third element NewReqIdCollection is a possibly modified request identifier collection.

If ReqIdCollection is empty, the atom no_request will be returned. If Msg does not correspond to any of the request identifiers in ReqIdCollection, the atom no_reply is returned.

If Delete equals true, the association with Label will have been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false, NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to check_response/3, receive_response/3, and wait_response/3. However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to check_response/3, it will always return no_reply.

####
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()

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

####
sync_notify(EventMgrRef, Event) -> ok

### Types

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.

####
receive_response(ReqId, Timeout) -> Result
OTP 24.0

### Types

ReqId = request_id()
Timeout = response_timeout()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), emgr_ref()}}
Result = Response | timeout

Receive a response corresponding to the request identifier ReqId- The request must have been made by send_request/3 to the gen_statem process. This function must be called from the same process from which send_request/3 was made.

Timeout specifies how long to wait for a response. If no response is received within the specified time, the function returns timeout. Assuming that the server executes on a node supporting aliases (introduced in OTP 24) the request will also be abandoned. That is, no response will be received after a timeout. Otherwise, a stray response might be received at a later time.

The return value Reply is defined in the return value of Module:handle_call/3.

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. If the event manager dies before or during the request this function returns {error,{Reason, EventMgrRef}}.

The difference between wait_response/2 and receive_response/2 is that receive_response/2 abandons the request at timeout so that a potential future response is ignored, while wait_response/2 does not.

####
receive_response(ReqIdCollection, Timeout, Delete) -> Result
OTP 25.0

### Types

ReqIdCollection = request_id_collection()
Timeout = response_timeout()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), emgr_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | timeout

Receive a response corresponding to a request identifier saved in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/3 or send_request/5, and all request must have been made by the process calling this function.

The Label in the response equals the Label associated with the request identifier that the response corresponds to. The Label of a request identifier is associated when adding the request id in a request identifier collection, or when sending the request using send_request/5.

Compared to receive_response/2, the returned result associated with a specific request identifier will be wrapped in a 3-tuple. The first element of this tuple equals the value that would have been produced by receive_response/2, the second element equals the Label associated with the specific request identifier, and the third element NewReqIdCollection is a possibly modified request identifier collection.

If ReqIdCollection is empty, the atom no_request will be returned.

Timeout specifies how long to wait for a response. If no response is received within the specified time, the function returns timeout. Assuming that the server executes on a node supporting aliases (introduced in OTP 24) all requests identified by ReqIdCollection will also be abandoned. That is, no responses will be received after a timeout. Otherwise, stray responses might be received at a later time.

The difference between receive_response/3 and wait_response/3 is that receive_response/3 abandons the requests at timeout so that potential future responses are ignored, while wait_response/3 does not.

If Delete equals true, the association with Label will have been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false, NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to receive_response/3, check_response/3, and wait_response/3. However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to receive_response/3, it will always block until a timeout determined by Timeout is triggered.

####
reqids_add(ReqId :: request_id(),
Label :: term(),
ReqIdCollection :: request_id_collection()) ->
NewReqIdCollection :: request_id_collection()
OTP 25.0

Saves ReqId and associates a Label with the request identifier by adding this information to ReqIdCollection and returning the resulting request identifier collection.

####
reqids_new() -> NewReqIdCollection :: request_id_collection()
OTP 25.0

Returns a new empty request identifier collection. A request identifier collection can be utilized in order the handle multiple outstanding requests.

Request identifiers of requests made by send_request/3 can be saved in a request identifier collection using reqids_add/3. Such a collection of request identifiers can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or, check_response/3.

reqids_size/1 can be used to determine the amount of request identifiers in a request identifier collection.

####
reqids_size(ReqIdCollection :: request_id_collection()) ->
integer() >= 0
OTP 25.0

Returns the amount of request identifiers saved in ReqIdCollection.

####
reqids_to_list(ReqIdCollection :: request_id_collection()) ->
[{ReqId :: request_id(), Label :: term()}]
OTP 25.0

Returns a list of {ReqId, Label} tuples which corresponds to all request identifiers with their associated labels present in the ReqIdCollection collection.

####
send_request(EventMgrRef :: emgr_ref(),
Handler :: handler(),
Request :: term()) ->
ReqId :: request_id()
OTP 23.0

Sends an asynchronous call request Request to event handler Handler installed in the event manager identified by EventMgrRef and returns a request identifier ReqId. The return value ReqId shall later be used with receive_response/2, wait_response/2, or check_response/2 to fetch the actual result of the request. Besides passing the request identifier directly to these functions, it can also be saved in a request identifier collection using reqids_add/3. Such a collection of request identifiers can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or check_response/3. If you are about to save the request identifier in a request identifier collection, you may want to consider using send_request/5 instead.

The call gen_event:receive_response(gen_event:send_request(EventMgrRef, Handler, Request), Timeout) can be seen as equivalent to gen_event:call(EventMgrRef, Handler, Request, Timeout), ignoring the error handling.

The event manager calls Module:handle_call/2 to handle the request.

Request is any term that is passed as one of the arguments to Module:handle_call/3.

####
send_request(EventMgrRef :: emgr_ref(),
Handler :: handler(),
Request :: term(),
Label :: term(),
ReqIdCollection :: request_id_collection()) ->
NewReqIdCollection :: request_id_collection()
OTP 25.0

Sends an asynchronous call request Request to event handler Handler installed in the event manager identified by EventMgrRef. The Label will be associated with the request identifier of the operation and added to the returned request identifier collection NewReqIdCollection. The collection can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or, check_response/3.

The same as calling gen_event:reqids_add(gen_event:send_request(EventMgrRef, Handler, Request), Label, ReqIdCollection), but calling send_request/5 is slightly more efficient.

####
start() -> Result

####
start(EventMgrName | Options) -> Result

####
start(EventMgrName, Options) -> Result
OTP 20.0

### Types

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.

####
start_link() -> Result

####
start_link(EventMgrName | Options) -> Result

####
start_link(EventMgrName, Options) -> Result
OTP 20.0

### Types

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 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 option {hibernate_after,HibernateAfterTimeout} is present, the gen_event process awaits any message for HibernateAfterTimeout milliseconds and if no message is received, the process goes into hibernation automatically (by calling proc_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.

####
start_monitor() -> Result
OTP 23.0

####
start_monitor(EventMgrName | Options) -> Result
OTP 23.0

####
start_monitor(EventMgrName, Options) -> Result
OTP 23.0

### Types

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,Mon}} | {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) and atomically sets up a monitor to the newly created process.

For a description of the arguments and return values, see start_link/0,1. Note that the return value on successful start differs from start_link/3,4. start_monitor/3,4 will return {ok,{Pid,Mon}} where Pid is the process identifier of the process, and Mon is a reference to the monitor set up to monitor the process. If the start is not successful, the caller will be blocked until the DOWN message has been received and removed from the message queue.

####
stop(EventMgrRef) -> ok

####
stop(EventMgrRef, Reason, Timeout) -> ok
OTP 18.0

### 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 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, the call exits the calling process with reason timeout.

If the process does not exist, the call exits the calling process with reason noproc, and with reason {nodedown,Node} if the connection fails to the remote Node where the server runs.

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()

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()

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.

####
wait_response(ReqId, WaitTime) -> Result
OTP 23.0

### Types

ReqId = request_id()
WaitTime = response_timeout()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), emgr_ref()}}
Result = Response | timeout

Wait for a response corresponding to the request identifier ReqId. The request must have been made by send_request/3 to the gen_statem process. This function must be called from the same process from which send_request/3 was made.

WaitTime specifies how long to wait for a response. If no response is received within the specified time, the function returns timeout and no cleanup is done, and thus the function can be invoked repeatedly until a reply is returned.

The return value Reply is defined in the return value of Module:handle_call/3.

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. If the event manager dies before or during the request this function returns {error,{Reason, EventMgrRef}}.

The difference between receive_response/2 and wait_response/2 is that receive_response/2 abandons the request at timeout so that a potential future response is ignored, while wait_response/2 does not.

####
wait_response(ReqIdCollection, WaitTime, Delete) -> Result
OTP 25.0

### Types

ReqIdCollection = request_id_collection()
WaitTime = response_timeout()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), emgr_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | timeout

Wait for a response corresponding to a request identifier saved in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/3 or send_request/5, and all request must have been made by the process calling this function.

The Label in the response equals the Label associated with the request identifier that the response corresponds to. The Label of a request identifier is associated when saving the request id in a request identifier collection, or when sending the request using send_request/5.

Compared to wait_response/2, the returned result associated with a specific request identifier or an exception associated with a specific request identifier will be wrapped in a 3-tuple. The first element of this tuple equals the value that would have been produced by wait_response/2, the second element equals the Label associated with the specific request identifier, and the third element NewReqIdCollection is a possibly modified request identifier collection.

If ReqIdCollection is empty, no_request will be returned. If no response is received before the WaitTime timeout has triggered, the atom timeout is returned. It is valid to continue waiting for a response as many times as needed up until a response has been received and completed by check_response(), receive_response(), or wait_response().

The difference between receive_response/3 and wait_response/3 is that receive_response/3 abandons requests at timeout so that a potential future responses are ignored, while wait_response/3 does not.

If Delete equals true, the association with Label will have been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false, NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to wait_response/3, check_response/3, and receive_response/3. However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to wait_response/3, it will always block until a timeout determined by WaitTime is triggered and then return no_reply.

####
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()

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()
Note

This callback is optional, so callback modules need not export it. If a release upgrade/downgrade with Change={advanced,Extra} specified in the .appup file is made when code_change/3 isn't implemented the event handler will crash with an undef error reason.

This function is called for an installed event handler that is to update its internal state during a release upgrade/downgrade, that is, when the instruction {update,Module,Change,...}, where Change={advanced,Extra}, is specified in the .appup file. For more information, see OTP Design Principles.

For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old version of the callback module Module. If no such attribute is defined, the version is the checksum of the Beam file.

State is the internal state of the event handler.

Extra is passed "as is" from the {advanced,Extra} part of the update instruction.

The function is to return the updated internal state.

####
Module:format_status(Status) -> NewStatus
OTP 25.0

### Types

Status = format_status()
NewStatus = format_status()
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.

If this callback is exported but fails, to hide possibly sensitive data, the default function will instead return the fact that format_status/1 has crashed.

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.
- The event handler terminates abnormally and gen_event logs an error.

This callback is used to limit the status of the event handler returned by sys:get_status/1,2 or sent to logger.

The callback gets a map Status describing the current status and shall return a map NewStatus with the same keys, but it may transform some values.

Two possible use cases for this callback is to remove sensitive information from the state to prevent it from being printed in log files, or to compact large irrelevant status items that would only clutter the logs.

    format_status(Status) ->
      maps:map(
        fun(state,State) ->
                maps:remove(private_key, State);
           (message,{password, _Pass}) ->
                {password, removed};
           (_,Value) ->
                Value
        end, Status).

####
Module:format_status(Opt, [PDict, State]) -> Status
OTP R14B

### Types

Opt = normal | terminate
PDict = [{Key, Value}]
State = term()
Status = term()
Warning

This callback is deprecated, in new code use format_status/1. If a format_status/1 callback exists, then this function will never be called.

Note

This callback is optional, so event handler modules need not export it. If a handler does not export this function, the gen_event module uses the handler state directly for the purposes described below.

This function is called by a gen_event process in the following situations:

- One of sys:get_status/1,2 is invoked to get the gen_event status. Opt is set to the atom normal for this case.
- The event handler terminates abnormally and gen_event logs an error. Opt is set to the atom terminate for this case.

This function is useful for changing the form and appearance of the event handler state for these cases. An event handler callback module wishing to change the the sys:get_status/1,2 return value as well as how its state appears in termination error logs, exports an instance of format_status/2 that returns a term describing the current state of the event handler.

PDict is the current value of the process dictionary of gen_event.

State is the internal state of the event handler.

The function is to return Status, a term that change the details of the current state of the event handler. Any term is allowed for Status. The gen_event module uses Status as follows:

- When sys:get_status/1,2 is called, gen_event ensures that its return value contains Status in place of the state term of the event handler.

- When an event handler terminates abnormally, gen_event logs Status in place of the state term of the event handler.

One use for this function is to return compact alternative state representations to avoid that large state terms are printed in log files.

####
Module:handle_call(Request, State) -> Result

### Types

Request = term()
State = term()
Result = {ok,Reply,NewState} | {ok,Reply,NewState,hibernate}
| {swap_handler,Reply,Args1,NewState,Handler2,Args2}
| {remove_handler, Reply}
Reply = term()
NewState = term()
Args1 = Args2 = term()
Handler2 = Module2 | {Module2,Id}
Module2 = atom()
Id = term()

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

Request is the Request argument of call/3,4.

State is the internal state of the event handler.

The return values are the same as for Module:handle_event/2 except that they also contain a term Reply, which is the reply to the client as the return value of call/3,4.

####
Module:handle_event(Event, State) -> Result

### Types

Event = term()
State = term()
Result = {ok,NewState} | {ok,NewState,hibernate}
| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler
NewState = term()
Args1 = Args2 = term()
Handler2 = Module2 | {Module2,Id}
Module2 = atom()
Id = term()

Whenever an event manager receives an event sent using notify/2 or sync_notify/2, this function is called for each installed event handler to handle the event.

Event is the Event argument of notify/2/sync_notify/2.

State is the internal state of the event handler.

- If {ok,NewState} or {ok,NewState,hibernate} is returned, the event handler remains in the event manager with the possible updated internal state NewState.

- If {ok,NewState,hibernate} is returned, the event manager also goes into hibernation (by calling proc_lib:hibernate/3), waiting for the next event to occur. It is sufficient that one of the event handlers return {ok,NewState,hibernate} for the whole event manager process to hibernate.

- If {swap_handler,Args1,NewState,Handler2,Args2} is returned, the event handler is replaced by Handler2 by first calling Module:terminate(Args1,NewState) and then Module2:init({Args2,Term}), where Term is the return value of Module:terminate/2. For more information, see swap_handler/3.

- If remove_handler is returned, the event handler is deleted by calling Module:terminate(remove_handler,State).

####
Module:handle_info(Info, State) -> Result

### Types

Info = term()
State = term()
Result = {ok,NewState} | {ok,NewState,hibernate}
| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler
NewState = term()
Args1 = Args2 = term()
Handler2 = Module2 | {Module2,Id}
Module2 = atom()
Id = term()
Note

This callback is optional, so callback modules need not export it. The gen_event module provides a default implementation of this function that logs about the unexpected Info message, drops it and returns {ok, State}.

This function is called for each installed event handler when an event manager receives any other message than an event or a synchronous request (or a system message).

Info is the received message.

In particular, this callback will be made when a process terminated after calling add_sup_handler/3. Any event handler attached to an event manager which in turn has a supervised handler should expect callbacks of the shape Module:handle_info({'EXIT', Pid, Reason}, State).

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

####
Module:init(InitArgs) -> {ok,State} | {ok,State,hibernate} | {error,Reason}

### Types

InitArgs = Args | {Args,Term}
Args = Term = term()
State = term()
Reason = term()

Whenever a new event handler is added to an event manager, this function is called to initialize the event handler.

If the event handler is added because of a call to add_handler/3 or add_sup_handler/3, InitArgs is the Args argument of these functions.

If the event handler replaces another event handler because of a call to swap_handler/3 or swap_sup_handler/3, or because of a swap return tuple from one of the other callback functions, InitArgs is a tuple {Args,Term}, where Args is the argument provided in the function call/return tuple and Term is the result of terminating the old event handler, see swap_handler/3.

If successful, the function returns {ok,State} or {ok,State,hibernate}, where State is the initial internal state of the event handler.

If {ok,State,hibernate} is returned, the event manager goes into hibernation (by calling proc_lib:hibernate/3), waiting for the next event to occur.

####
Module:terminate(Arg, State) -> term()

### Types

Arg = Args | {stop,Reason} | stop | remove_handler
| {error,{'EXIT',Reason}} | {error,Term}
Args = Reason = Term = term()
Note

This callback is optional, so callback modules need not export it. The gen_event module provides a default implementation without cleanup.

Whenever an event handler is deleted from an event manager, this function is called. It is to be the opposite of Module:init/1 and do any necessary cleaning up.

If the event handler is deleted because of a call to delete_handler/3, swap_handler/3, or swap_sup_handler/3, Arg is the Args argument of this function call.

Arg={stop,Reason} if the event handler has a supervised connection to a process that has terminated with reason Reason.

Arg=stop if the event handler is deleted because the event manager is terminating.

The event manager terminates if it is part of a supervision tree and it is ordered by its supervisor to terminate. Even if it is not part of a supervision tree, it terminates if it receives an 'EXIT' message from its parent.

Arg=remove_handler if the event handler is deleted because another callback function has returned remove_handler or {remove_handler,Reply}.

Arg={error,Term} if the event handler is deleted because a callback function returned an unexpected value Term, or Arg={error,{'EXIT',Reason}} if a callback function failed.

State is the internal state of the event handler.

The function can return any term. If the event handler is deleted because of a call to gen_event:delete_handler/3, the return value of that function becomes the return value of this function. If the event handler is to be replaced with another event handler because of a swap, the return value is passed to the init function of the new event handler. Otherwise the return value is ignored.

###
See Also

supervisor(3), sys(3)