如何在 Erlang 中使用 gen_event 调用？
How to call using gen_event in Erlang?
我正在使用 gen_event 行为，当我尝试发出 gen_event:call 时，出现以下错误：
> =CRASH REPORT==== 22-Dec-2019::19:17:43.030000 === crasher:
> initial call: gen_event:init_it/6
> pid: <0.215.0>
> registered_name: hev
> exception exit: {undef,[{fm,state,[<0.215.0>],[]},
> {erl_eval,do_apply,6,
> [{file,"erl_eval.erl"},{line,684}]},
> {shell,exprs,7,[{file,"shell.erl"},{line,686}]},
> {shell,eval_exprs,7,
> [{file,"shell.erl"},{line,642}]},
> {shell,eval_loop,3,
> [{file,"shell.erl"},{line,627}]}]}
> in function gen_event:terminate_server/4 (gen_event.erl, line 354)
> ancestors: [<0.212.0>]
> message_queue_len: 1
> messages: [{'EXIT',<0.212.0>,normal}]
> links: []
> dictionary: []
> trap_exit: true
> status: running
> heap_size: 610
> stack_size: 27
> reductions: 279 neighbours:
我的事件管理器和事件处理程序得到生成，我可以成功发出 notify（我返回 ok）但我不能 call:
模块
-module(hev).
-export([start/0,append/2,state/1]).
-export([init/1,terminate/2,code_change/3,handle_call/2,handle_event/2]).
-record(state,{
xs=[]
}).
-behaviour(gen_event).
%callbacks
init([])->
{ok,#state{xs=[1]}}.
**API**
start()->
{ok,Pid}=gen_event:start_link({local,?MODULE}),
gen_event:add_handler(Pid,some_handler,[]),
Pid.
append(Pid,Elem)->
gen_event:notify(Pid,{append,Elem}).
state(Pid)->
gen_event:call(Pid,state).
处理程序
handle_event({append,Elem},State=#state{xs=XS})->
{ok,#state{xs=[Elem|XS]}};
handle_call(state,State})->
{ok,State,State};
handle_call(Event,State)->
{ok,nada_for_you,State}.
P.S 我还没有发布所有需要的方法（code_change、终止等），但它们存在。
I have not posted all the requried methods (code_change,terminate..etc) but they exist.
1) 无论如何它们都是可选的。检查文档中的大绿色注释，例如terminate().
2) 至于你的错误信息：
pid: <0.215.0>
registered_name: hev
exception exit: {undef,[{fm,state,[<0.215.0>],[]},
似乎是说有一个进程（pid=<0.215.0>）注册了名称hev，试图执行一个名为fm:state()的函数有一个参数，但没有在任何地方定义函数 fm:state/1，因此出现 undef 异常。因为您没有发布名为 fm 的模块，所以该错误与您发布的代码无关。
3) 您的代码还指定了一个名为 some_handler 的模块，该模块不存在：
gen_event:add_handler(Pid,some_handler,[]),
4) 你这里有一个基本的语法错误：
handle_call(state,State})->
5) 您调用了几个参数数量错误的函数。
您需要更勤奋地发布实际会产生您遇到的错误的代码。
这是一个使用 gen_event 创建计数器的简单示例：
-module(counter).
-behaviour(gen_event).
-compile(export_all).
%% Callback functions:
init(StartingCount) -> % Called by gen_event:add_handler()
State = StartingCount,
{ok, State}.
terminate(_Reason, State) ->
io:format("Terminating state was: ~w~n", [State]).
% Calls to gen_event:notify() cause this function to execute:
handle_event({increase, Change}, State) ->
NewState = State+Change,
{ok, NewState};
handle_event({decrease, Change}, State) ->
NewState = State-Change,
{ok, NewState}.
% Calls to gen_event:call() cause this function to execute:
handle_call(get_count, State) ->
Reply = io_lib:format("Reply from handle_call(): count is ~w~n", [State]),
{ok, Reply, State};
handle_call({increase_and_get_count, Change}, State) ->
NewState = State+Change,
Reply = io_lib:format("Reply from handle_call(): count is ~w~n", [NewState]),
{ok, Reply, NewState}.
%% User interface functions:
start(StartingCount) ->
ServerName = gen_event_counter,
CallbackModule = counter,
{ok, _Pid} = gen_event:start_link({local, ServerName}),
%Name of process running gen_event server is: gen_event_counter
ok = gen_event:add_handler(ServerName, CallbackModule, StartingCount).
%StartingCount is passed to init() callback
stop() ->
ok = gen_event:stop(gen_event_counter),
stopped.
send_request_with_notify(Request) ->
gen_event:notify(gen_event_counter, Request). % returns immediately, does not wait for a reply.
% Request = {increase, 1}, {decrease, 2}, etc.
% Request is passed as first arg to handle_event().
send_request_with_call(Request) ->
Reply = gen_event:call(gen_event_counter, counter, Request), % waits for a reply
% Request is passed as first arg to handle_call()
io:format("send_request_with_call() returned => ~s", [Reply]).
在shell中：
~/erlang_programs/gen_event$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(counter).
counter.erl:3: Warning: export_all flag enabled - all functions will be exported
{ok,counter}
2> counter:start(0).
ok
3> counter:send_request_with_call(get_count).
send_request_with_call() returned => Reply from handle_call(): count is 0
ok
4> counter:send_request_with_notify({increase, 2}).
ok
5> counter:send_request_with_call(get_count).
send_request_with_call() returned => Reply from handle_call(): count is 2
ok
6> counter:send_request_with_call({increase_and_get_count, 5}).
send_request_with_call() returned => Reply from handle_call(): count is 7
ok
7> counter:stop().
Terminating state was: 7
stopped
8>
修复代码中的所有错误后：
-module(hev).
-compile(export_all).
-behaviour(gen_event).
-record(state,{
xs=[]
}).
%callbacks
init(no_args)->
{ok, #state{xs=[1]} }.
handle_event({append,Elem}, #state{xs=XS} ) ->
io:format("hev:handle_event() called~n"),
{ok, #state{xs=[Elem|XS]}}.
handle_call(get_state, State)->
Reply = State,
{ok, Reply, State};
handle_call(_Other, State)->
Reply = nada_for_you,
{ok, Reply, State}.
%**API**
start()->
gen_event:start_link({local, ?MODULE}), %Sets the gen_event server name to ?MODULE
% Server Callback Args for
% Name module init()
gen_event:add_handler(?MODULE, ?MODULE, no_args).
%Tells the gen_event server named ?MODULE to look for the callback functions
%in a module also named ?MODULE
append(Elem)->
% Server Request
% Name (matches against 1st arg in handle_event() )
gen_event:notify(?MODULE, {append, Elem}).
get_state()->
% Server Calback Request
% Name module (matches against 1st arg in handle_call() )
gen_event:call(?MODULE, ?MODULE, get_state).
other_calls() ->
gen_event:call(?MODULE, ?MODULE, {set_state, [1, 2, 3]}).
stop() ->
ok = gen_event:stop(?MODULE),
stopped.
在shell中：
~/erlang_programs/gen_event$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(hev).
hev.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,hev}
2> hev:start().
ok
3> hev:get_state().
{state,[1]}
4> hev:append(45).
ok
hev:handle_event() called
5> hev:get_state().
{state,[45,1]}
6> hev:other_calls().
nada_for_you
7> hev:stop().
stopped
8>
请注意，notify() 会导致所有使用 add_handler() 添加的模块中的 handle_event() 函数执行，而 call() 会针对特定模块的 handle_call() 函数。
我正在使用 gen_event 行为，当我尝试发出 gen_event:call 时，出现以下错误：
> =CRASH REPORT==== 22-Dec-2019::19:17:43.030000 === crasher:
> initial call: gen_event:init_it/6
> pid: <0.215.0>
> registered_name: hev
> exception exit: {undef,[{fm,state,[<0.215.0>],[]},
> {erl_eval,do_apply,6,
> [{file,"erl_eval.erl"},{line,684}]},
> {shell,exprs,7,[{file,"shell.erl"},{line,686}]},
> {shell,eval_exprs,7,
> [{file,"shell.erl"},{line,642}]},
> {shell,eval_loop,3,
> [{file,"shell.erl"},{line,627}]}]}
> in function gen_event:terminate_server/4 (gen_event.erl, line 354)
> ancestors: [<0.212.0>]
> message_queue_len: 1
> messages: [{'EXIT',<0.212.0>,normal}]
> links: []
> dictionary: []
> trap_exit: true
> status: running
> heap_size: 610
> stack_size: 27
> reductions: 279 neighbours:
我的事件管理器和事件处理程序得到生成，我可以成功发出 notify（我返回 ok）但我不能 call:
模块
-module(hev).
-export([start/0,append/2,state/1]).
-export([init/1,terminate/2,code_change/3,handle_call/2,handle_event/2]).
-record(state,{
xs=[]
}).
-behaviour(gen_event).
%callbacks
init([])->
{ok,#state{xs=[1]}}.
**API**
start()->
{ok,Pid}=gen_event:start_link({local,?MODULE}),
gen_event:add_handler(Pid,some_handler,[]),
Pid.
append(Pid,Elem)->
gen_event:notify(Pid,{append,Elem}).
state(Pid)->
gen_event:call(Pid,state).
处理程序
handle_event({append,Elem},State=#state{xs=XS})->
{ok,#state{xs=[Elem|XS]}};
handle_call(state,State})->
{ok,State,State};
handle_call(Event,State)->
{ok,nada_for_you,State}.
P.S 我还没有发布所有需要的方法（code_change、终止等），但它们存在。
I have not posted all the requried methods (code_change,terminate..etc) but they exist.
1) 无论如何它们都是可选的。检查文档中的大绿色注释，例如terminate().
2) 至于你的错误信息：
pid: <0.215.0> registered_name: hev exception exit: {undef,[{fm,state,[<0.215.0>],[]},
似乎是说有一个进程（pid=<0.215.0>）注册了名称hev，试图执行一个名为fm:state()的函数有一个参数，但没有在任何地方定义函数 fm:state/1，因此出现 undef 异常。因为您没有发布名为 fm 的模块，所以该错误与您发布的代码无关。
3) 您的代码还指定了一个名为 some_handler 的模块，该模块不存在：
gen_event:add_handler(Pid,some_handler,[]),
4) 你这里有一个基本的语法错误：
handle_call(state,State})->
5) 您调用了几个参数数量错误的函数。
您需要更勤奋地发布实际会产生您遇到的错误的代码。
这是一个使用 gen_event 创建计数器的简单示例：
-module(counter).
-behaviour(gen_event).
-compile(export_all).
%% Callback functions:
init(StartingCount) -> % Called by gen_event:add_handler()
State = StartingCount,
{ok, State}.
terminate(_Reason, State) ->
io:format("Terminating state was: ~w~n", [State]).
% Calls to gen_event:notify() cause this function to execute:
handle_event({increase, Change}, State) ->
NewState = State+Change,
{ok, NewState};
handle_event({decrease, Change}, State) ->
NewState = State-Change,
{ok, NewState}.
% Calls to gen_event:call() cause this function to execute:
handle_call(get_count, State) ->
Reply = io_lib:format("Reply from handle_call(): count is ~w~n", [State]),
{ok, Reply, State};
handle_call({increase_and_get_count, Change}, State) ->
NewState = State+Change,
Reply = io_lib:format("Reply from handle_call(): count is ~w~n", [NewState]),
{ok, Reply, NewState}.
%% User interface functions:
start(StartingCount) ->
ServerName = gen_event_counter,
CallbackModule = counter,
{ok, _Pid} = gen_event:start_link({local, ServerName}),
%Name of process running gen_event server is: gen_event_counter
ok = gen_event:add_handler(ServerName, CallbackModule, StartingCount).
%StartingCount is passed to init() callback
stop() ->
ok = gen_event:stop(gen_event_counter),
stopped.
send_request_with_notify(Request) ->
gen_event:notify(gen_event_counter, Request). % returns immediately, does not wait for a reply.
% Request = {increase, 1}, {decrease, 2}, etc.
% Request is passed as first arg to handle_event().
send_request_with_call(Request) ->
Reply = gen_event:call(gen_event_counter, counter, Request), % waits for a reply
% Request is passed as first arg to handle_call()
io:format("send_request_with_call() returned => ~s", [Reply]).
在shell中：
~/erlang_programs/gen_event$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(counter).
counter.erl:3: Warning: export_all flag enabled - all functions will be exported
{ok,counter}
2> counter:start(0).
ok
3> counter:send_request_with_call(get_count).
send_request_with_call() returned => Reply from handle_call(): count is 0
ok
4> counter:send_request_with_notify({increase, 2}).
ok
5> counter:send_request_with_call(get_count).
send_request_with_call() returned => Reply from handle_call(): count is 2
ok
6> counter:send_request_with_call({increase_and_get_count, 5}).
send_request_with_call() returned => Reply from handle_call(): count is 7
ok
7> counter:stop().
Terminating state was: 7
stopped
8>
修复代码中的所有错误后：
-module(hev).
-compile(export_all).
-behaviour(gen_event).
-record(state,{
xs=[]
}).
%callbacks
init(no_args)->
{ok, #state{xs=[1]} }.
handle_event({append,Elem}, #state{xs=XS} ) ->
io:format("hev:handle_event() called~n"),
{ok, #state{xs=[Elem|XS]}}.
handle_call(get_state, State)->
Reply = State,
{ok, Reply, State};
handle_call(_Other, State)->
Reply = nada_for_you,
{ok, Reply, State}.
%**API**
start()->
gen_event:start_link({local, ?MODULE}), %Sets the gen_event server name to ?MODULE
% Server Callback Args for
% Name module init()
gen_event:add_handler(?MODULE, ?MODULE, no_args).
%Tells the gen_event server named ?MODULE to look for the callback functions
%in a module also named ?MODULE
append(Elem)->
% Server Request
% Name (matches against 1st arg in handle_event() )
gen_event:notify(?MODULE, {append, Elem}).
get_state()->
% Server Calback Request
% Name module (matches against 1st arg in handle_call() )
gen_event:call(?MODULE, ?MODULE, get_state).
other_calls() ->
gen_event:call(?MODULE, ?MODULE, {set_state, [1, 2, 3]}).
stop() ->
ok = gen_event:stop(?MODULE),
stopped.
在shell中：
~/erlang_programs/gen_event$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(hev).
hev.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,hev}
2> hev:start().
ok
3> hev:get_state().
{state,[1]}
4> hev:append(45).
ok
hev:handle_event() called
5> hev:get_state().
{state,[45,1]}
6> hev:other_calls().
nada_for_you
7> hev:stop().
stopped
8>
请注意，notify() 会导致所有使用 add_handler() 添加的模块中的 handle_event() 函数执行，而 call() 会针对特定模块的 handle_call() 函数。