gen_event:add_handler/3
gen_event:add_sup_handler/3

-record(handler, {module             :: atom(),id = false,state,supervised = false :: 'false' | pid()}).

%% server_add_handler(Handler, Args, MSL) -> {Ret, MSL'}.
%%   where MSL = [#handler{}]
%%   Ret goes to the top level MSL' is the new internal state of the
%%   event handlerserver_add_handler({Mod,Id}, Args, MSL) ->Handler = #handler{module = Mod,id = Id},server_add_handler(Mod, Handler, Args, MSL);
server_add_handler(Mod, Args, MSL) -> Handler = #handler{module = Mod},server_add_handler(Mod, Handler, Args, MSL).server_add_handler(Mod, Handler, Args, MSL) ->case catch Mod:init(Args) of{ok, State} ->{false, ok, [Handler#handler{state = State}|MSL]};{ok, State, hibernate} ->{true, ok, [Handler#handler{state = State}|MSL]};Other ->{false, Other, MSL}end.%% Set up a link to the supervising process.
%% (Ought to be unidirected links here, Erl5.0 !!)
%% NOTE: This link will not be removed then the
%% handler is removed in case another handler has
%% own link to this process.
server_add_sup_handler({Mod,Id}, Args, MSL, Parent) ->link(Parent),Handler = #handler{module = Mod,id = Id,supervised = Parent},server_add_handler(Mod, Handler, Args, MSL);
server_add_sup_handler(Mod, Args, MSL, Parent) -> link(Parent),Handler = #handler{module = Mod,supervised = Parent},server_add_handler(Mod, Handler, Args, MSL).

%% First terminate the supervised (if exists) handlers and
%% then inform other handlers.
%% We do not know if any handler really is interested but it
%% may be so !
handle_exit(From, Reason, MSL, SName) ->MSL1 = terminate_supervised(From, Reason, MSL, SName),{_,MSL2}=server_notify({'EXIT', From, Reason}, handle_info, MSL1, SName),MSL2.terminate_supervised(Pid, Reason, MSL, SName) ->F = fun(Ha) when Ha#handler.supervised =:= Pid ->do_terminate(Ha#handler.module,Ha,{stop,Reason},Ha#handler.state,{parent_terminated, {Pid,Reason}},SName,shutdown),false;(_) ->trueend,lists:filter(F, MSL).do_terminate(Mod, Handler, Args, State, LastIn, SName, Reason) ->Res = (catch Mod:terminate(Args, State)),report_terminate(Handler, Reason, Args, State, LastIn, SName, Res),Res.

terminate_server(Reason, Parent, MSL, ServerName) ->stop_handlers(MSL, ServerName),do_unlink(Parent, MSL),exit(Reason).%% unlink the supervisor process of all supervised handlers.
%% We do not want a handler supervisor to EXIT due to the
%% termination of the event manager (server).
%% Do not unlink Parent !
do_unlink(Parent, MSL) ->lists:foreach(fun(Handler) when Handler#handler.supervised =:= Parent ->true;(Handler) when is_pid(Handler#handler.supervised) ->unlink(Handler#handler.supervised),true;(_) ->trueend,MSL).

gen_event:delete_handler/3
gen_event:swap_sup_handler/3