What's the best way to do something periodically in Erlang?
I have a process that needs to do some work every fifteen seconds. I'm currently doing it like this:
-behavior(gen_server).
interval_milliseconds ()-> 15000.
init()->
{ok,
_State = FascinatingStateData,
_TimeoutInterval = interval_milliseconds ()
}.
%% This gets called automatically as a result of our handlers
%% including the optional _TimeoutInterval value in the returned
%% Result
handle_info(timeout, StateData)->
{noreply,
_State = do_some_work(StateData),
_TimeoutInterval = interval_milliseconds ()
}.
This works, but it's extremely brittle: if I want to teach my server a new message, when I write any new handler function, I have to remember to include the optional timeout interval in its return value. That is, say if I'm handling a synchronous call, I need to do this:
%% Someone wants to know our state; tell them
handle_call(query_state_data, _From, StateData)->
{reply, StateData, _NewStateData = whatever (), interval_milliseconds ()};
instead of
%% Someone wants to know our state; tell them
handle_call(query_state_data, _From, StateData)->
{reply, StateData, _NewStateData = whatever ()};
As you might guess, I've made that very mistake a number of times. It's nasty, because once the code handles that query_state_data message, the timeouts no longer get generated, and the whole server grinds to a halt. (I can "defibrillate" it manually by getting a shell on the machine and sending a "timeout" message by hand, but ... eww.)
Now, I could try to remember to always specify that optional Timeout parameter in my Result value. But that doesn't scale: I'll forget someday, and will be staring at this bug once again. So: what's a better way?
I don't think I want to write an actual loop that runs forever, and spends most of its time sleeping; that seems counter to the spirit of OTP.
最好的方法是：
init([]) ->
Timer = erlang:send_after(1, self(), check),
{ok, Timer}.
handle_info(check, OldTimer) ->
erlang:cancel_timer(OldTimer),
do_task(),
Timer = erlang:send_after(1000, self(), check),
{noreply, Timer}.
Use timer:send_interval/2. Eg:
-behavior(gen_server).
interval_milliseconds()-> 15000.
init()->
timer:send_interval(interval_milliseconds(), interval),
{ok, FascinatingStateData}.
%% this clause will be called every 15 seconds
handle_info(interval, StateData)->
State2 = do_some_work(StateData)
{noreply, State2}.
使用计时器模块:)
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