-- |This module can execute events at specified time. It uses a two thread
-- system that allows the STM adding and deleting of new threads without
-- requiring later IO actions. For a simpler system that uses relative times
-- see Control.Event.Relative
module Control.Event (
EventId
,EventSystem
,noEvent
,initEventSystem
,addEvent
,addEventSTM
,cancelEvent
,cancelEventSTM
,evtSystemSize
) where
import Prelude hiding (lookup, catch)
import Control.Concurrent (forkIO, myThreadId, ThreadId, threadDelay)
import Control.Concurrent.STM
import Control.Exception
import Control.Monad (forever, when)
import Data.Dynamic
import Data.List (partition, deleteBy)
import Data.Map (Map, empty, findMin, deleteFindMin, insertLookupWithKey, adjust, size, singleton, toList, insert, updateLookupWithKey, delete, lookup, fold)
import Data.Time
type EventNumber = Int
type EventSet = (EventNumber, Map EventNumber (IO ()))
singletonSet :: (IO ()) -> EventSet
singletonSet a = (1, singleton 0 a)
-- |IDs useful for canceling previously scheduled events.
data EventId = EvtId UTCTime EventNumber deriving (Eq, Ord, Show)
-- |A value indicating there is no such event.
-- Canceling this event returns True and has no other effect.
noEvent :: EventId
noEvent = EvtId never (-1)
never :: UTCTime
never = UTCTime (toEnum (-1)) (-1)
-- |The event system must be initilized using initEventSystem.
-- More than one event system can be instantiated at once
-- (eg. for non-interference).
data EventSystem = EvtSys {
esEvents :: TVar (Map UTCTime EventSet), -- Pending Events
esThread :: TVar (Maybe ThreadId), -- Id of thread for TimerReset exceptions
esAlarm :: TVar UTCTime, -- Time of soonest event
esNewAlarm :: TVar Bool, -- An event w/ earlier expiration was added
esExpired :: TVar [[EventSet]]
}
-- |The only way to get an event system is to initilize one
-- This sets internal TVars and sparks three threads:
-- trackAlarm: Ensured esAlarm remains
-- monitorExpiredQueue: Executes all events on the expired queue
-- expireEvents: When the alarm sounds, moves expired events to expired queue
initEventSystem :: IO EventSystem
initEventSystem = do
evts <- newTVarIO empty
tid <- newTVarIO Nothing
alm <- newTVarIO never
new <- newTVarIO False
exp <- newTVarIO []
let evtSys = EvtSys evts tid alm new exp
forkIO $ forever $ trackAlarm evtSys
forkIO $ forever $ monitorExpiredQueue exp
forkIO $ expireEvents evtSys
return evtSys
-- |Main thread. Delays till the alarm time then executes any expired events.
-- Asynchronous 'TimerReset' exceptions might occur to indicate a new,
-- earlier, alarm time.
expireEvents :: EventSystem -> IO ()
expireEvents es = mask $ \restore -> do
tid <- myThreadId
forever $ catch (restore (setTID (Just tid) es >> expireEvents' es))
(\TimerReset -> return ())
where
setTID i es = atomically (writeTVar (esThread es) i)
-- |Worker function for expireEvents - the parent simply catches the exceptions
expireEvents' :: EventSystem -> IO ()
expireEvents' evtSys = do
usDelay <- determineDelay
threadDelay usDelay
runExpire evtSys
where
determineDelay :: IO Int
determineDelay = do
alm <- atomically (do
evts <- readTVar (esEvents evtSys)
case findMinM evts of
Nothing -> retry
Just (c,_) -> return c )
now <- getCurrentTime
return $ timeDiffToMicroSec $ diffUTCTime alm now
findMinM :: Map UTCTime EventSet -> Maybe (UTCTime,EventSet)
findMinM m | size m == 0 = Nothing
| otherwise = Just $ findMin m
-- |Determines which events are expired, running all their actions
runExpire :: EventSystem -> IO ()
runExpire evtSys = do
now <- getCurrentTime
atomically (do evts <- readTVar (esEvents evtSys)
let (exp, newMap) = getEarlierKeys now evts
newAlarm = getAlarm newMap
writeTVar (esAlarm evtSys) newAlarm
writeTVar (esEvents evtSys) newMap
exps <- readTVar (esExpired evtSys)
writeTVar (esExpired evtSys) (exp:exps) )
where
getEarlierKeys :: UTCTime -> Map UTCTime EventSet -> ([EventSet], Map UTCTime EventSet)
getEarlierKeys clk m =
case deleteFindMinM m of
Just ((k,es), m') ->
if k < clk
then let (exp, lastMap) = getEarlierKeys clk m'
in (es:exp, lastMap)
else ([], m)
Nothing -> ([], m)
getAlarm m | size m == 0 = never
| otherwise = fst $ findMin m
deleteFindMinM :: Map k a -> Maybe ((k, a), Map k a)
deleteFindMinM m = if size m == 0 then Nothing else Just (deleteFindMin m)
-- |Execute expired events
monitorExpiredQueue :: TVar [[EventSet]] -> IO ()
monitorExpiredQueue exp = do
exp <- atomically (do
e <- readTVar exp
case e of
(a:as) -> writeTVar exp [] >> return e
_ -> retry )
mapM_ (mapM_ runEvents) exp
-- |Runs all provided events (which must have expired)
runEvents :: EventSet -> IO ()
runEvents (_,set) = do
let actions = map snd (toList set)
mapM_ forkIO actions
-- |Add an *action* to be performed at *time* by *system*. Returns a unique ID.
addEvent :: EventSystem -> UTCTime -> IO () -> IO EventId
addEvent sys clk act = atomically (addEventSTM sys clk act)
-- |Atomic version of addEvent
addEventSTM :: EventSystem -> UTCTime -> IO () -> STM EventId
addEventSTM sys clk act = do
evts <- readTVar (esEvents sys)
let (old, newMap) = insertLookupWithKey (\_ _ o -> insertEvent o) clk (singletonSet act) evts
num = case old of
Nothing -> 0
Just (n,_) -> n
eid = EvtId clk num
writeTVar (esEvents sys) newMap
alm <- readTVar (esAlarm sys)
when (clk < alm || alm == never)
(writeTVar (esAlarm sys) clk >> writeTVar (esNewAlarm sys) True)
return eid
where
insertEvent :: EventSet -> EventSet
insertEvent (num,set) | num == maxBound = error "maxBound events at given time, something is broken."
| otherwise =
(num+1, insert num act set)
-- |Cancel an event from the system, returning True on success.
cancelEvent :: EventSystem -> EventId -> IO Bool
cancelEvent sys eid = atomically (cancelEventSTM sys eid)
-- |Atomic version of cancelEvent
cancelEventSTM :: EventSystem -> EventId -> STM Bool
cancelEventSTM sys eid@(EvtId clk num) = do
evts <- readTVar (esEvents sys)
let newMap :: Map UTCTime EventSet
prev :: Maybe EventSet
(prev,newMap) = insertLookupWithKey (\_ _ (cnt, old) -> (cnt,delete num old)) clk undefined evts
ret = case prev of
Nothing -> False -- error "Canceling an event that never existed."
Just (_,p) -> case lookup clk newMap of
Nothing -> False
Just (_,m) -> (size p /= size m)
when (eid /= noEvent) (writeTVar (esEvents sys) newMap)
return (eid == noEvent || ret)
-- |Returns the number of pending events.
evtSystemSize :: EventSystem -> STM Int
evtSystemSize sys = do
evts <- readTVar (esEvents sys)
return $ foldr (\(_,m) n -> n + size m) 0 evts
-- |Tracks the alarm time and the earliest event. If an earlier event is added
-- the alarm time is updated and TimerReset is thrown to the expireEvent thread
trackAlarm :: EventSystem -> IO ()
trackAlarm sys = do
tid <- atomically (do
newAlm <- readTVar (esNewAlarm sys)
if newAlm then writeTVar (esNewAlarm sys) False else retry
tid <- readTVar (esThread sys)
i <- case tid of
Just i -> return i
Nothing -> retry
return i )
throwTo tid TimerReset
-- |Returns the time difference in microseconds (potentially returning maxBound <= the real difference)
timeDiffToMicroSec :: NominalDiffTime -> Int
timeDiffToMicroSec = floor . (* 10^6)
data TimerReset = TimerReset deriving (Eq, Ord, Show, Typeable)
instance Exception TimerReset