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|
{- notification broadcaster
-
- This is used to allow clients to block until there is a new notification
- that some thing occurred. It does not communicate what the change is,
- it only provides blocking reads to wait on notifications.
-
- Multiple clients are supported. Each has a unique id.
-
- Copyright 2012 Joey Hess <joey@kitenet.net>
-
- License: BSD-2-clause
-}
module Utility.NotificationBroadcaster (
NotificationBroadcaster,
NotificationHandle,
NotificationId,
newNotificationBroadcaster,
newNotificationHandle,
notificationHandleToId,
notificationHandleFromId,
sendNotification,
waitNotification,
checkNotification,
) where
import Common
import Control.Concurrent.STM
{- One TMVar per client, which are empty when no notification is pending,
- and full when a notification has been sent but not yet seen by the
- client. The list TMVar is never empty, so never blocks. -}
type NotificationBroadcaster = TMVar [TMVar ()]
newtype NotificationId = NotificationId Int
deriving (Read, Show, Eq, Ord)
{- Handle given out to an individual client. -}
data NotificationHandle = NotificationHandle NotificationBroadcaster NotificationId
newNotificationBroadcaster :: IO NotificationBroadcaster
newNotificationBroadcaster = atomically $ newTMVar []
{- Allocates a notification handle for a client to use.
-
- An immediate notification can be forced the first time waitNotification
- is called on the handle. This is useful in cases where a notification
- may be sent while the new handle is being constructed. Normally,
- such a notification would be missed. Forcing causes extra work,
- but ensures such notifications get seen.
-}
newNotificationHandle :: Bool -> NotificationBroadcaster -> IO NotificationHandle
newNotificationHandle force b = NotificationHandle
<$> pure b
<*> addclient
where
addclient = atomically $ do
s <- if force
then newTMVar ()
else newEmptyTMVar
l <- takeTMVar b
putTMVar b $ l ++ [s]
return $ NotificationId $ length l
{- Extracts the identifier from a notification handle.
- This can be used to eg, pass the identifier through to a WebApp. -}
notificationHandleToId :: NotificationHandle -> NotificationId
notificationHandleToId (NotificationHandle _ i) = i
notificationHandleFromId :: NotificationBroadcaster -> NotificationId -> NotificationHandle
notificationHandleFromId = NotificationHandle
{- Sends a notification to all clients. -}
sendNotification :: NotificationBroadcaster -> IO ()
sendNotification b = do
l <- atomically $ readTMVar b
mapM_ notify l
where
notify s = atomically $
whenM (isEmptyTMVar s) $
putTMVar s ()
{- Used by a client to block until a new notification is available since
- the last time it tried. -}
waitNotification :: NotificationHandle -> IO ()
waitNotification (NotificationHandle b (NotificationId i)) = do
l <- atomically $ readTMVar b
atomically $ takeTMVar (l !! i)
{- Used by a client to check if there has been a new notification since the
- last time it checked, without blocking. -}
checkNotification :: NotificationHandle -> IO Bool
checkNotification (NotificationHandle b (NotificationId i)) = do
l <- atomically $ readTMVar b
maybe False (const True) <$> atomically (tryTakeTMVar (l !! i))
|
