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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>
-
- Licensed under the GNU GPL version 3 or higher.
-}
module Utility.NotificationBroadcaster (
NotificationBroadcaster,
NotificationHandle,
NotificationId,
newNotificationBroadcaster,
newNotificationHandle,
notificationHandleToId,
notificationHandleFromId,
sendNotification,
waitNotification,
) where
import Common
import Control.Concurrent.STM
import Control.Concurrent.MSampleVar
{- One MSampleVar per client. The TMVar is never empty, so never blocks. -}
type NotificationBroadcaster = TMVar [MSampleVar ()]
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 = do
s <- if force
then newSV ()
else newEmptySV
atomically $ do
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 = writeSV 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
readSV (l !! i)
|
