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