我已經放在一起實施下面的一個例子。其基本思想是:
- 使用fsnotify軟件包監視文件更改。
- 使用
sourceFileRange
來流式傳輸文件以前未消耗的部分。
- 使用
MVar
讓fsnotify回調信號Source
繼續讀取。
這裏假定源文件只添加到永遠不會刪除或縮短。
import Control.Concurrent (forkIO, threadDelay)
import Control.Concurrent.MVar (MVar, newEmptyMVar, putMVar,
takeMVar)
import Control.Exception (IOException, try)
import Control.Monad (forever, void, when)
import Control.Monad.IO.Class (liftIO)
import Data.ByteString (ByteString)
import qualified Data.ByteString as S
import Data.Conduit (MonadResource, Source, bracketP,
runResourceT, ($$), ($=))
import Data.Conduit.Binary (sourceFileRange)
import qualified Data.Conduit.List as CL
import Data.IORef (IORef, modifyIORef, newIORef,
readIORef)
import Data.Time (getCurrentTime)
import Filesystem (canonicalizePath)
import Filesystem.Path.CurrentOS (decodeString, directory)
import System.FSNotify (Event (..), startManager,
stopManager, watchDir)
tryIO :: IO a -> IO (Either IOException a)
tryIO = try
sourceFileForever :: MonadResource m => FilePath -> Source m ByteString
sourceFileForever fp' = bracketP startManager stopManager $ \manager -> do
fp <- liftIO $ canonicalizePath $ decodeString fp'
baton <- liftIO newEmptyMVar
liftIO $ watchDir manager (directory fp) (const True) $ \event -> void $ tryIO $ do
fpE <- canonicalizePath $
case event of
Added x _ -> x
Modified x _ -> x
Removed x _ -> x
when (fpE == fp) $ putMVar baton()
consumedRef <- liftIO $ newIORef 0
loop baton consumedRef
where
loop :: MonadResource m => MVar() -> IORef Integer -> Source m ByteString
loop baton consumedRef = forever $ do
consumed <- liftIO $ readIORef consumedRef
sourceFileRange fp' (Just consumed) Nothing $= CL.iterM counter
liftIO $ takeMVar baton
where
counter bs = liftIO $ modifyIORef consumedRef (+ fromIntegral (S.length bs))
main :: IO()
main = do
let fp = "foo.txt"
writeFile fp "Hello World!"
_ <- forkIO $ runResourceT $ sourceFileForever fp $$ CL.mapM_ (liftIO . print)
forever $ do
now <- getCurrentTime
appendFile fp $ show now ++ "\n"
threadDelay 1000000