src/Pure/General/file_watcher.scala

manage changes of external files;
tuned;

/*  Title:      Pure/General/file_watcher.scala
    Author:     Makarius

Watcher for file-system events.
*/

package isabelle


import java.io.{File => JFile}
import java.nio.file.FileSystems
import java.nio.file.{WatchKey, WatchEvent, Path => JPath}
import java.nio.file.StandardWatchEventKinds.{ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY}

import scala.collection.JavaConversions


object File_Watcher
{
  def apply(handle: Set[JFile] => Unit, delay: => Time = Time.seconds(0.5)): File_Watcher =
    new File_Watcher(handle, delay)


  /* internal state */

  sealed case class State(
    dirs: Map[JFile, WatchKey] = Map.empty,
    changed: Set[JFile] = Set.empty)
}

class File_Watcher private(handle: Set[JFile] => Unit, delay: Time)
{
  private val state = Synchronized(File_Watcher.State())
  private val watcher = FileSystems.getDefault.newWatchService()


  /* registered directories */

  def register(dir: JFile): Unit =
    state.change(st =>
      st.dirs.get(dir) match {
        case Some(key) if key.isValid => st
        case _ =>
          val key = dir.toPath.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY)
          st.copy(dirs = st.dirs + (dir -> key))
      })

  def deregister(dir: JFile): Unit =
    state.change(st =>
      st.dirs.get(dir) match {
        case None => st
        case Some(key) =>
          key.cancel
          st.copy(dirs = st.dirs - dir)
      })


  /* changed directory entries */

  private val delay_changed = Standard_Thread.delay_last(delay)
  {
    val changed = state.change_result(st => (st.changed, st.copy(changed = Set.empty)))
    handle(changed)
  }

  private val watcher_thread = Standard_Thread.fork("File_Watcher", daemon = true)
  {
    try {
      while (true) {
        val key = watcher.take
        val has_changed =
          state.change_result(st =>
            {
              val (remove, changed) =
                st.dirs.collectFirst({ case (dir, key1) if key == key1 => dir }) match {
                  case Some(dir) =>
                    val events =
                      JavaConversions.collectionAsScalaIterable(
                        key.pollEvents.asInstanceOf[java.util.List[WatchEvent[JPath]]])
                    val remove = if (key.reset) None else Some(dir)
                    val changed =
                      (Set.empty[JFile] /: events.iterator) {
                        case (set, event) => set + dir.toPath.resolve(event.context).toFile
                      }
                    (remove, changed)
                  case None =>
                    key.pollEvents
                    key.reset
                    (None, Set.empty[JFile])
                }
              (changed.nonEmpty, st.copy(dirs = st.dirs -- remove, changed = st.changed ++ changed))
            })
        if (has_changed) delay_changed.invoke()
      }
    }
    catch { case Exn.Interrupt() => }
  }


  /* shutdown */

  def shutdown()
  {
    watcher_thread.interrupt
    watcher_thread.join
    delay_changed.revoke
  }
}