scope jdk_watchService_start.

static new Flag #started;
static new MultiMap<WatchKey, O> #listeners; // values: voidfunc(File)
static int #interval = 50; // larger interval helps show less duplicate events (https://stackoverflow.com/questions/16777869/java-7-watchservice-ignoring-multiple-occurrences-of-the-same-event/34818620#34818620)

static WatchService jdk_watchService_start() {
  final WatchService service = jdk_defaultWatchService();
  if (started.raise()) thread "Watch Service" {
    while licensed { 
      WatchKey key = service.poll(10, TimeUnit.SECONDS);
      if (key != null) {
        Path dir = cast key.watchable();
        sleep(interval);
        L<WatchEvent<?>> events = key.pollEvents();
        new LinkedHashSet<File> changedFiles;
        for (WatchEvent<?> event : reversedIterator(events)) pcall {
          final Path changed = (Path) event.context();
          File full = newFile(dir.toFile(), changed.toFile().getPath());
          changedFiles.add(full);
          //print("WatchService Changed: " + full + ", " + event.kind());
        }
        for (File f : changedFiles)
          pcallFAll(listeners.get(key), f);
        key.reset();
      }
    }
  }
  ret service;
}

end scope

Began life as a copy of #1019342