fileWatcherMiddleware.js

const middleware = () => store => {
  const fileWatcher = new FileWatcher();
  fileWatcher.on('file-changed', filename => {
    store.dispatch({ type: 'FILE_CHANGED', filename });
  });

  // Make sure we're watching files that may be included in the store's initial state
  const initialFiles = store.getState().activeFiles;
  fileWatcher.watchFiles(initialFiles);

  return next => action => {
    // Get the state before and after the action was performed
    const previousFiles = store.getState().activeFiles;
    next(action);
    const nextFiles = store.getState().activeFiles;

    // See what changed before and after
    const filesToUnwatch = _.difference(previousFiles, nextFiles);
    const filesToWatch = _.difference(nextFiles, previousFiles);

    // Respond to changes
    fileWatcher.unwatchFiles(filesToUnwatch);
    fileWatcher.watchFiles(filesToWatch);
  };
};