class ActionView::CacheExpiry

def clear_cache_if_necessary
  @mutex.synchronize do
    watched_dirs = dirs_to_watch
    return if watched_dirs.empty?
    if watched_dirs != @watched_dirs
      @watched_dirs = watched_dirs
      @watcher = @watcher_class.new([], watched_dirs) do
        clear_cache
      end
      @watcher.execute
    else
      @watcher.execute_if_updated
    end
  end
end