instance method clear_cache_if_necessary

# File actionview/lib/action_view/cache_expiry.rb, line 22
    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