For the guard-tired

watchist.rb

require 'thread'

# I am very Watchist. Use me in your config.ru like so:
# Watchist.start_and_mount_in_rack(self)
class Watchist
  CHMUX = Mutex.new
  $watchist_channels = []
  $watchist_logger = Logger.new($stderr)
  $watchist_logger.progname = "watchist"
  
  def start(dir_path=Dir.pwd)
    dir_path = File.expand_path(dir_path)
    require 'rb-fsevent'
    Thread.new do
      begin
        ctr = 0
        fsevent = FSEvent.new
        fsevent.watch(dir_path, file_events: true) do |file_paths|
          destinations = CHMUX.synchronize { $watchist_channels.dup }
          file_paths.each do |file_path|
            next if file_path.include?('/.') # Dirty check for anything hidden
            ctr += 1
            $watchist_logger.debug { "Dispatching change in %s to %d listeners" % [file_path, destinations.length] }
            destinations.each { |q| q << {seq: ctr, path: file_path} }
          end
        end 
        fsevent.run
      rescue StandardError => e
        $stderr.puts e.inspect
        raise e
      end
    end
    true
  end
  
  def self.event_source_script
    return '' if ENV['RACK_ENV'] == 'production'
    return <<-EOF
    <script>
    function replaceQSParams(urlStr, overrides) {
      var url = new URL(urlStr, window.location);
      var qsp = new URLSearchParams(url.search);
      Object.keys(overrides).forEach(function(key) {
        qsp.set(key, overrides[key]);
      });
      url.search = qsp.toString();
      return url.toString();
    }

    function reloadLinkElem(linkElem, seqNumber) {
      var replacementLinkElem = linkElem.cloneNode()
      replacementLinkElem.href = replaceQSParams(replacementLinkElem.href, {watchist: seqNumber})
      linkElem.parentNode.insertBefore(replacementLinkElem, linkElem.nextSibling);
      window.setTimeout(function() {
        linkElem.parentNode.removeChild(linkElem);
      }, 100);
    }
    function reloadStyles(seqNumber) {
      var links = document.querySelectorAll("link");
      links.forEach(reloadLinkElem);
    }

    var evtSource = new EventSource("/_watchist_");
    evtSource.addEventListener("file_change", function(e) {
      var obj = JSON.parse(e.data);
      var changed_path = obj.path;
      if(changed_path.match(/css$/)) {
        console.debug("Filesystem CSS change, reloading stylesheets");
        reloadStyles(obj.seq);
      }
    }, false);
    </script>
    EOF
  end

  class Hijacker
    TERM = "\r\n"
    
    def initialize(q)
      @q = q
    end
     
    def write_with_chunked_encoding(socket, chunk)
      chunk_payload = [chunk.bytesize.to_s(16), TERM, chunk, TERM].join
      socket.write(chunk_payload)
    end
        
    def call(socket)
      $watchist_logger.debug { "Listener joined" }
      write_with_chunked_encoding socket, "event: attach\n"
      write_with_chunked_encoding socket, "data: {hello: 1}"
      write_with_chunked_encoding socket, "\n\n"
      socket.flush
      loop do
        begin
          change_event = @q.pop(nb=true)
          write_with_chunked_encoding socket, "event: file_change\n"
          write_with_chunked_encoding socket, "data: %s" % JSON.dump(change_event)
          write_with_chunked_encoding socket, "\n\n"
          socket.flush
        rescue ThreadError # empty
          sleep 0.1
        end
      end
    rescue Errno::EPIPE
    ensure
      io.close rescue nil
      $watchist_logger.debug { "Listener leaving, detaching the queue" }
      CHMUX.synchronize do
        $watchist_channels.delete(@q)
      end
    end
  end

  def initialize(app)
    @app = app
  end

  def call(env)
    req = Rack::Request.new(env)
    if req.fullpath.start_with?('/_watchist_') && ENV['RACK_ENV'] != 'production'
      $watchist_started ||= start
      watchist_event_stream
    else
      @app.call(env)
    end
  end
  
  def watchist_event_stream
    q = Queue.new
    $watchist_channels << q
    [200, {"Content-Type" => "text/event-stream", "Transfer-Encoding" => "chunked", "rack.hijack" => Hijacker.new(q)}, []]
  end
end