Replace directory_watcher with listen.

Directory_watcher consumed ~25% CPU on big Jekyll projects (depending on
the number of watched files), since it polled for changes every second.

Listen is easier on CPU, as it uses directory change notifications
provided by OS (currently OS X and Linux), falling back to polling when
they are not available.

jekyll.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
 
   s.add_runtime_dependency('liquid', "~> 2.5.2")
   s.add_runtime_dependency('classifier', "~> 1.3")
-  s.add_runtime_dependency('directory_watcher', "~> 1.4.1")
+  s.add_runtime_dependency('listen', "~> 1.3.1")
   s.add_runtime_dependency('maruku', "~> 0.5")
   s.add_runtime_dependency('pygments.rb', "~> 0.5.0")
   s.add_runtime_dependency('commander', "~> 4.1.3")

lib/jekyll/commands/build.rb

@@ -31,25 +31,24 @@ module Jekyll
       #
       # Returns nothing.
       def self.watch(site, options)
-        require 'directory_watcher'
+        require 'listen'
+        require 'pathname'
 
         source = options['source']
-        destination = options['destination']
+        destination = Pathname.new(options['destination'])
+                              .relative_path_from(Pathname.new(source))
+                              .to_path
 
         Jekyll.logger.info "Auto-regeneration:", "enabled"
 
-        dw = DirectoryWatcher.new(source, :glob => self.globs(source, destination), :pre_load => true)
-        dw.interval = 1
-
-        dw.add_observer do |*args|
+        Listen.to(source, :ignore => %r{#{Regexp.escape(destination)}}) do |modified, added, removed|
           t = Time.now.strftime("%Y-%m-%d %H:%M:%S")
-          print Jekyll.logger.formatted_topic("Regenerating:") + "#{args.size} files at #{t} "
+          n = modified.length + added.length + removed.length
+          print Jekyll.logger.formatted_topic("Regenerating:") + "#{n} files at #{t} "
           self.process_site(site)
           puts  "...done."
         end
 
-        dw.start
-
         unless options['serving']
           trap("INT") do
             puts "     Halting auto-regeneration."