enables the evented monitor in new applications

railties/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   New applications are generated with the evented file system monitor enabled
+    on Linux and Mac OS X.
+
+    *Xavier Noria*
+
 *   Add dummy files for apple-touch-icon.png and apple-touch-icon.png. GH#23427
 
     *Alexey Zabelin*

railties/lib/rails/generators/app_base.rb

@@ -390,6 +390,10 @@ module Rails
         !options[:skip_spring] && !options.dev? && Process.respond_to?(:fork) && !RUBY_PLATFORM.include?("cygwin")
       end
 
+      def os_supports_listen_out_of_the_box?
+        RbConfig::CONFIG['host_os'] =~ /darwin|linux/
+      end
+
       def run_bundle
         bundle_command('install') if bundle_install?
       end

railties/lib/rails/generators/rails/app/templates/Gemfile

@@ -38,6 +38,9 @@ group :development do
   gem 'web-console', '~> 3.0'
   <%- end -%>
 <%- end -%>
+<% if  os_supports_listen_out_of_the_box? -%>
+  gem 'listen', '~> 3.0.5'
+<% end  -%>
 <% if  spring_install? -%>
   # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
   gem 'spring'

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -58,5 +58,5 @@ Rails.application.configure do
 
   # Use an evented file watcher to asynchronously detect changes in source code,
   # routes, locales, etc. This feature depends on the listen gem.
-  # config.file_watcher = ActiveSupport::EventedFileUpdateChecker
+  <%= '# ' unless os_supports_listen_out_of_the_box? %>config.file_watcher = ActiveSupport::EventedFileUpdateChecker
 end

railties/test/generators/app_generator_test.rb

@@ -479,6 +479,28 @@ class AppGeneratorTest < Rails::Generators::TestCase
     end
   end
 
+  def test_inclusion_of_listen_related_gems
+    run_generator
+    if RbConfig::CONFIG['host_os'] =~ /darwin|linux/
+      assert_gem 'listen'
+    else
+      assert_file 'Gemfile' do |content|
+        assert_no_match(/listen/, content)
+      end
+    end
+  end
+
+  def test_evented_file_update_checker_config
+    run_generator
+    assert_file 'config/environments/development.rb' do |content|
+      if RbConfig::CONFIG['host_os'] =~ /darwin|linux/
+        assert_match(/^\s*config.file_watcher = ActiveSupport::EventedFileUpdateChecker/, content)
+      else
+        assert_match(/^\s*# config.file_watcher = ActiveSupport::EventedFileUpdateChecker/, content)
+      end
+    end
+  end
+
   def test_template_from_dir_pwd
     FileUtils.cd(Rails.root)
     assert_match(/It works from file!/, run_generator([destination_root, "-m", "lib/template.rb"]))