Support React on Rails by merging Webpacker Lite

See #594

From a long discussion on #464, this issue will summarize. I'll soon be
posting proposed changes to the README.md.

Summary of changes

* Move base url out from manifest.json to manifest.rb
* Assign env variables to dev server settings so it can be overridden at
  runtime.
* The keys for dev_server should use same format as of now as documented
  in Paths on the README.md. Note that
* hot is a new setting to indicate that the dev_server is used with hot
  reloading, which means that CSS should be inlined to be hot loaded.
  The presence of dev_server means that the webpack-dev-server is used for
  the given env.

development:
  // put the created files to the /public/webpack/development directory
  public_output_path: webpack/development

 //# if dev_server is not provided, then dev_server is not used
 dev_server:
   hot: true # This is a new setting
   static: false
   host: localhost
   https: false

Gemfile

@@ -9,3 +9,13 @@ gem "rubocop", ">= 0.47", require: false
 group :test do
   gem "minitest", "~> 5.0"
 end
+
+if ENV["USE_PRY"]
+  gem "awesome_print"
+  gem "pry"
+  gem "pry-byebug"
+  gem "pry-doc"
+  gem "pry-rails"
+  gem "pry-rescue"
+  gem "pry-stack_explorer"
+end

Gemfile.lock

@@ -48,12 +48,19 @@ GEM
       tzinfo (~> 1.1)
     arel (7.1.4)
     ast (2.3.0)
+    awesome_print (1.8.0)
+    binding_of_caller (0.7.2)
+      debug_inspector (>= 0.0.1)
     builder (3.2.3)
+    byebug (9.0.6)
+    coderay (1.1.1)
     concurrent-ruby (1.0.5)
+    debug_inspector (0.0.3)
     erubis (2.7.0)
     globalid (0.3.7)
       activesupport (>= 4.1.0)
     i18n (0.8.1)
+    interception (0.5)
     loofah (2.0.3)
       nokogiri (>= 1.5.9)
     mail (2.6.4)
@@ -71,6 +78,24 @@ GEM
     parser (2.4.0.0)
       ast (~> 2.2)
     powerpack (0.1.1)
+    pry (0.10.4)
+      coderay (~> 1.1.0)
+      method_source (~> 0.8.1)
+      slop (~> 3.4)
+    pry-byebug (3.4.2)
+      byebug (~> 9.0)
+      pry (~> 0.10)
+    pry-doc (0.10.0)
+      pry (~> 0.9)
+      yard (~> 0.9)
+    pry-rails (0.3.6)
+      pry (>= 0.10.4)
+    pry-rescue (1.4.5)
+      interception (>= 0.5)
+      pry
+    pry-stack_explorer (0.4.9.2)
+      binding_of_caller (>= 0.7)
+      pry (>= 0.9.11)
     rack (2.0.1)
     rack-test (0.6.3)
       rack (>= 1.0)
@@ -106,6 +131,7 @@ GEM
       ruby-progressbar (~> 1.7)
       unicode-display_width (~> 1.0, >= 1.0.1)
     ruby-progressbar (1.8.1)
+    slop (3.6.0)
     sprockets (3.7.1)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
@@ -121,17 +147,25 @@ GEM
     websocket-driver (0.6.5)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.2)
+    yard (0.9.9)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
+  awesome_print
   bundler (~> 1.12)
   minitest (~> 5.0)
+  pry
+  pry-byebug
+  pry-doc
+  pry-rails
+  pry-rescue
+  pry-stack_explorer
   rails
   rake (>= 11.1)
   rubocop (>= 0.47)
   webpacker!
 
 BUNDLED WITH
-   1.14.6
+   1.15.3

README.md

@@ -367,8 +367,14 @@ development:
     host: 0.0.0.0
     port: 8080
     https: false
+    hot: false
 ```
 
+If you have hot turned to `true`, then the `stylesheet_pack_tag` generates no output, as you will want
+to configure your styles to be inlined in your JavaScript for hot reloading. During production and testing, the
+`stylesheet_pack_tag` will create the appropriate HTML tags.
+
+
 #### Resolved Paths
 
 If you are adding webpacker to an existing app that has most of the assets inside
@@ -601,8 +607,12 @@ and
 
 #### React
 
-You may consider using [react-rails](https://github.com/reactjs/react-rails) or
-[webpacker-react](https://github.com/renchap/webpacker-react) for more advanced react integration. However here is how you can do it yourself:
+The most widely used React integration is [shakacode/react_on_rails](https://github.com/shakacode/react_on_rails) which includes support for server rendering, redux and react-router.
+
+Other alternatives include [react-rails](https://github.com/reactjs/react-rails) and 
+[webpacker-react](https://github.com/renchap/webpacker-react) for more advanced react integration. 
+
+If you're not concerned with view helpers to pass props or server rendering, can do it yourself:
 
 ```erb
 <%# views/layouts/application.html.erb %>
@@ -1103,6 +1113,8 @@ git push heroku master
 Webpacker lazily compiles assets in test env so you can write your tests without any extra
 setup and everything will just work out of the box.
 
+Note, [React on Rails] users should set configuration value `compile` to false, as React on Rails
+handle compilation for test and production environments.
 
 Here is a sample system test case with hello_react example component:
 

lib/install/config/webpack/shared.js

@@ -27,8 +27,7 @@ module.exports = {
 
   output: {
     filename: '[name].js',
-    path: output.path,
-    publicPath: output.publicPath
+    path: output.path
   },
 
   module: {
@@ -39,7 +38,6 @@ module.exports = {
     new webpack.EnvironmentPlugin(JSON.parse(JSON.stringify(env))),
     new ExtractTextPlugin(env.NODE_ENV === 'production' ? '[name]-[hash].css' : '[name].css'),
     new ManifestPlugin({
-      publicPath: output.publicPath,
       writeToFileEmit: true
     })
   ],

lib/install/config/webpacker.yml

@@ -34,6 +34,7 @@ development:
     host: localhost
     port: 8080
     https: false
+    hot: false
 
 test:
   <<: *default

lib/webpacker.rb

@@ -2,14 +2,15 @@ module Webpacker
   extend self
 
   def bootstrap
-    Webpacker::Env.load
-    Webpacker::Configuration.load
-    Webpacker::Manifest.load
+    Webpacker::Env.load_instance
+    Webpacker::Configuration.load_instance
+    Webpacker::DevServer.load_instance
+    Webpacker::Manifest.load_instance
   end
 
   def compile
     Webpacker::Compiler.compile
-    Webpacker::Manifest.load
+    Webpacker::Manifest.load_instance
   end
 
   def env
@@ -19,6 +20,7 @@ def env
 
 require "webpacker/env"
 require "webpacker/configuration"
+require "webpacker/dev_server"
 require "webpacker/manifest"
 require "webpacker/compiler"
 require "webpacker/railtie" if defined?(Rails)

lib/webpacker/configuration.rb

@@ -4,12 +4,21 @@
 
 class Webpacker::Configuration < Webpacker::FileLoader
   class << self
+    def reset
+      @defaults = nil
+      super
+    end
+
     def entry_path
       source_path.join(fetch(:source_entry_path))
     end
 
+    def public_output_path
+      fetch(:public_output_path)
+    end
+
     def output_path
-      public_path.join(fetch(:public_output_path))
+      public_path.join(public_output_path)
     end
 
     def manifest_path
@@ -45,19 +54,30 @@ def fetch(key)
     end
 
     def data
-      load if Webpacker.env.development?
-      raise Webpacker::FileLoader::FileLoaderError.new("Webpacker::Configuration.load must be called first") unless instance
+      load_instance if Webpacker.env.development?
+      raise Webpacker::FileLoader::FileLoaderError.new("Webpacker::Configuration.load_data must be called first") unless instance
       instance.data
     end
 
     def defaults
       @defaults ||= HashWithIndifferentAccess.new(YAML.load(default_file_path.read)[Webpacker.env])
     end
+
+    # Uses the webpack dev server host if appropriate
+    def output_path_or_url
+      if Webpacker::DevServer.dev_server?
+        Webpacker::DevServer.base_url
+      else
+        # Ensure we start with a slash so that the asset helpers don't prepend the default asset
+        # pipeline locations.
+        public_output_path.starts_with?("/") ? public_output_path : "/#{public_output_path}"
+      end
+    end
   end
 
   private
-    def load
-      return super unless File.exist?(@path)
+    def load_data
+      return Webpacker::Configuration.defaults unless File.exist?(@path)
       HashWithIndifferentAccess.new(YAML.load(File.read(@path))[Webpacker.env])
     end
 end

lib/webpacker/dev_server.rb

@@ -0,0 +1,78 @@
+# Same convention as manifest/configuration.rb
+
+# Loads webpacker configuration from config/webpacker.yml
+
+require "webpacker/configuration"
+
+class Webpacker::DevServer < Webpacker::FileLoader
+  class << self
+    def dev_server?
+      !dev_server_values.nil?
+    end
+
+    # read settings for dev_server
+    def hot?
+      return false unless dev_server?
+      if ENV["WEBPACKER_HMR"].present?
+        val = ENV["WEBPACKER_HMR"].downcase
+        return true if val == "true"
+        return false if val == "false"
+        raise new ArgumentError("WEBPACKER_HMR value is #{ENV['WEBPACKER_HMR']}. Set to TRUE|FALSE")
+      end
+      fetch(:hot)
+    end
+
+    def host
+      fetch(:host)
+    end
+
+    def port
+      fetch(:port)
+    end
+
+    def https?
+      fetch(:https)
+    end
+
+    def protocol
+      https? ? "https" : "http"
+    end
+
+    def file_path
+      Webpacker::Configuration.file_path
+    end
+
+    # Uses the hot_reloading_host if appropriate
+    def base_url
+      "#{protocol}://#{host}:#{port}"
+    end
+
+    private
+
+    def dev_server_values
+      data.fetch(:dev_server, nil)
+    end
+
+    def fetch(key)
+      return nil unless dev_server?
+      dev_server_values.fetch(key, dev_server_defaults[key])
+    end
+
+    def data
+      load_instance if Webpacker.env.development?
+      unless instance
+        raise Webpacker::FileLoader::FileLoaderError.new("Webpacker::DevServer.load_data must be called first")
+      end
+      instance.data
+    end
+
+    def dev_server_defaults
+      @defaults ||= Webpacker::Configuration.defaults[:dev_server]
+    end
+  end
+
+  private
+    def load_data
+      Webpacker::Configuration.instance.data
+    end
+end

lib/webpacker/env.rb

@@ -14,7 +14,7 @@ def file_path
   end
 
   private
-    def load
+    def load_data
       environments = File.exist?(@path) ? YAML.load(File.read(@path)).keys : [].freeze
       return ENV["NODE_ENV"] if environments.include?(ENV["NODE_ENV"])
       return Rails.env if environments.include?(Rails.env)

lib/webpacker/file_loader.rb

@@ -4,21 +4,44 @@ class NotFoundError < StandardError; end
   class FileLoaderError < StandardError; end
 
   class_attribute :instance
-  attr_accessor :data
+  attr_accessor :data, :mtime, :path
 
   class << self
-    def load(path = file_path)
+    def load_instance(path = file_path)
+      # Assume production is 100% cached and don't reload if file's mtime not changed
+      cached = self.instance && # if we have a singleton
+        (env == "production" || # skip if production bc always cached
+          (File.exist?(path) && self.instance.mtime == File.mtime(path))) # skip if mtime not changed
+
+      return if cached
       self.instance = new(path)
     end
+
+    def file_path
+      raise FileLoaderError.new("Subclass of Webpacker::FileLoader should override this method")
+    end
+
+    def reset
+      self.instance = nil
+      load_instance
+    end
+
+    private
+
+    # Prefer the NODE_ENV to the rails env.
+    def env
+      ENV["NODE_ENV"].presence || Rails.env
+    end
   end
 
   private
     def initialize(path)
       @path = path
-      @data = load
+      @mtime = File.exist?(path) ? File.mtime(path) : nil
+      @data = load_data
     end
 
-    def load
+    def load_data
       {}.freeze
     end
 end