Add a test helper

README.md

@@ -222,6 +222,25 @@ To use Webpacker with [Elm](http://elm-lang.org), create a new app with `rails n
 The Elm library and core packages will be added via Yarn and Elm itself. An example `Main.elm` app
 is also added to your project in `app/javascript` so that you can experiment with Elm right away.
 
+## Testing
+
+Webpacker provides a `Webpacker::TestHelper`, which includes a setup to
+pre-compile webpacker assets when running tests.
+
+```rb
+# Test the example react component message
+require "webpacker/test_helper"
+require "application_system_test_case"
+
+class HomesTest < ApplicationSystemTestCase
+  include Webpacker::TestHelper
+  test "can see the hello message" do
+    visit root_url
+    assert_selector "h5", text: "Hello! David"
+  end
+end
+```
+
 ## Troubleshooting
 
 *  If you get this error `ENOENT: no such file or directory - node-sass` on Heroku

lib/tasks/installers.rake

@@ -1,4 +1,4 @@
-INSTALLERS = {
+installers = {
   "Angular": :angular,
   "Elm": :elm,
   "React": :react,
@@ -7,7 +7,7 @@ INSTALLERS = {
 
 namespace :webpacker do
   namespace :install do
-    INSTALLERS.each do |name, task_name|
+    installers.each do |name, task_name|
       desc "Install everything needed for #{name}"
       task task_name => ["webpacker:verify_install"] do
         template = File.expand_path("../install/#{task_name}.rb", __dir__)

lib/tasks/webpacker/compile.rake

@@ -1,6 +1,5 @@
 require "webpacker/env"
 require "webpacker/configuration"
-REGEX_MAP = /\A.*\.map\z/
 
 namespace :webpacker do
   desc "Compile javascript packs using webpack for production with digests"
@@ -25,12 +24,6 @@ namespace :webpacker do
   end
 end
 
-# Compile packs prior to system and controller tests running
-if Rake::Task.task_defined?("test:system")
-  Rake::Task["test:system"].enhance(["webpacker:compile_before_test"])
-  Rake::Task["test:controllers"].enhance(["webpacker:compile_before_test"])
-end
-
 # Compile packs after we've compiled all other assets during precompilation
 if Rake::Task.task_defined?("assets:precompile")
   Rake::Task["assets:precompile"].enhance do

lib/tasks/webpacker/install.rake

@@ -1,12 +1,12 @@
-WEBPACKER_APP_TEMPLATE_PATH = File.expand_path("../../install/template.rb", __dir__)
+install_template_path = File.expand_path("../../install/template.rb", __dir__)
 
 namespace :webpacker do
   desc "Install webpacker in this application"
   task install: [:check_node, :check_yarn] do
     if Rails::VERSION::MAJOR >= 5
-      exec "#{RbConfig.ruby} ./bin/rails app:template LOCATION=#{WEBPACKER_APP_TEMPLATE_PATH}"
+      exec "#{RbConfig.ruby} ./bin/rails app:template LOCATION=#{install_template_path}"
     else
-      exec "#{RbConfig.ruby} ./bin/rake rails:template LOCATION=#{WEBPACKER_APP_TEMPLATE_PATH}"
+      exec "#{RbConfig.ruby} ./bin/rake rails:template LOCATION=#{install_template_path}"
     end
   end
 end

lib/webpacker/test_helper.rb

@@ -0,0 +1,25 @@
+require "rake"
+require "webpacker"
+
+module Webpacker::TestHelper
+  extend ActiveSupport::Concern
+
+  included do
+    setup :compile_webpack_assets
+  end
+
+  def compile_webpack_assets
+    Rails.cache.fetch(["webpacker", "manifest", checksum]) do
+      @load_rakefile ||= Rake.load_rakefile(Rails.root.join("Rakefile"))
+      Rake::Task["webpacker:compile"].invoke
+      Rake::Task["webpacker:compile"].reenable
+    end
+  end
+
+  private
+
+    def checksum
+      files = Dir["#{Webpacker::Configuration.source}/**/*", "package.json", "yarn.lock"].reject { |f| File.directory?(f) }
+      files.map { |f| File.mtime(f).utc.to_i }.max.to_s
+    end
+end