[close #381] Avoid Sprockets deprecations

- Use both `register_engine` and `register_transformer` in railtie.rb. 
- In the test postprocessor use an object that responds to `call` and `new.render`.

.travis.yml

@@ -15,6 +15,7 @@ gemfile:
   - gemfiles/Gemfile-sprockets-2-12
   - gemfiles/Gemfile-sprockets-2-8
   - gemfiles/Gemfile-sprockets-3-0
+  - gemfiles/Gemfile-sprockets-4-0
   - gemfiles/Gemfile-sprockets-rails-2-2
   - gemfiles/Gemfile-sprockets-rails-master
   - gemfiles/Gemfile-sass-3-1

gemfiles/Gemfile-sprockets-4-0

@@ -0,0 +1,7 @@
+source "https://rubygems.org"
+
+# Specify your gem"s dependencies in sass-rails.gemspec
+gemspec path: ".."
+
+gem "rails"
+gem "sprockets", "~> 4.x"

lib/sass/rails/railtie.rb

@@ -54,8 +54,17 @@ class Railtie < ::Rails::Railtie
       end
 
       config.assets.configure do |env|
-        env.register_engine '.sass', Sass::Rails::SassTemplate
-        env.register_engine '.scss', Sass::Rails::ScssTemplate
+        if env.respond_to?(:register_engine)
+          env.register_engine '.sass', Sass::Rails::SassTemplate, silence_deprecation: true
+          env.register_engine '.scss', Sass::Rails::ScssTemplate, silence_deprecation: true
+        end
+
+        if env.respond_to?(:register_transformer)
+          env.register_transformer 'text/sass', 'text/css',
+            Sprockets::SassProcessor.new(importer: SassImporter, sass_config: app.config.sass)
+          env.register_transformer 'text/scss', 'text/css',
+            Sprockets::ScssProcessor.new(importer: SassImporter, sass_config: app.config.sass)
+        end
 
         env.context_class.class_eval do
           class_attribute :sass_config

test/fixtures/engine_project/engine_project.gemspec

@@ -7,11 +7,11 @@ require "engine_project/version"
 Gem::Specification.new do |s|
   s.name        = "engine_project"
   s.version     = EngineProject::VERSION
-  s.authors     = ["TODO: Your name"]
-  s.email       = ["TODO: Your email"]
-  s.homepage    = "TODO"
-  s.summary     = "TODO: Summary of EngineProject."
-  s.description = "TODO: Description of EngineProject."
+  s.authors     = ["Your name"]
+  s.email       = ["Your email"]
+  s.homepage    = ""
+  s.summary     = "Summary of EngineProject."
+  s.description = "Description of EngineProject."
 
   s.files = Dir["{app,config,db,lib}/**/*", "MIT-LICENSE", "Rakefile", "README.rdoc"]
   s.test_files = Dir["test/**/*"]

test/fixtures/scss_project/config/initializers/postprocessor.rb

@@ -1,5 +1,25 @@
-Rails.application.config.assets.configure do |env|
-  env.register_postprocessor 'text/css', :postprocessor do |context, css|
-    css.gsub /@import/, 'fail engine'
+
+class SassRailsTestPostProcessor
+  def initialize(filename, &block)
+    @filename = filename
+    @source   = block.call
+  end
+
+  def render(context)
+    self.class.run(@source)
+  end
+
+  def self.run(source)
+    source.gsub /@import/, 'fail engine'
   end
+
+  def self.call(input)
+    source = input[:data]
+    result = run(source)
+    { data: result }
+  end
+end
+
+Rails.application.config.assets.configure do |env|
+  env.register_postprocessor 'text/css', SassRailsTestPostProcessor
 end