Add `config.generators.after_generate` for processing to generated files

Register a callback that will get called right after generators has
finished.

This is useful if users want to process generated files.
For example, can execute an autocorrect of RuboCop for generated files
as like following.

```ruby
config.generators.after_generate do |files|
  system("bundle exec rubocop --auto-correct " + files.join(" "), exception: true)
end
```

railties/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Add `config.generators.after_generate` for processing to generated files.
+
+    Register a callback that will get called right after generators has finished.
+
+    *Yuji Yaginuma*
+
 *   Make test file patterns configurable via Environment variables
 
     This makes test file patterns configurable via two environment variables:

railties/lib/rails/configuration.rb

@@ -108,7 +108,7 @@ module Rails
 
     class Generators #:nodoc:
       attr_accessor :aliases, :options, :templates, :fallbacks, :colorize_logging, :api_only
-      attr_reader :hidden_namespaces
+      attr_reader :hidden_namespaces, :after_generate_callbacks
 
       def initialize
         @aliases = Hash.new { |h, k| h[k] = {} }
@@ -118,6 +118,7 @@ module Rails
         @colorize_logging = true
         @api_only = false
         @hidden_namespaces = []
+        @after_generate_callbacks = []
       end
 
       def initialize_copy(source)
@@ -131,6 +132,10 @@ module Rails
         @hidden_namespaces << namespace
       end
 
+      def after_generate(&block)
+        @after_generate_callbacks << block
+      end
+
       def method_missing(method, *args)
         method = method.to_s.sub(/=$/, "").to_sym
 

railties/lib/rails/generators.rb

@@ -80,6 +80,7 @@ module Rails
         templates_path.concat config.templates
         templates_path.uniq!
         hide_namespaces(*config.hidden_namespaces)
+        after_generate_callbacks.replace config.after_generate_callbacks
       end
 
       def templates_path #:nodoc:
@@ -94,6 +95,10 @@ module Rails
         @options ||= DEFAULT_OPTIONS.dup
       end
 
+      def after_generate_callbacks # :nodoc:
+        @after_generate_callbacks ||= []
+      end
+
       # Hold configured generators fallbacks. If a plugin developer wants a
       # generator group to fallback to another group in case of missing generators,
       # they can add a fallback.
@@ -269,6 +274,7 @@ module Rails
         if klass = find_by_namespace(names.pop, names.any? && names.join(":"))
           args << "--help" if args.empty? && klass.arguments.any?(&:required?)
           klass.start(args, config)
+          run_after_generate_callback if config[:behavior] == :invoke
         else
           options     = sorted_groups.flat_map(&:last)
           suggestion  = Rails::Command::Spellchecker.suggest(namespace.to_s, from: options)
@@ -281,6 +287,11 @@ module Rails
         end
       end
 
+      def add_generated_file(file) # :nodoc:
+        (@@generated_files ||= []) << file
+        file
+      end
+
       private
         def print_list(base, namespaces) # :doc:
           namespaces = namespaces.reject { |n| hidden_namespaces.include?(n) }
@@ -314,6 +325,15 @@ module Rails
         def file_lookup_paths # :doc:
           @file_lookup_paths ||= [ "{#{lookup_paths.join(',')}}", "**", "*_generator.rb" ]
         end
+
+        def run_after_generate_callback
+          if defined?(@@generated_files) && !@@generated_files.empty?
+            @after_generate_callbacks.each do |callback|
+              callback.call(@@generated_files)
+            end
+            @@generated_files = []
+          end
+        end
     end
   end
 end

railties/lib/rails/generators/actions/create_migration.rb

@@ -19,6 +19,11 @@ module Rails
           exists? && File.binread(existing_migration) == render
         end
 
+        def invoke!
+          invoked_file = super
+          File.exist?(@destination) ? invoked_file : relative_existing_migration
+        end
+
         def revoke!
           say_destination = exists? ? relative_existing_migration : relative_destination
           say_status :remove, :red, say_destination

railties/lib/rails/generators/migration.rb

@@ -62,13 +62,14 @@ module Rails
         dir, base = File.split(destination)
         numbered_destination = File.join(dir, ["%migration_number%", base].join("_"))
 
-        create_migration numbered_destination, nil, config do
+        file = create_migration numbered_destination, nil, config do
           if ERB.instance_method(:initialize).parameters.assoc(:key) # Ruby 2.6+
             ERB.new(::File.binread(source), trim_mode: "-", eoutvar: "@output_buffer").result(context)
           else
             ERB.new(::File.binread(source), nil, "-", "@output_buffer").result(context)
           end
         end
+        Rails::Generators.add_generated_file(file)
       end
     end
   end

railties/lib/rails/generators/named_base.rb

@@ -22,7 +22,7 @@ module Rails
       no_tasks do
         def template(source, *args, &block)
           inside_template do
-            super
+            Rails::Generators.add_generated_file(super)
           end
         end
 

railties/test/application/generators_test.rb

@@ -217,5 +217,37 @@ module ApplicationTests
       output = rails("generate", "model", "post", "title:string", "body:string", "--force")
       assert_no_match(/The name 'Post' is either already used in your application or reserved/, output)
     end
+
+    test "generators with after_generate callback" do
+      model_file = File.join(app_path, "app/models/post.rb")
+      with_config do |c|
+        c.generators.after_generate do |files|
+          expected = %w(
+            db/migrate/20000101000000_create_posts.rb
+            app/models/post.rb
+            test/models/post_test.rb
+            test/fixtures/posts.yml
+            app/controllers/posts_controller.rb
+            app/views/posts/index.html.erb
+            app/views/posts/edit.html.erb
+            app/views/posts/show.html.erb
+            app/views/posts/new.html.erb
+            app/views/posts/_form.html.erb
+            test/controllers/posts_controller_test.rb
+            test/system/posts_test.rb
+            app/helpers/posts_helper.rb
+          )
+          assert_equal expected, files
+
+          File.open(model_file, "a") { |f| f.write("# Add comment to model") }
+        end
+      end
+
+      travel_to Time.utc(2000, 1, 1)  do
+        rails("generate", "scaffold", "post", "title:string")
+      end
+
+      assert_match(/# Add comment to model/, File.read(model_file))
+    end
   end
 end

railties/test/generators/create_migration_test.rb

@@ -73,6 +73,15 @@ class CreateMigrationTest < Rails::Generators::TestCase
     assert_predicate @migration, :identical?
   end
 
+  def test_invoke_return_existing_file_when_exists_identical
+    migration_exists!
+    create_migration
+
+    invoked_file = nil
+    quietly { invoked_file = @migration.invoke! }
+    assert_equal @existing_migration.relative_existing_migration, invoked_file
+  end
+
   def test_invoke_when_exists_not_identical
     migration_exists!
     create_migration { "different content" }