Extracts `render :update` blocks from controllers

extract_render_update.rb

require 'parser/current'

class ExtractRenderUpdate < Parser::Rewriter
  attr_reader :current_action
  attr_reader :extracted_sources

  def on_block(node)
    replace_with_render_file(node)
    super
  end

  def on_def(node)
    @current_action = node.children.first
    super
  end

  protected
  def replace_with_render_file(node)
    if render_update?(node.children.first)
      @extracted_sources ||= {}
      raise "Action #{current_action} already taken !" if @extracted_sources[current_action]
      @extracted_sources[current_action] = node.children[2..-1].map{|p| p.loc.expression.source}.join("\n")

      replace(node.loc.expression, "render")
    end
  end

  def render_update?(node)
    receiver, method_name, *args = *node
    receiver.nil? && method_name == :render && args[0] && args[0].to_a[0] == :update
  end
end

filepaths = ARGV
filepaths.each do |filepath|
  view_folderpath = filepath.sub('controllers', 'views').sub('_controller.rb', '')
  raise "#{view_folderpath} does not exist" unless Dir.exists?(view_folderpath)

  # find the action and the render :update or render(:update)
  #   extract the block,
  #   put it into a new file called action.rjs
  #   replace with a call to render the file
  buffer = Parser::Source::Buffer.new(filepath).read
  parser = Parser::CurrentRuby.new
  ast = parser.parse(buffer)

  rewriter = ExtractRenderUpdate.new
  rewritten_source = rewriter.rewrite(buffer, ast)
  rewriter.extracted_sources.each do |action, source|
    view_filepath = File.join(view_folderpath, "#{action}.js.erb")
    puts "File #{view_filepath} already exists!" if File.exists?(view_filepath)
    File.open(view_filepath, 'w+') {|f| f << source}
  end

  File.open(filepath, 'w') {|f| f << rewritten_source}
end