在Ruby on Rails中使用Markdown的方法

实现 Markdown 语法和代码语法高亮分别是用的 Redcarpet 和 pygments.rb 两个 Gem：

    https://github.com/vmg/redcarpet
    https://github.com/tmm1/pygments.rb
    https://github.com/richleland/pygments-css
    http://pygments.org/docs/lexers/

在/Gemfile中添加如下两行：

gem 'redcarpet'
gem 'pygments.rb'

需要说明的是，pygments.rb 依赖于 Python，所以确保机器已经安装了 Python 2.x。

然后在/app/controllers/comments_controller.rb中添加相应 redcarpet 和 pygments.rb 的代码：

class ApplicationController < ActionController::Base
 # Prevent CSRF attacks by raising an exception.
 # For APIs, you may want to use :null_session instead.
 protect_from_forgery with: :exception

 helper_method [:markdown]

 # Highlight code with Pygments
 class HTMLwithPygments < Redcarpet::Render::HTML
  def block_code(code, language)
   language = "text" if language.blank?
   sha = Digest::SHA1.hexdigest(code)
   Rails.cache.fetch ["code", language, sha].join("-") do
    Pygments.highlight(code, :lexer => language)
   end
  end
 end

 protected

 # Markdown with Redcarpet
 def markdown(text)
  renderer = HTMLwithPygments.new({
   :filter_html => true,
   :hard_wrap => true,
   :link_attributes => {:rel => 'external nofollow'}
  })

  options = {
   :autolink => true,
   :no_intra_emphasis => true,
   :fenced_code_blocks => true,
   :lax_html_blocks => true,
   :strikethrough => true,
   :superscript => true,
   :tables => true
  }

  Redcarpet::Markdown.new(renderer, options).render(text).html_safe
 end
end

最后在 View 中就可以直接调用 markdown 方法来处理博客正文了：

<%= markdown @post.content %>

语法规则类似 Github 上的 Markdown，码字效率大大提高。