Domain-Specific Languages

Example 8-11. A simple DSL for generating XML output

Code View:
class XML
  # Create an instance of this class, specifying a stream or object to
  # hold the output. This can be any object that responds to <<(String).
  def initialize(out)
    @out = out  # Remember where to send our output
  end

  # Output the specified object as CDATA, return nil.
  def content(text)
    @out << text.to_s
    nil
  end

  # Output the specified object as a comment, return nil.
  def comment(text)
    @out << "<!-- #{text} -->"
    nil
  end

  # Output a tag with the specified name and attributes.
  # If there is a block invoke it to output or return content.
  # Return nil.
  def tag(tagname, attributes={})
    # Output the tag name
    @out << "<#{tagname}"

    # Output the attributes
    attributes.each {|attr,value| @out << " #{attr}='#{value}'" }
    
    if block_given?
      # This block has content
      @out << '>'             # End the opening tag
      content = yield         # Invoke the block to output or return content
      if content              # If any content returned
        @out << content.to_s  # Output it as a string
      end
      @out << "</#{tagname}>" # Close the tag
    else 
      # Otherwise, this is an empty tag, so just close it.
      @out << '/>'
    end
    nil # Tags output themselves, so they don't return any content
  end

  # The code below is what changes this from an ordinary class into a DSL.
  # First: any unknown method is treated as the name of a tag.
  alias method_missing tag

  # Second: run a block in a new instance of the class.
  def self.generate(out, &block)
    XML.new(out).instance_eval(&block)
  end
end

pagetitle = "Test Page for XML.generate"
XML.generate(STDOUT) do
  html do
    head do
      title { pagetitle }
      comment "This is a test"
    end
    body do
      h1(:style => "font-family:sans-serif") { pagetitle }
      ul :type=>"square" do
        li { Time.now }
        li { RUBY_VERSION }
      end
    end
  end
end


 

This code doesn't look like XML, and it only sort of looks like Ruby. Here's the output it generates (with some line breaks added for legibility):

<html><head>
<title>Test Page for XML.generate</title>
<!-- This is a test -->
</head><body>
<h1 style='font-family:sans-serif'>Test Page for XML.generate</h1>
<ul type='square'>
<li>2007-08-19 16:19:58 -0700</li>
<li>1.9.0</li>
</ul></body></html>

 

 

posted @ 2009-04-21 11:10  麦飞  阅读(236)  评论(2)    收藏  举报