follesoe/RubyXmlDslExercise.md

## RubyXmlDslExercise.md

      
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              RubyXmlDslExercise.md
            
          
    Simple Ruby DSL to generate XML

Implemented as an exercise to learn about metaprogramming in Ruby. The DSL is implemented using instance_eval and method_missing.
Usage

require 'xmldsl'

Twitter = Struct.new :name, :avatar, :text
twitters = []
5.times { twitters << Twitter.new("Jonas", "/profile.png", "Hello World!") }

xml = XML.generate do 
  html do    
    body do 
      twitters.each do |tw|
        img :src => tw.avatar, :alt => tw.name, :width => 25, :height => 25
        content [" : ", tw.text]
        br
        br        
      end
      form :action => '/page', :method => 'post' do
          input :type => 'text', :name => 'str', :maxlength => 3, :size => 3
          input :type => 'submit', :value => 'page'
      end      
    end
  end
end

open('sample_twitters.html', 'w'){|f| f.puts xml}
puts xml
Output

<html>
   <body >
      <img width='25' height='25' src='/profile.png' alt='TwitterHandle'/>
       : Hello Ruby DSL!
      <br />
      <br />
      <img width='25' height='25' src='/profile.png' alt='TwitterHandle'/>
       : Hello Ruby DSL!
      <br />
      <br />
      <img width='25' height='25' src='/profile.png' alt='TwitterHandle'/>
       : Hello Ruby DSL!
      <br />
      <br />
      <img width='25' height='25' src='/profile.png' alt='TwitterHandle'/>
       : Hello Ruby DSL!
      <br />
      <br />
      <img width='25' height='25' src='/profile.png' alt='TwitterHandle'/>
       : Hello Ruby DSL!
      <br />
      <br />
      <form action='/page' method='post'>
         <input type='text' maxlength='3' size='3' name='str'/>
         <input type='submit' value='page'/>
      </form>
   </body>
</html>
Implementation

class XML  
  def self.generate &block
    XML.new.xml_generate &block
  end

  def xml_generate &block
    instance_eval &block    
  end

  def initialize
    @xml = ""
    @indent = 0   
  end
 
  def method_missing name, *args, &block
    @xml += "#{add_indent}<#{name} #{attributes(*args)}"   
    if(block) 
      @xml += ">\n"
      @indent += 3
      block.call
      @indent -= 3
      @xml += "#{add_indent}</#{name}>\n"
    else
      @xml += "/>\n"
    end
  end

  def add_indent 
    " "*@indent 
  end

  def attributes *args
    (args[0] || {}).map { |key, value| "#{key}='#{value}'"}.join(" ")
  end

  def content txt 
    @xml += "#{add_indent}#{[txt].join}\n"
  end
end