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