From: Phrogz <phrogz@...>
Date: 2008-01-13T14:25:04+09:00
Subject: Re: Stream Parsing with REXML

On Jan 12, 9:58 am, Marc Hoeppner <marc.hoepp...@molbio.su.se> wrote:
> Hi (again, sort of) :)
>
> I am still on my quest to write a program that parses a large XML file.
> After having tried to do it in tree mode, I had to realize that the
> performance was simply abysmal. So back to the drawing board. But, and
> here is the thing...I could find a good straight-forward tutorial on how
> to write a stream parser using REXML. The official tutorial is pretty
> much mute on that part and the only other example I found (or rather was
> pointed to -http://www.janvereecken.com/2007/4/11/event-driven-xml-parser-in-ruby)
> was way too complex for someone like me who is still pretty much a
> beginner in ruby.
>
> So, what I am looking for is either a brief description of how to write
> an event driven parser or else a link to a good and simple tutorial.

xml = <<ENDXML
oh hai
<root>
  <kid1 foo="bar" jim="jam" />
  <kid2 foo="sausage">
    <grandkid1>I ate &lt;raw&gt; sausages&tm;!</grandkid1>
    <grandkid2>
      <![CDATA[
        I ate <raw> sausages&tm;!
      ]]>
    </grandkid2>
  </kid2>
</root>
whoa!
ENDXML

class Listener
  def initialize
    # Use instance variables to mantain information
    # between different callbacks.
    @nest_level = 0
  end

  def tag_start( name, attr_hash )
    @nest_level += 1
    print "  " * @nest_level
    puts "<#{name} ...> #{attr_hash.inspect}"
  end

  def text( str )
    print "  " * @nest_level
    p str
  end

  # Treat CDATA sections just like text
  alias_method :cdata, :text

  def tag_end( name )
    print "  " * @nest_level
    puts "</#{name}>"
    @nest_level -= 1
  end
end

require 'rexml/document'
require 'stringio' # for this demo
stream_handler = Listener.new
pseudo_file = StringIO.new( xml )
REXML::Document.parse_stream( pseudo_file, stream_handler )

#=> OUTPUT:
#=> "oh hai\n"
#=>   <root ...> {}
#=>   "\n  "
#=>     <kid1 ...> {"jim"=>"jam", "foo"=>"bar"}
#=>     </kid1>
#=>   "\n  "
#=>     <kid2 ...> {"foo"=>"sausage"}
#=>     "\n    "
#=>       <grandkid1 ...> {}
#=>       "I ate <raw> sausages&tm;!"
#=>       </grandkid1>
#=>     "\n    "
#=>       <grandkid2 ...> {}
#=>       "\n      "
#=>       "\n        I ate <raw> sausages&tm;!\n      "
#=>       "\n    "
#=>       </grandkid2>
#=>     "\n  "
#=>     </kid2>
#=>   "\n"
#=>   </root>
#=> "\nwhoa!\n"