From: Robert Klemme <shortcutter@...>
Date: 2009-03-26T19:53:48+09:00
Subject: Re: program design - template design pattern

2009/3/26 Srijayanth Sridhar <srijayanth@gmail.com>:
> While a lot depends on several other aspects of your project, this specific
> problem depends on a principle question:
>
> Do you have any control over the feed object?

I second that.  Also, what type is "feed"?  That's not clear to me.

> If you have any control, then you could make a polymorphic superclass called
> Feed and go on from there where each separate feed has its own related
> methods to deal with its idiosyncrasies and polymorphic methods that deal
> with common tasks. For ex:
>
> class Feed
>   def initialize
>   end
>   def parse_content
>   end

One could also add

  # download the feed
  def fetch(uri)
     ....
  end

> end
>
> class BbcFeed < Feed
>   def parse_content
>      # do something relevant here
>      # probably return a string/hash/or content object
>   end
>   # define other methods specific to BbcFeed etc
> end
>
> Once this is done, you just have to create the right object at runtime,
> perhaps have a factory object detailing this.

Or stuff the relationship between feed URI's and class that deals with
it in a Hash:

FEEDS = {
  'http://foo.bar/feed.rdf' => BbcFeed,
  'http://baz.boz.buz/path.rdf' => BbcFeed,
  'http://what.ever.you/like' => OtherFeed,
}.freeze

Then you can do

FEEDS.each |uri, cl|
  feed_handler = cl.new
  feed_handler.fetch
  feed_handler.parse_content
end

> If you don't have control, there isn't a way to run away from doing checks
> on the object. Depending on how you determine the regex to use, you could
> have specific solutions. Perhaps each feed has a different URL that you
> could use to determine the parsing, or perhaps something else in the
> xml...either way, you have to know what object you are dealing with, or you
> have to write a parser that deals with it.

Feed type checks could be done on the URI or the content.  Depending
on that a different approach needs to be followed but the basic
pattern stays the same.

Kind regards

robert

-- 
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