From: Brian Candler <b.candler@...>
Date: 2009-07-21T00:06:59+09:00
Subject: Re: Ruby Object listener?

Maran Chandrasekar wrote:
> Dear folks,
>   I have a Hash Object. If any thing added, removed or modified in that
> Hash. I want to trigger a operation. How could I do that?

You cannot detect the general case of anything "modified" in that Hash, 
unless you build objects which know they are modified and know who to 
notify. e.g.

  a = {1=>"foo", 2=>"bar"}

  a[2].replace("baz")

The second operation is a method call to the String object which mutates 
that String. The Hash never sees it, and is still pointing to the same 
object both before and after the call. Furthermore, multiple hashes 
could be pointing to the same object. A standard String has no idea 
which Hash(es) might contain it.

If you are only interested in modifications to the Hash itself, not the 
objects it contains, then you can intercept all calls which might modify 
it (perhaps using delegate.rb). Note that this includes a large number 
of methods, some of which take blocks, like delete_if.

If you want to notify multiple clients, then have a look at observer.rb 
in the standard library.

If you don't need immediate notification, and are prepared to poll 
occasionally, then you could consider doing a Marshal.dump of the Hash, 
taking an SHA1 digest of that, and seeing if the digest is the same or 
different.

>> require 'digest/sha1'
=> true
>> a = {1=>"foo", 2=>"bar"}
=> {1=>"foo", 2=>"bar"}
>> Digest::SHA1.hexdigest(Marshal.dump(a))
=> "d28ca1b04097274124675d63b227ecc15c71a0c5"
>> Digest::SHA1.hexdigest(Marshal.dump(a))
=> "d28ca1b04097274124675d63b227ecc15c71a0c5"
>> a[2].replace("baz")
=> "baz"
>> Digest::SHA1.hexdigest(Marshal.dump(a))
=> "c6661b1010def28517ef5da33a9e540f324bff5c"
>> 
-- 
Posted via http://www.ruby-forum.com/.