From: Anders Borch <spam@...>
Date: 2003-07-03T18:08:25+09:00
Subject: Re: Auto-serialization (as in Vapor, sql-serialize, etc.): detecting changes to instance variables

I haven't come up with a good solution to this myself, I'm merely 
suggesting, that maybe you are going about this the wrong way. I once 
read that if you are experiencing extreme difficulty with the 
implementation you are attempting then maybe you are attempting the 
wrong implementation. Very often there is an easy solution to an 
otherwise hard problem.

Andrey Kulinich wrote:
> Anders Borch wrote:
> 
>> I think you should not allow anyone to modify your instance variables 
>> without going through your accessor methods. It ruins everything that 
>> encapsulation gives you.
> 
> 
> Sometimes you can't control modification.
> 
> class Session
>   attr_accessor :modified
>   def initialize
>     @data = {}
>     @modified = false
>   end
> 
>   def []( key )
       # this may not ALWAYS be appropriate, but if you are
       # concerned with encapsulation you may want to consider it...
       d = @data[key].dup
       d.freeze
       return d

>     @data[key]
>   end
> 
>   def []=( key, value )
>     @data[key] = value
>     @modified = true
>   end
> end
> 
> session = Session.new
> session[:arr] = []
> session.modified # true. all ok
> session.modified = false
> 
> session[:arr].push( 1 )
> session.modified # false :(
> 
>> Using the method_missing to test if the object has changed after each 
>> method call *could* solve your problem. 
> 
> 
> There is a trouble with method_missing: you can't redirect methods 
> 'to_s', '==' and other Object's methods, because they are defined in 
> Wrapper.
> 

then overload ==, to_s and the rest of Object's methods to make sure 
they are passed on to the wrapped object.

while freezing the return value of [] may not be a good idea, 
overloading the rest of Object's methods is certainly a nice and easy 
solution to most of your problems.

/Anders

-- 
dc -e 
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