From: Alex Fenton <alex@...>
Date: 2006-08-28T07:55:11+09:00
Subject: Re: Detecting when an object's instance variables are modified

Paul Murton wrote:
> Hi,
> 
> I have a whole load of objects which I would like to periodically dump 
> as YAML. However, in order to save on resources, I would prefer to only 
> dump those whose instance variables have changed since the last dump.

If you're interested in knowing whether instance variables have changed since a known point in the past (eg when you last dumped the object), rather than the moment they change, something like this might work:

module InstanceVariableSnapshooter
  def clean!
    @__snapshot__ = ivar_hash
  end

  def clean?
    ivar_hash == @__snapshot__
  end

  def dirty?
    not clean?
  end

  private
  def ivar_hash
    instance_variables.inject({}) do | iv_hash, i_var |
      next iv_hash if i_var == '@__snapshot__'
      iv_hash[i_var] = instance_variable_get(i_var)
      iv_hash
    end
  end
end


Call clean! when the object is dumped to file, then call dirty? later to see if anything has changed. It's probably better to work via setter methods if you can, but this works direct with the variables.

Also, test whether this is less expensive than the dumping operation you're trying to avoid

alex