From: Avdi Grimm <avdi@...>
Date: 2008-05-24T04:22:06+09:00
Subject: Re: Ruby observer module

On Fri, May 23, 2008 at 2:49 PM, S2 <x@y.z> wrote:
> I also have another question: why does the subject (the notifying class)
> need to call "changed"? Wouldn't it be sufficient to call notify_observers?
> I mean, if I call notify_observers in the Subject class, doesn't that imply
> has something has changed?

Consider the case:

  def do_stuff
    step_that_might_change_self()
    another_step_that_might_change_self()
    yet_another_step_that_might_change_self()
  end

Any of the methods called by do_stuff MIGHT change the data, but then
again they might not.  do_stuff should appear as an atomic operation
from outside the object, so we don't want to call notify_observers in
all three methods.  What we really want to do is do all of the steps
and then, when they are finished, notify our observers - but only if
we've actually been changed.  So we write it like this:

  def do_stuff
    step_that_might_change_self()
    another_step_that_might_change_self()
    yet_another_step_that_might_change_self()
   notify_observers
  end

And then it is each step's responsibility to call #changed if and only
if they actually made a change.  If a change was made by one or more
steps, our observers are notified ONCE at the end.

-- 
Avdi

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