From: wys@... (Clemens Wyss)
Date: 2001-01-03T16:40:02+09:00
Subject: [ruby-talk:8558] Re: Observing multiple observables

Kevin Smith <sent@qualitycode.com> wrote in
<0G6K002IIN4WZV@mta5.snfc21.pbi.net>: 
<<SNIP>>
>
>The definition of add_observer would change to:
>
>def add_observer(object, methodSymbol = :update)
>
>When it comes time to actually do the update, it 
>calls:
>
>object.method(methodSymbol).call(*args)
>
<<SNIP>>

...why not go one step further (to the SmallTalk onChange:send:-update:-
paradigm) and extend add_observer to: 
    	def add_observer(oberverObj, aspectOfInterest = nil, callback =
    :update) 

aspectOfInterest is the aspect of the observable object which the client
(observer) is interested in (ie a Person's name or a Person's address): 

class Observer

  def add_observer(observerObj, aspect = nil, callback = :update)
    ...
  end
  
  def update(aspect = nil)
    ...
  end

end

class Person
  include Observer
  ...
end

class PersonObserver
  def initialize(p)
    p.add_observer(self, 'name', :name_of_person_changed)
    ...
  end

  def name_of_person_changed(object, newValue = nil)
  end
end

p = Person.new()
PersonObserver.new(p)


NOTE: the callback method (cuold) expect the newValue of the aspect)!
Instead of adding default parameters to add_observer we could define a
new method: add_observer_for(_aspect) 

Clemens