Re: [Rubycocoa-talk] adding to attr_accessor.

[Mark H. <di...@ma...>]

On Jul 14, 2005, at 2:53 PM, Dave Howell wrote:

> Climb over one hill, find another behind it. I think this is mostly  
> a plain Ruby question, but let's see what happens.
>
> I have a RubyCocoa app I'm working on, a To Do list manager. I can  
> pass a "ToDo" task to a subclassed NSWindowManager, and voila! I  
> have a window with that task in it. Happy me! Make changes, make  
> changes, have changes written back to the task, et cetera.
>
> If I open a *second* window on the same task, however, they're each  
> writing back to the task but not aware of what the other window's  
> doing. Obviously (I hope?) my task needs to know what windows are  
> open onto it, and when it changes, it should tell them all to update.
>
> [Because of RubyCocoa's incomplete support for Bindings and  
> whatnot, I have abandoned trying to do this with CoreData. I figure  
> I'm probably better off learning this one the "old fashioned way,"  
> especially since it's not really that big a project.]
>
> My plan is to give each task a "ViewsOfMe" array property. Whenever  
> a task window controller opens a window, it tells the task it did,  
> and when it closes, ditto. The task would tell all registered  
> window controllers to update whenever one of its relevant  
> properties is changed.
>
> This seemed like a good plan up to this point.
>
>
> But my Task class starts out like this:
>
>         attr_accessor :title, :description, :priority [8 more  
> symbols here....]
>
> I *could* just replace all this with
>
>     def title
>         @title
>     end
>     def title=
>         @title=title
>         #put update windows code here
>     end
>     [repeat 10 more times]
>
> But that seems horribly unRubyesque. I figured I'd just modify/mix- 
> in/replace attr_accessor with one that also sets up the window  
> update triggers. OTOH, I'm absolutely mystified how to modify  
> attr_accessor to stick the updating code in the right spot. I  
> cannot find any reference for how attr actually works. I couldn't  
> even find a file that contained its code. One doc said it was in  
> "objects.c" but I can't find that either. LOL.
>
> Part of what makes this tricky is that I can't just add something  
> to attr, I have to change something it already does.
>
> Thoughts? Suggestions?

Roll your own attr_accessor. attr_accessor is just a method:  
Class#attr_accessor.[1] Define one that creates methods that you  
want. In this case, you want instance variables set, and a hook that  
will let you notify observers.

Here's a quick example done in irb:

class Class
   def observable_attr(*names)
     # Make sure we have the right scope
     self.class_eval do
       names.each do |name|
         define_method name do  # the getter
           instance_variable_get :"@#{name}"
         end
         define_method :"#{name}=" do |value|  # the setter
           instance_variable_set :"@#{name}", value
           update_observers(name)
         end
       end
     end
   end
end
     ==>nil
class Foo
   def update_observers(name) # the hook method
     puts "updating observers regarding '#{name}'"
   end
   observable_attr :foo, :bar, :baz
end
     ==>[:foo, :bar, :baz]
f = Foo.new
     ==>#<Foo:0x121eaa0>
f.foo, f.bar, f.baz = 23, 42, 93
updating observers regarding 'foo'
updating observers regarding 'bar'
updating observers regarding 'baz'
     ==>[23, 42, 93]
[f.foo, f.bar, f.baz]
     ==>[23, 42, 93]

[1] actually, it's Module#attr_accessor, but that's another story.