From: Timothy Hunter <TimHunter@...>
Date: 2006-09-06T07:17:46+09:00
Subject: Re: How to store/load persistent data?

Josef Wolf wrote:
> It is clear to me that the ruby object is removed by the garbage
> collector.  But it is not clear what happens to other recources (a
> TkCanvas in this case) allocated by this object. Check this out:
>
>   #! /usr/bin/ruby
>
>   require 'tk'
>
>   root=TkRoot.new
>
>   class Foo
>     def initialize
>       canvas = TkCanvas.new.pack
>       rect = TkcRectangle.new(canvas, 0, 0, 50, 50, "fill"=>"white")
>     end
>   end
>
>   begin
>     x=Foo.new()
>   end
>
>   GC.start
>
>   # Object destroyed but canvas and rectangle are still alive
>
>   Tk.mainloop
>
> I can't see a solution for this problem since I am a novice for ruby/OO/Tk.
> How do you gurus handle such situations?
>
>   

There is a well-known Ruby idiom for coding block-scoped resources. 
There used to be a good page describing this in RubyGarden but alas that 
page seems to have disappeared. Anyway, here's the general example from 
that page:

 > def Resource.open( identifier )  # :yield: resource
 >     resource = Resource.new( identifier )
 >     if block_given?
 >         begin
 >             yield resource
 >         ensure
 >             resource.close
 >         end
 >     else
 >         return resource
 >     end
 > end


That is, create/open/whatever the resource is and yield to a block. 
Within the block use the resource. When the block returns, close/destroy 
the resource. The open method in the IO class is prototypical.