From: Kfir Lavi <lavi.kfir@...>
Date: 2009-05-10T02:20:04+09:00
Subject: Wait until a Singleton instantiated?

Hi,
I have singleton class I want to observe with another class. The
problem is that the singleton class will create the instance of the
class that wants to observe it.
Here is an example:
require 'singleton'
require 'observer'

class A
  include Singleton
  include Observable

  def initialize
    B.new
    sleep 10
  end
end

class B
  def initialize
    # register for A updates
    A.instance.add_observer(self) # will fail, because it waits for A
  end
end

A.instance

You need to stop the program with Control-C and you get:

^C/usr/lib/ruby/1.8/singleton.rb:149:in `sleep': Interrupt
        from /usr/lib/ruby/1.8/singleton.rb:149:in `_instantiate?'
        from /usr/lib/ruby/1.8/singleton.rb:105:in `instance'
        from c.rb:21:in `initialize'
        from c.rb:10:in `new'
        from c.rb:10:in `initialize'
        from /usr/lib/ruby/1.8/singleton.rb:94:in `new'
        from /usr/lib/ruby/1.8/singleton.rb:94:in `instance'
        from c.rb:25

Command terminated

The problem is that A didn't finish initializing, and B in order to
add_observer waits for A to finish initialize. So both A and B
sleeping waiting for each other.

I thought to solve it by creating a thread that will wait for A to
finish initialize.
Something like this:
Thread.new(self) do |me|
  while A.instance.not_initialize? do
    sleep 2
  end
  A.instance.add_observer(self)
end

But how do I monitor if A finished initialize?
Also, is there a better way of doing this? Idiom?

Thanks,
Kfir