# Wait until a Singleton instantiated?

**URL:** <https://www.ruby-forum.com/t/wait-until-a-singleton-instantiated/167592>\
**Category:** Ruby\
**Created:** [May 9, 2009, 7:20pm UTC](https://www.ruby-forum.com/t/wait-until-a-singleton-instantiated/167592 "2009-05-09T19:20:36Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Kfir\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/e47c2d/32.png) [@Kfir\_L](https://www.ruby-forum.com/u/Kfir_L)\
**Post date:** [May 9, 2009, 7:20pm UTC](https://www.ruby-forum.com/t/wait-until-a-singleton-instantiated/167592/1 "2009-05-09T19:20:36Z")

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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

---

<div class="post-metadata">

**Author:** ![Kfir\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/e47c2d/32.png) [@Kfir\_L](https://www.ruby-forum.com/u/Kfir_L)\
**Post date:** [May 9, 2009, 11:03pm UTC](https://www.ruby-forum.com/t/wait-until-a-singleton-instantiated/167592/2 "2009-05-09T23:03:48Z")

</div>

On Sat, May 9, 2009 at 1:20 PM, Kfir L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> include Observable
> 
> def initialize  
> B.new  
> sleep 10  
> end  
> end

Why are you instantiating B here? This is odd for several reasons:

1. The whole point of the observer pattern is that the observable  
object doesn’t shouldn’t know about the actual observers or who  
created them.

2. Your initialize method creates an instance of B and then ignores  
it, it’s subject to be GCed since there are no references to it.

So why not:

class A  
include Singleton  
include Observable

def initialize  
sleep 10  
end  
end

class B  
def initialize  
A.instance.add\_observer(self)  
end  
end

a\_observer = B.new

–  
Rick DeNatale

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