From: Robert Klemme <shortcutter@...>
Date: 2009-04-09T01:10:11+09:00
Subject: Re: performing an action only the first time a function called

On 08.04.2009 16:29, James French wrote:

Please do not hijack other threads.

> Could anyone suggest an elegant way of doing something like:
> 
> $directXSDKBootstrapped = false
> 
> def bootstrapDirectXSDK
>   if !$directXSDKBootstrapped
>     bootstrapBuildToolsDirectory("DirectX/#{DIRECTX_VERSION}")
>     $directXSDKBootstrapped = true
>     $directXSDKBootstrapped.freeze

Freezing has no effect here because it does not prevent reassignment to 
the variable.

>   end
> end
> 
> Obviously the intention is to only do something the first time it is called Maybe some little reusable util?I have a number of different bootstrap functions I want to apply this to.

Why can't you just invoke the initialization once and be done?  If this 
is in a file which is required then Ruby will take care of this 
automatically.

Another solution:

class OnlyOnce
   def initialize(&b)
     raise "Need a block!" unless b
     @block = b
   end

   def get
     if @block
       @val = block.call
       @block = nil
     end

     @val
   end
end

$directXSDKBootstrapped = OnlyOnce.new do
   bootstrapBuildToolsDirectory("DirectX/#{DIRECTX_VERSION}")
end

Cheers

	robert