From: Josh Cheek <josh.cheek@...>
Date: 2010-06-03T01:18:31+09:00
Subject: Re: having a class method called only one time ?

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2010/6/2 Une B=E9vue <unbewusst.sein@google.com.invalid>

> I'd like having a class method called only one time ?
> Is it possible ?
>
> suppose i do have a class "HFSFile" and i need to know the host ip.
>
> i don't want to call the method giving host ip each time i need it but
> only the first time even i do instantiate several HFSFile, mostly
> because it's time consuming :
>
> @h[:host_ip] =3D `curl -s http://whatismyip.org`.chomp
>
> it is even in a while loop because sometimes curl returns ''.
>
>
> --
>  =AB Les faiblesses des hommes font la force des femmes. =BB
>   (Voltaire)
>
>

This is basically the same as what other people have said, but uses begin
... end to do the entire calculation in one method. Also, addresses that it
should only calculate once, across all instances.


class HFSFile

  # assumes all instances of this class have the same host_ip
  # frozen just b/c a lot of objects are going to be using it
  def self.host_ip
    @@host_ip ||=3D begin
      puts "calculating host_ip"
      ip =3D `curl -s http://whatismyip.org`.chomp until ip && !ip.empty?
      ip.freeze
    end
  end

  # instance forwards to class
  # if there is a better way to do this, someone let me know
  def host_ip
    self.class.host_ip
  end

end

hfs1 =3D HFSFile.new
puts hfs1.host_ip
puts hfs1.host_ip

hfs2 =3D HFSFile.new
puts hfs2.host_ip
puts hfs2.host_ip

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Also, the delightful Ruby Best Practices discusses lazy evaluation in
chapter 5. You can download a copy at http://rubybestpractices.com/ and if
you feel you got something worthwhile, you might consider purchasing it as
well :)

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And depending on where you are going with this, you could also consider
rake, which is built around tasks and dependencies, and won't execute a tas=
k
more than one time (per user invocation).

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