Re: Am I justified to use a global variable if it must be used in all scopes?

On May 27, 2012, at 5:33 AM, Phil Stone wrote:

> Hello,
> Ever since I started learning Ruby, I've been strongly discouraged
> from using global variables
> (http://www.caliban.org/ruby/rubyguide.shtml#style)
> (http://www.ruby-forum.com/topic/187685#819233).
>=20
> In a program I'm making, I have a variable (which stores an instance =
of
> a class) that is declared outside of any functions or classes. =
However,
> I need to be able to call its methods from various class and function
> scopes, and nothing but a global variable would let me do that. This
> variable is simply what the program is centered around, and must be
> accessed and edited by various classes. I'm not sure why a global
> variable would be such a bad habit in this case, especially since it =
is
> only one.
>=20
> Can I justify using it?

Avoid using globals as much as possible, variables or constants alike. =
The are very hard to look for where they are coming from, impossible to =
mock and painful while unit testing in general as they keep persistent =
state not being reset for individual test cases.

There are many alternatives to using globals, depending on your =
particular case. The best would be to design your app in a way that an =
instance can request what it needs from some other instance, possibly =
made available at instantiation (or via dependency injection). As others =
suggested, you can use a singleton for a class that provides state for =
the whole app, which I personally dislike. Or just class methods.

This is what I sometimes use when I need a quick and dirty way to do the =
job, to be refactored at the earliest opportunity:

=3D=3D=3D main script
class App
  def start
    ...
  end

  def self.config
    @config ||=3D Config.new
  end
end

App.new.start
=3D=3D=3D end of main script
=3D=3D=3D player.rb
class Player
  def config
    App::config
  end

  def perform
    return if config.idle_mode?
    ...
  end
end
=3D=3D=3D end of player.rb

While behaving exactly as a global config would, it makes it possible to =
mock config on several levels in your unit tests. Besides, you can =
clearly see where the data is coming from.

Hope this makes sense to you,
Gennady.

>=20
> --=20
> Posted via http://www.ruby-forum.com/.
>=20