Simple example of using class variable from a mixin

ruby_fiddle.rb

module MX
  
  @testvar = 'abc'
  
  def ModTest
    'This is ModTest.'
  end
  
  def self.ModVarTest
    "This is ModVarTest. #{@testvar}"
  end
  
  def self.SetVar(v)
    @testvar = v
  end
end

class A
  extend MX
  def self.Test
    puts 'This is test.'
  end
  
  def self.TestMixin
    puts self.ModTest
  end
  
  def self.TestMixinVar
    puts MX.ModVarTest
  end
  
  def self.SetMixinVar(v)
    MX.SetVar(v)
  end
end

class B
  extend MX

  def self.AlsoTestMixinVar
    puts MX.ModVarTest
  end
end

A.Test
A.TestMixin
A.TestMixinVar
B.AlsoTestMixinVar

A.SetMixinVar('fghj')
A.TestMixinVar
B.AlsoTestMixinVar

MX.SetVar('234')
A.TestMixinVar
B.AlsoTestMixinVar