From: Brian Candler <b.candler@...>
Date: 2010-02-10T06:33:55+09:00
Subject: Re: Mixin module with class variables and class methods

John Lane wrote:
> Ok so I get that point. I've now got working code, well almost. I mix 
> the module into multiple classes and each gets their own instance of the 
> variable, which is not what I need.

If everything which mixes in this module shares the same state, then you 
can use a module instance variable.

If you want each subtree of related classes to share state, then one 
option is to use a class instance variable and use the class hierarchy 
to find where the value is held. Along the lines of:

  def name
    defined?(@name) ? @name : super
  end

Another option is for each class to have its own @name instance 
variable, but for them all to refer to the same object (a Hash in your 
case); in self.included you initialize @name to the same as @name in the 
parent class, if it exists.

But in your case class One and Two are unrelated but you want them to 
share state anyway, so I'd go with a module instance variable. This 
passes your test:

module TestMixin

  @list = "a"

  def self.list
    @list
  end

  def self.included(base)
    puts "TestMixin included in #{base}"

    base.extend ClassMethods
  end

  module ClassMethods

    def list
      TestMixin.list
    end

    def class_method(str)
      self.list << " #{str}"
      puts "#{str} added for class #{self} : #{list}"
    end

  end

  def list
    TestMixin.list
  end

  def instance_method
    puts "the list for class #{self.class} is #{list}"
  end

end

class One

  include TestMixin

  class_method 'b'
  class_method 'c'

end

class Two

  include TestMixin

  class_method 'd'
  class_method 'e'

end

class Three < Two

  class_method 'f'
  class_method 'g'

end

one = One.new

two = Two.new

three = Three.new

one.instance_method

two.instance_method

three.instance_method
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