eigenfun.rb

# see http://blog.codegram.com/2011/4/understanding-class-instance-variables-in-ruby
# and http://vimeo.com/33481092 (Ruby's Object Model & Eigenclasses)


module MyMixin
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    # the current class (also called definee or klass) determines on
    # which class 'def' will act on
    #
    # definee => MyMixin::ClassMethods
    #
    def set(value)
      # self points to the current object (also called callee), it
      # determines on which object a method is called
      #
      # self => either A or B
      #
      @@my_value = value
    end

    def get
      @@my_value
    end
  end
end


class A
  include MyMixin

  set 'Test from A'
end

class B
  include MyMixin

  set 'Test from B'
end


# we have overwritten MyMixin::ClassMethods::@@my_value
A.get # => "Test from B"


# methods are defined in module
A.singleton_methods # => [:set, :get]
A.singleton_methods(false) # => []

A.class_variables # => []

MyMixin::ClassMethods.class_variables # => [:@@my_value]


## Summary:
#
# 'set', 'get' methods are defined in:   A's singleton class
# 'my_value' is defined in: MyMixin::ClassMethods as class var

###################### ######################



module YourMixin
  def self.included(base)

    # define_singleton_method essentially means the following:
    #
    # base.singleton_class.send :define_method, :set do ...; end
    #
    # 'define_method' cares only about 'self' not the current class
    #
    # We're creating a singleton method without using 'class' or 'self'
    # which means, we are not forced into a new scope
    base.define_singleton_method :set do |value|
      @my_value = value
    end

    base.define_singleton_method :get do
      @my_value
    end

  end

end


class C
  include YourMixin

  set 'Test from C'
end

class D
  include YourMixin

  set 'Test from D'
end

C.get # => "Test from C"

# methods are defined in the class' singleton class
C.singleton_methods(false) # => [:set, :get]

C.class_variables          # => []
C.instance_variables       # => [:@my_value]



## Summary:
#
# 'set', 'get' methods are defined in:   C's singleton class
# 'my_value' is defined as C's class instance variable

###################### ######################



module HerMixin
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def set(value)
      class_variable_set '@@my_value', value
    end

    def get
      class_variable_get '@@my_value'
    end
  end
end



class E
  include HerMixin

  set 'Test from E'
end

class F
  include HerMixin

  set 'Test from F'
end



E.get # => "Test from E"

E.singleton_methods(false) # => []
E.class_variables          # => [:@@my_value]


## Summary:
#
# 'set', 'get' methods are defined in:   HerMixin module
# 'my_value' is defined in: E as class var


###################### ######################



module HisMixin
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def set(value)
      @my_value = value
    end

    def get
      @my_value
    end
  end
end



class G
  include HisMixin

  set 'Test from G'
end

class H
  include HisMixin

  set 'Test from H'
end


H.get # => "Test from H"

H.singleton_methods(false) # => []
H.class_variables          # => []
H.instance_variables       # => [:@my_value]


## Summary:
#
# 'set', 'get' methods are defined in:   HisMixin module
# 'my_value' is defined as H's class instance variable