Question

I have a number of classes that inherit from one superclass.
The superclass as a module defined. Inside of a module is a self.included(base) method that sets some instance variables.

So something like this:

module MyModule
  def self.included(base)
    base.instance_variable_set("@my_instance_variable", {})
  end
end

class MySuperClass
  include MyModule
end

class ClassA < MySuperClass
end

class ClassB < MySuperClass
end

Unless I explicitly include MyModule in ClassA and ClassB then my instance variable will not get set in these two classes.

Is there a way of making sure the modules self.included(base) method is executed in each sub class without the need to explicitly include the module? Since it’s already included in the superclass.

Answer 1

Class instance variable are private to the class. Inherited classes can’t access them directly. There is a couple of ways here.

1. Define accessor

module MyModule
  def self.included(base)
    base.instance_variable_set :@my_instance_variable, {}
    base.extend ClassMethods
  end

  module ClassMethods
    def my_instance_variable
      # self is ClassA here, so we need to call superclass
      superclass.instance_variable_get :@my_instance_variable # => {}
    end
  end
end

class MySuperClass
  include MyModule
end

class ClassA < MySuperClass; end

ClassA.my_instance_variable # => {}

2. More metaprogrammatic

But there’s a hook that gets called every time when a class is inherited from you. You can set variables at this point. Check this out.

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

  module ClassMethods
    # make it a class method. It will be called on target classes later.
    def enhance klass 
      klass.instance_variable_set("@my_instance_variable", {})
    end
  end
end

class MySuperClass
  include MyModule

  # this hook will get called on 'class ClassA < MySuperClass`
  def self.inherited klass
    # set class instance var on child class
    enhance klass
  end
end

class ClassA < MySuperClass; end

ClassA.instance_variable_get '@my_instance_variable' # => {}

NOTE: in this sample each inherited class gets its own class instance var (and base class doesn’t get one). This might be more appropriate for your case, might be not.