inherited callback

using_the_inherited_callback.rb

class ShippingOption
  def self.children
    @children ||= []
  end

  def self.inherited(child_class)
    children << child_class
  end

  def self.for(weight, international)
    children.select {|cls| cls.can_ship?(weight, international)}
  end
end

class MediaMail < ShippingOption
  def self.can_ship?(weight, international)
    !international
  end
end

class PriorityFlatRate < ShippingOption
  def self.can_ship?(weight, international)
    !international && weight < (4 * 16)
  end
end

options = ShippingOption.for(3 * 16, false)
puts options