decorator.rb

class Sward
  attr_reader :options

  def initialize
    @options = {}
  end

  def cost
    raise NotImplementedError.new("#{self.class}##{__method__} が実装されていません")
  end
  
  def set_option(key, val)
    @options[key] = val
  end
end

class SwardMaterial # < Sward 別に継承している必要はない => 継承より委譲
  def initialize(sward)
    @sward = sward
  end
  
  def set(material)
    @sward.set_option(:material, material)
    @sward
  end
end

class AddAttribute # < Sward 別に継承している必要はない => 継承より委譲 < Sward
  def initialize(sward)
    @sward = sward
  end
  
  def set(attribute)
    new_options = (@sward.options[:attribute] || []).push(attribute)
    @sward.set_option(:attribute, new_options)
    @sward
  end
end


sward = Sward.new()
sward = SwardMaterial.new(sward).set('steel')
sward = AddAttribute.new(sward).set('fire')
sward = AddAttribute.new(sward).set('frozen')

p sward # #<Sward:0x000014b7a9599348 @options={:material=>"steel", :attribute=>["fire", "frozen"]}>