command_pattern.rb

class UIButton
  attr_reader :command

  def initialize(command)
    @command = command
  end

  def press_button
    @command.execute
  end
end

class AddItemToCartCommand
  attr_reader :item, :cart

  def initialize(item, cart)
    @item = item
    @cart = cart
  end

  def execute
    @cart.add_item(@item)
  end

  def unexecute
    @cart.remove_item(@item)
  end
end