self_shunt_spec.rb

class Scanner
  def initialize(display)
    @display = display
  end
  def scan
    @display.last_item = Item.new("Cornflakes")
  end
end

Item = Struct.new(:name)

describe Scanner do
  attr_accessor :last_item
  specify 'scan adds an item to the display' do
    Scanner.new(self).scan
    last_item.should == Item.new('Cornflakes')
  end
end