Example of a mock test and the same test with a spy

mock_versus_spy.rb

# Checking an interaction with a mock
it "should notify bidders in a mocky way" do
  bidder = mock( :bidder )
  bidder.should_receive( :bid_changed ).with( '10.00' )
  auctioneer = Auctioneer.new
  auctioneer.add_bidder( bidder )
  
  auctioneer.accept_bid( '10.00' )
end

# Checking an interaction with a spy
it "should notify bidders" do
  bidder = stub( :bidder ).as_null_object
  auctioneer = Auctioneer.new
  auctioneer.add_bidder( bidder )

  auctioneer.accept_bid( '10.00' )
  
  assert bidder.received_message?( :bid_changed, '10.00' )
end