as_null_object

as_null_object working

It only listen for the messages we tell it to expect and ignore any other messages.

For example:

spec/codebreaker/game_spec.rb

module Codebreaker
 describe Game do
   describe "#start" do
     before(:each) do
       @output = double('output').as_null_object
       @game = Game.new(@output)
     end

     it "sends a welcome message" do
       @output.should_receive(:puts).with('Welcome to Codebreaker!')
       @game.start
     end
     it "prompts for the first guess" do
       @output.should_receive(:puts).with('Enter Guess:')
       @game.start
     end
   end
 end
end

In first example we are expecting ‘Welcone to Codebreaker!’ while in second example we expect ‘Enter Guess:’

and in before(:each) first line we are using as_null_object which allowing us to only check if expected string exists in game.start method then it will pass.

lib/codebreaker/game.rb

module Codebreaker
 class Game
   def initialize(output)
     @output = output
   end
   def start
     @output.puts 'Welcome to Codebreaker!'
     @output.puts 'Enter Guess:'
   end
 end
end