rspec spies: spy on unstubbed method

spy_spec.rb

# This spec demonstrates how rspec's and_call_original
# allows spying on real objects without affecting their
# behaviour. In the following output from rspec, note
# that real output ("Moo!") actually goes to $stdout,
# even though the latter's puts method was stubbed.
#
# Cow
#   before it speaks
#     is considered quiet
#   when it speaks
# Moo!
#     prints Moo! to the standard output stream
# Moo!
#     is not considered quiet
# 
# Finished in 0.00257 seconds
# 3 examples, 0 failures

require 'spec_helper'

class Cow
  def initialize(io = $stdout)
    @io = io
    @quiet = true
  end

  def say
    @io.puts "Moo!"
    @quiet = false
  end

  def quiet?
    @quiet
  end
end

describe Cow do
  let(:stdout) do
    $stdout.tap { |io| allow(io).to receive(:puts).and_call_original }
  end
  subject { Cow.new(stdout) }

  context "before it speaks" do
    it "is considered quiet" do
      expect(subject).to be_quiet
    end
  end

  context "when it speaks" do
    before(:each) { subject.say }

    it "prints Moo! to the standard output stream" do
      expect(stdout).to have_received(:puts).with("Moo!")
    end

    it "is not considered quiet" do
      expect(subject).to_not be_quiet
    end
  end
end