Testing system calls

default_formatter.rb

require 'stringio'

class DefaultFormatter
  attr_reader :output

  def initialize(output = StringIO.new)
    @output = output
  end

  def display(message)
    output.puts
    output.puts "=" * 80
    output.puts ">> #{message}"
    output.puts "=" * 80
    output.puts
  end
end

whoopsie.rb

require './default_formatter'

class Whoopsie
  attr_reader :formatter

  def initialize(thing, options = {})
    @formatter = options[:formatter] || DefaultFormatter.new
  end

  def do_special_work
    notify_screen("Amazing stuff happening here")
    # actual amazing work
  end

  private
  def notify_screen(message)
    formatter.display(message)
  end
end

whoopsie_spec.rb

require 'rspec'
require './whoopsie'

describe Whoopsie do
  describe "#do_special_work" do
    let(:whoopsie) do
      described_class.new('thing', formatter: formatter)
    end
    let(:formatter) { double(:formatter) }

    it "displays message" do
      formatter.should_receive(:display).with('Amazing stuff happening here')
      whoopsie.do_special_work
    end
  end
end