Sandi Metz's Magic Tricks of Testing

magic_tricks_of_testing.rb

require 'minitest/autorun'
require 'minitest/mock'

# From the Queen herself, Sandi Metz
# https://www.youtube.com/watch?v=URSWYvyc42M
# http://jnoconor.github.io/images/unit-testing-chart-sandi-metz.png

class Gear
  attr_reader :chainring, :cog, :wheel, :observer

  def initialize chainring: nil, cog: nil, wheel: nil, observer: Obs.new
    @chainring = chainring
    @cog = cog
    @wheel = wheel
    @observer = observer
  end

  # incoming query (w/ sent-to-self & outgoing query)
  def gear_inches
    ratio * wheel.diameter
  end

  # incoming command (w/ sent-to-self w/ outgoing command)
  def set_cog new_cog
    @cog = new_cog
    changed
    cog
  end

  private
  def ratio
    chainring / cog.to_f
  end

  def changed
    observer.changed(chainring, cog)
  end
end

class Wheel
  attr_reader :rim, :tire

  def initialize rim, tire
    @rim = rim
    @tire = tire
  end

  # incoming query
  def diameter
    rim + (tire * 2)
  end
end

class Obs
  # incoming command
  def changed _, __
    # lots of public side-effects here, like
    # saving a bunch of data in the db
  end
end


class WheelTest < MiniTest::Test
  # incoming query
  def test_calculates_diamter
    wheel = Wheel.new(26, 1.5)

    assert_in_delta(29, #<< assert result
                    wheel.diameter,
                    0.01)
  end
end

class GearTest < MiniTest::Test
  # incoming query
  def test_calculates_gear_inches
    gear = Gear.new(
             chainring: 52,
             cog: 11,
             wheel: Wheel.new(26, 1.5))

    assert_in_delta(137.1, #<< assert result
                    gear.gear_inches,
                    0.01)

    # not expecting or asserting private #ratio
    # not expecting or asserting Wheel's #diameter
  end


  ## set_cog tests
  # incoming command
  def test_set_cog
    gear = Gear.new
    gear.set_cog 27
    assert(27, gear.cog) #<< assert direct public side effects
  end

  # outgoing command
  def test_notifies_observers_when_cogs_change
    observer = MiniTest::Mock.new # mock, for speed
    gear = Gear.new(
             chainring: 52,
             cog: 11,
             observer: observer)

    observer.expect(:changed, true, [52,27]) # setup mock's expect
    gear.set_cog 27
    observer.verify #<< expect message got sent
  end
end

Run tests with:
$ ruby magic_tricks_of_testing.rb