9_40.rb

# query vs command messages
  # ...
    # ...
class Gear
  attr_reader :chainring, :cog, :wheel, :observer
  def initialize(chainring:, cog:, wheel:, observer:)
    @chainring = chainring
    @cog       = cog
    @wheel     = wheel
    @observer  = observer
  end

  def gear_inches
    ratio * wheel.diameter
  end

  def ratio
    chainring / cog.to_f
  end

  def set_cog(new_cog)
    @cog = new_cog
    changed
  end

  def set_chainring(new_chainring)
    @chainring = new_chainring
    changed
  end

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


##########################################################
require "minitest"
require 'minitest/mock'
require "minitest/reporters"
require "minitest/asciidoc_plugin"

Minitest::Reporters.use! Minitest::Reporters::Asciidoc.new

class DiameterDouble
  def diameter
    10
  end
end

# Using Mocks (Expectations that a message is sent) to
# test command messages.
class GearTest < Minitest::Test
  def setup
    @observer = Minitest::Mock.new
    @gear     = Gear.new(
                  chainring: 52,
                  cog:       11,
                  wheel:     DiameterDouble.new,
                  observer:  @observer)
  end

  def test_notifies_observers_when_cogs_change
    @observer.expect(:changed, true, [52, 27])
    @gear.set_cog(27)
    @observer.verify
  end

  def test_notifies_observers_when_chainrings_change
    @observer.expect(:changed, true, [42, 11])
    @gear.set_chainring(42)
    @observer.verify
  end
end

Minitest.run
 **[rubyclass]#GearTest#**
   **[green]#PASS#** __(0.00s)__  test_notifies_observers_when_chainrings_change
   **[green]#PASS#** __(0.00s)__  test_notifies_observers_when_cogs_change
 Finished in 0.00020s
 2 tests, 0 assertions, [green]#0 failures, 0 errors#, [yellow]#0 skips#