Chapter_3.rb

# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Needs to Manage Dependencies

class Gear
  attr_reader :chainring, :cog, :rim, :tire

  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog  = cog
    @rim = rim
    @tire = tire
  end

  def ratio
    chainring / cog.to_f
  end

  def gear_inches
    # Gear knows about wheel
    # Gear knows that wheel takes rim and tire
    # Gear expect wheel to respond to diameter
    ratio * Wheel.new(rim, tire).diameter

    # ...
    # if wheel changes, this method is broken
  end
end

class Wheel
  attr_reader :rim, :tire

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

  def diameter
    rim + (tire * 2)
  end
end

# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Inject Dependencies

# 1 -------------------GOOD

class Gear
  attr_reader :charing, :cog, :wheel

  def initialize(charing, cog, wheel)
    @chainring = chainring
    @cog  = cog
    @wheel = wheel
  end

  def gear_inches
    ratio * wheel.diameter
  end

end


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Isolate Instance Creation -- ONLY IF YOU CAN'T INJECT DEPENDENCY

# 2a -------------------OKAY

class Gear
  attr_reader :chainring, :cog, :rim, :tire

  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog = cog
    @wheel = Wheel.new(rim, tire)
  end

  # reduce dependency in gear_inches
  def gear_inches
    ratio * wheel.diameter
  end
end

# 2b -------------------OKAY
class Gear
  attr_reader :chainring, :cog, :rim, :tire

  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog = cog
    @rim = rim
    @tire = tire
  end

  # reduce dependency in gear_inches
  def gear_inches
    ratio * wheel.diameter
  end

  def wheel
    @wheel ||= Wheel.new(rim, tire)
  end
end

# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Isolate Vulnerable External Messages

# 3 -------------------GOOD

def gear_inches
  foo = some_intermediate_result * diameter
end

def diameter
  wheel.diameter
end

# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Remove Argument-Order Dependencies

# 4 -------------------BAD

class Gear
  attr_reader :chainring, :cog, :wheel

  def initialize(chainring, cog, wheel)
    @chainring = chainring
    @cog = cog
    @wheel = wheel
  end
end

Gear.new(52, 11, WHeel.new(26, 1.5)).gear_inches

# 4 -------------------GOOD

class Gear
  attr_reader :chainring, :cog, :wheel

  def initialize(args)
    @chainring = args[:chainring]
    @cog = args[:cog]
    @wheel = args[:wheel]
  end
end
Gear.new({:chainring => 52, :cog => 11, :wheel => Wheel.new(26, 1.5)}).gear_inches


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Define Defaults

# 5 -------------------GOOD

def initialize(args)
  # CANNOT set the key to nil or false, #fetch can
  @chainring = args[:chainring] || 40
  @cog = args[:cog] || 18
  @wheel = args[:wheel]
end

# 5 -------------------BEST

def initialize(args)
  @chainring = args.fetch(:chainring, 40)
  @cog = args.fetch(:cog, 18)
  @wheel = args[:wheel]
end

# 5 -------------------BEST

def initialize(args)
  # If the defaults are large, implement defaults method
  args = defaults.merge(args)
  @chainring = args[:chainring]
end

def defaults
  {:chainring => 40, :cog => 18}
end


# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# Isolate Multiparameter Initialization w/Modules

# 6 -------------------GOOD

module SomeFramework
  class Gear
    attr_reader :chainring, :cog, :wheel
    def initialize(chainring, cog, wheel)
      @chainring = chainring
      @cog = cog
      @wheel = wheel
    end
  end
end

module GearWrapper
  def self.gear(args)
    SomeFramework::Gear.new(args[:chainring],
                            args[:cog],
                            args[:wheel])
  end
end

GearWrapper.gear(
    :chainring => 52,
    :cog => 11,
    :wheel => Wheel.new(26, 1.5)).gear_inches