POODR, Chp 6 Acquiring Behavior Through Inheritance, Sandi Metz

poodr_chp_6.rb

class Bicycle
  attr_reader :size, :chain, :tire_size

  def initialize(args={})
    @size = args[:size]
    @chain = args[:chain] || default_chain
    @tire_size = args[:tire_size] || default_tire_size
    post_initialize(args)
  end

  def spares
    {  tire_size: tire_size,
       chain: chain}.merge(local_spares)
  end

  def default_tire_size
    raise NotImplementedError
  end

  def post_initialize(args)
    nil
  end

  def local_spares
    {}
  end

  def default_chain
    '10-speed'
  end
end

class RoadBike < Bicycle
  attr_reader :tape_color

  def post_initialize(args)
    @tape_color = args[:tape_color]
  end

  def local_spares
    {tape_color: tape_color}
  end

  def default_tire_size
    '23'
  end
end

class MountainBike < Bicycle
  attr_reader :front_shock, :rear_shock

  def post_initialize(args)
    @front_shock = args[:front_shock]
    @rear_shock = args[:rear_shock]
  end

  def local_spares
    {rear_shock: rear_shock}
  end

  def default_tire_size
    '2.1'
  end
end


road_bike = RoadBike.new(
              size:       'M',
              tape_color: 'red' )
  
road_bike.tire_size     #  = > '23'
road_bike.chain         #  = > '10-speed'

mountain_bike = MountainBike.new(
                  size:         'S',
                  front_shock:  'Manitou',
                  rear_shock:   'Fox')
                  
mountain_bike.tire_size #  = > '2.1'
mountain_bike.chain     #  = > '10-speed'

Notes:

• subclasses are specializations of their superclasses (e.g. they should do everything their superclass does plus more)
  • wait until you have 3 subclasses to implement an abstract superclass, if you can
  • promote abstractions to the superclass (rather than demote concrete to subclasses)
• "This technique of defining a basic structure in the superclass and sending messages to acquire subclass-specific contributions is known as the template method pattern."
  • superclass should implement all behavior it expects (even if just raising an error)
  • provide hooks in superclass to prevent tight coupling with super

ref: https://github.com/skmetz/poodr/blob/master/chapter_6.rb#L678

class RecumbentBike < Bicycle
  attr_reader :flag

  def post_initialize(args)
    @flag = args[:flag]
  end

  def local_spares
    {flag: flag}
  end

  def default_chain
    '9-speed'
  end

  def default_tire_size
    '28'
  end
end

bent = RecumbentBike.new(flag: 'tall and orange')
bent.spares
# -> {:tire_size => "28",
#     :chain     => "10-speed",
#     :flag      => "tall and orange"}