POODR: Chapter 2 - Designing Classes With Single Repsonsibility

Chapter_2.rb

# 1 -------------------BAD
class Gear
  attr_reader :chainring, :cog

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

  def ratio
    chainring / cog.to_f
  end
end

puts Gear.new(52, 11).ratio
puts Gear.new(30, 27).ratio

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

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
    ratio * (rim + (tire * 2))
  end
end

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

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# Hide Instance Variables

# 2 -------------------BAD
class Gear
  def initialize(chainring, cog)
    @chainring = chainring
    @cog = cog
  end

  def ratio
    @chainring / @cog.to_f
  end
end

# 2 -------------------GOOD

class Gear
  attr_reader :chainring, :cog

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

  def ratio
    chainring / cog.to_f
  end

end

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# Hide Data Structures

# 3 -------------------BAD

class ObscuringReferences
  attr_reader :data

  def initialize(data)
    @data = data
  end

  def diameters
    data.collect do |cell|
      cell[0] + (cell[1] * 2)
    end
  end

end

# rim and tire sizes (now in millimeters!) in a 2d array
@data = [[622, 20], [622, 23], [559, 30], [559, 40]]


# 3 -------------------GOOD
class RevealingReferences
  attr_reader :wheels
  def initialize(data)
    @wheels = wheelify(data)
  end

  def diameters
    wheels.collect do |wheel|
      wheel.rim + (wheel.tire * 2)
    end
  end

  Wheel = Struct.new(:rim, :tire)

  def wheelify(data)
    data.collect do |cell|
      Wheel.new(cell[0], cell[1])
    end
  end
end


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# Enforce Single Responsibility

# 4 -------------------GOOD
def diameters
  wheels.collect { |wheel| diameter(wheel) }
end

def diameter(wheel)
  wheel.rim + (wheel.tire * 2)
end

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# Isolating Extra Responsibilities

# 5 -------------------OKAY
class Gear
  attr_reader :chainring, :cog, :wheel
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog  = cog
    @wheel = Wheel.new(rim, tire)
  end

  def ratio
    chainring / cog.to_f
  end

  def gear_inches
    ratio * wheel.diameter
  end

  # This says that wheel exists only in the context of gear.
  Wheel = Struct.new(:rim, :tire) do
    def diameter
      rim + (tire * 2)
    end
  end
end

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# Final Wheel

class Gear
  attr_reader :chainring, :cog, :wheel

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

  def ratio
    chainring / cog.to_f
  end

  def gear_inches
    ratio * wheel.diameter
  end
end

class Wheel
  attr_reader :rim, :tire

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

  def diameter
    rim + (tire * 2)
  end

  def circumference
    diameter * Math::PI
  end
end

@wheel = Wheel.new(26, 1.5)
puts @wheel.circumference