Reading: Practical object-oriented design in ruby (ch. 3)

This work is due by 8:00pm on Thursday, 12 April 2018.

Read Chapter 3 of Practical Object-Oriented Design in Ruby.

For the journal send an email message to me titled “CSC 322.01 2018S Journal on Practical object-oriented design in ruby (ch. 3) (Your Name)” (without the quotation marks) and with answers to the following questions.

1. What do you see as Metz’s primary points in this chapter?

2. What important points or ideas is Metz illustrating in the following example?

    class Gear
      attr_reader :chainring, :cog, :rim, :tire
      def initialize(chainring, cog, rim, tire)
        @chainring = chainring
          @cog       = cog
        @rim       = rim
        @tire      = tire
      end
       
      def gear_inches
        ratio * wheel.diameter
        # Old code: ratio * Wheel.new(rim,tire).diameter
      end
       
      def wheel
        @wheel ||= Wheel.new(rim, tire)
      end
         
      # ...
    end
   

3. Metz advocates a very different approach to parameters (or at least parameters to the initialize) than we’ve used in the past. Explain the approach the Metz advocates.

4. Explain the concept of “dependency injection” in your own words.

If you did not turn in the reading journal on time, you should do the rewrite the following code so that

• (a) Wheel objects take a hash as a parameter;
• (b) you remove the dependency on Gear so that you can use different kinds of objects that (i) are built from chainrings and cogs (perhaps EuropeanGear or some such) and (ii) provide a gear_inches method; and
• (c) Wheel objects build the thing that computes gear_inches lazily.

You may assume that there is no other code that depends on Gear and Wheel. (You don’t have to support legacy code.)

(In essence, you are showing that you understood the key points from the reading plus one or two others.)

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

  def gear_inches(diameter)
    ratio * diameter
  end

  def ratio
    chainring / cog.to_f
  end
#  ...
end

class Wheel
  attr_reader :rim, :tire, :gear
  def initialize(rim, tire, chainring, cog)
    @rim       = rim
    @tire      = tire
    @gear      = Gear.new(chainring, cog)
  end

  def diameter
    rim + (tire * 2)
  end

  def gear_inches
    gear.gear_inches(diameter)
  end
#  ...
end

Work time