person.rb

class Person
  @@klass_name = "Person"

  def initialize(name)
    @name = name
  end

  def self.klass_name
    @@klass_name
  end

  attr_reader :name
  attr_writer :name

  def say_hi
    @@klass_name = "Person stuff"
    puts "#{@@klass_name}:#{name} says hi"
  end

  def klass_name
    @@klass_name
  end
end

person_log

→ irb -r ./person.rb
2.1.6 :001 > a = Person.new "A"
 => #<Person:0x007fc3aa507c68 @name="A">
2.1.6 :002 > b = Person.new "B"
 => #<Person:0x007fc3aa4f68a0 @name="B">
2.1.6 :003 > a.name
 => "A"
2.1.6 :004 > b.name
 => "B"
2.1.6 :005 > a.say_hi
Person stuff:A says hi
 => nil
2.1.6 :006 > b.klass_name
 => "Person stuff"
2.1.6 :007 > exit

test.rb

      # 3 aspects of a test
      # To test whether the code you write performs what you wants it to do.
      #
      # - Pre-condition
      # - Action
      # - Asserting that the result is what you expected.

      ## Add
      # Pre-condition
      # def add(a, b)
      #
      # - Need 2 params
      # - Prepare param
      param_a = 1
      param_b = 2
      # Action
      # run the code that you wrote
      # - Assert
      raise if result != 3
      # With rspec Action + Assertion
      expect(add(a, b)).to eq(3)


      ## def dice
      ##   rand(6) + 1
      ## end

      ## class SnakeGame
        ## @position = 1
        ##
        ## def play
        ##   @position += dice
        ## end
        ##
        ## def positon
        ##   @position
        ## end
      ## end


      # Precondition
      game = SnakeGame.new
      expect(Dice).to receive(:dice).and_return 2 #assertion/pre-condition

      # Action
      game.play
      result = game.position

      # Assert
      expect(result).to eq(3)

universe.rb

class Universe
  attr_accessor :stuffs

  def initialize
    @stuffs = []
    # @ this
    # @@ this.class
  end

  def self.get(type)
    if type == "a"
      @a_single_universe ||= Universe.new
    elsif type == "b"
      @b_single_universe ||= Universe.new
    end
    # @@single_universe = Universe.new unless @@single_universe
  end

  def put(a)
    stuffs << a
  end
end

universe_log

→ irb -r ./universe.rb
2.1.6 :001 > a = Universe.get "a"
 => #<Universe:0x007fce03612c08 @stuffs=[]>
2.1.6 :002 > a.put 1
 => [1]
2.1.6 :003 > a.put 2
 => [1, 2]
2.1.6 :004 > a.put 3
 => [1, 2, 3]
2.1.6 :005 > a
 => #<Universe:0x007fce03612c08 @stuffs=[1, 2, 3]>
2.1.6 :006 > b = Universe.get "b"
 => #<Universe:0x007fce035d9fe8 @stuffs=[]>
2.1.6 :007 > b.put 1
 => [1]
2.1.6 :008 > b.put 2
 => [1, 2]
2.1.6 :009 > a
 => #<Universe:0x007fce03612c08 @stuffs=[1, 2, 3]>
2.1.6 :010 > b
 => #<Universe:0x007fce035d9fe8 @stuffs=[1, 2]>
2.1.6 :011 > Universe.get "a"
 => #<Universe:0x007fce03612c08 @stuffs=[1, 2, 3]>
2.1.6 :012 > exit