patterns/structural/flyweight.rb

# The Flyweight pattern describes how to share objects to allow their use at
# fine granularity without prohibitive cost.

# Flyweight Object
class Lamp
  attr_reader :color

  def initialize(color)
    @color = color
  end
end

class TreeBranch
  attr_reader :branch_number

  def initialize(branch_number)
    @branch_number = branch_number
  end

  def hang(lamp)
    puts "Hang #{lamp.color} lamp on branch #{branch_number}"
  end
end

# Flyweight Factory
class LampFactory
  attr_reader :lamps

  def initialize
    @lamps = {}
  end

  def find_lamp(color)
    if lamps.key?(color)
      lamp = lamps[color]
    else
      lamp = Lamp.new(color)
      lamps[color] = lamp
    end
    lamp
  end

  def total_number_of_lamps_made
    lamps.size
  end
end

class ChristmasTree
  attr_reader :lamp_factory, :lamps_hung

  def initialize(lamp_factory = LampFactory.new)
    @lamp_factory = lamp_factory
    @lamps_hung = 0

    dress_up_the_tree
  end

  def hang_lamp(color, branch_number)
    TreeBranch.new(branch_number).hang(lamp_factory.find_lamp(color))
    lamps_hung += 1
  end

  def dress_up_the_tree
    yield self if block_given?
  end
end

# Usage
christmas_tree = ChristmasTree.new
christmas_tree.dress_up_the_tree do
  hang_lamp('red', 1)
  hang_lamp('blue', 1)
  hang_lamp('yellow', 1)
  hang_lamp('red', 2)
  hang_lamp('blue', 2)
  hang_lamp('yellow', 2)
  hang_lamp('red', 3)
  hang_lamp('blue', 3)
  hang_lamp('yellow', 3)
  hang_lamp('red', 4)
  hang_lamp('blue', 4)
  hang_lamp('yellow', 4)
  hang_lamp('red', 5)
  hang_lamp('blue', 5)
  hang_lamp('yellow', 5)
  hang_lamp('red', 6)
  hang_lamp('blue', 6)
  hang_lamp('yellow', 6)
  hang_lamp('red', 7)
  hang_lamp('blue', 7)
  hang_lamp('yellow', 7)
end