5 Composition Techniques

1_inheritance.rb

class Animal
  def run
    puts 'running'
  end
end

Animal.new.run #=> running

class Wolf < Animal
  def follow_pack
    puts 'following'
  end
end

Wolf.new.follow_pack #=> following

Wolf.new.run #=>running

# How do we get the #follow_pack behavior in Dolphin?
class Dolphin < Animal, Wolf?
end
# SyntaxError: (irb):14: syntax error, unexpected ',', expecting ';' or '\n'
# class Dolphin < Animal, Wolf?
#                        ^
#   from /Users/mikepack/.rvm/rubies/ruby-2.1.3/bin/irb:11:in `<main>'

# We could define a super class...
class Pack < Animal
  def follow_pack
    puts 'following'
  end
end

class Dolphin < Pack
end

Dolphin.new.follow_pack #=> following

# This doesn't really make sense...
Pack.new.follow_pack #=> following

2_mixins.rb

class Animal
  def run
    puts 'running'
  end
end

# We can use this module to include behavior in multiple classes...
module Pack
  def follow_pack
    puts 'following'
  end
end

class Wolf < Animal
  include Pack
end

class Dolphin < Animal
  include Pack
end

Wolf.new.follow_pack #=> following

module Hunter
  def hunt
    puts 'hunting'
  end
end

class Wolf < Animal
  # We can include multiple mixins
  include Pack, Hunter
end

Wolf.new.hunt #=> hunting

Wolf.new.follow_pack #=> following

3_delegation.rb

class Programmer
  def program
    puts 'doin my thang'
  end
end

require 'delegate'

class Manager < SimpleDelegator
  def make_charts
    puts 'sales are up'
  end
end

class Manager
  def make_charts
    puts 'sales are up'
  end
end

p = Programmer.new
p.program #=> doin my thang

# SimpleDelegators will delegate any methods they don't have
# to the object you pass into .new
m = Manager.new(p)
m.make_charts #=> sales are up
m.program #=> doin my thang
m.__getobj__ #=> #<Programmer:0x007fcbc202efc8>
m.__getobj__ == p #=> true

4_extension.rb

class Manager
  def make_charts
    puts 'sales are up'
  end
end

class ExecutivePresentation
  def initialize
    @manager = Manager.new
  end

  def start
    @manager.make_charts
  end
end

ExecutivePresentation.new.start #=> sales are up

# Say the executives ask the manager to also report about
# developer productivity, on the fly...

module Productivity
  def recap_productivity
    puts 'programmers are smarter'
  end
end

class ExecutivePresentation
  def initialize
    @manager = Manager.new
  end

  def start
    @manager.make_charts
    @manager.extend Productivity
    @manager.recap_productivity
  end
end

ExecutivePresentation.new.start
# => sales are up
# => programmers are smarter

5_refinements.rb

module Productivity
  refine Manager do
    def recap_productivity
      puts 'programmers are smarter'
    end
  end
end

class ExecutivePresentation
  using Productivity

  def initialize
    @manager = Manager.new
  end

  def start
    @manager.make_charts
    @manager.recap_productivity
  end
end

ExecutivePresentation.new.start
# => sales are up
# => programmers are smarter